Fetal development
Encyclopedia
Prenatal or antenatal development is the process in which a human embryo or fetus (or foetus) gestates
during pregnancy
, from fertilization until birth
. Often, the terms fetal development, foetal development, or embryology
are used in a similar sense.
After fertilization the embryogenesis
starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age
, the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age.
When semen
is deposited in the vagina
, the spermatozoa travel through the cervix
and body of the uterus and into the Fallopian tube
s. Fertilization of the ovum
(egg cell
) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material (DNA
). The egg then polarizes
, repelling any additional sperm. The resulting combination is called a zygote
, a new and genetically unique human organism. The term "conception" refers variably to either fertilization or to formation of the conceptus
after uterine implantation, and this terminology is controversial.
Prior to fertilization, each ovum contains a complete human genome
, including a single X but no Y chromosome
. Likewise, each spermatozoon contains a complete set of autosomes and a single sex chromosome, either X or Y. The resulting human zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm. If the spermatozoon contributes a Y chromosome then the zygote will develop as a male. Unlike the X chromosome
, the Y chromosome contains very little genetic information. However it does contain a gene, SRY
, which will switch on androgen production at a later stage, leading to the development of a male
body type. In contrast, the mitochondrial genetic information of the zygote comes entirely from the mother via the ovum.
The embryo
spends the next few days traveling down the Fallopian tube
. It starts out as a single cell zygote
and then divides several times to form a ball of cells called a morula
. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst
. Up to this point there is no growth in the overall size of the embryo, as it is confined within a glycoprotein shell, known as the zona pellucida
. Instead, each division produces successively smaller cells.
The blastocyst
reaches the uterus
at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida occurs. This process is analogous to zona hatching
, a term that refers to the emergence of the blastocyst from the zona pellucida, when incubated in vitro
. This allows the trophectoderm cells of the blastocyst to come into contact with, and adhere to, the endometrial
cells of the uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as the placenta
and the membranes. The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation (Wilcox et al. 1999). The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".
Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage
, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:
Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny
." For example, the backbone is a common structure among all vertebrates such as fish
, reptile
s and mammal
s, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain
, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.
Thus, the first week of embryonic age is already week three counting with gestational age.
Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.
The following table summarizes the various expression systems during week number x of gestation.
Embryonic age: -2 to -1 weeks old. (Week 1–2 of gestational age are theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.)
Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.
Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.
Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.
Embryonic age: Week nr 4. 3 weeks old. 22–28 days from fertilization.
Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.
Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.
Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.
Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.
Embryonic age: Weeks nr 8–10. 7–9 weeks old.
Embryonic age: Weeks nr 11–14. 10–13 weeks old.
Embryonic age: Week nr 17. 16 weeks old.
Embryonic age: Week nr 21. 20 weeks old.
Embryonic age: Week nr 25. 24 weeks old.
Embryonic age: Week nr 29. 28 weeks old.
Embryonic age: Week nr 33. 32 weeks old.
Embryonic age: Weeks nr 34–37. 33–36 weeks old.
The development is continued postnatally with adaptation to extrauterine life
and child development stages
.
, and is often given as the weight put in relation to what would be expected by the gestational age. A baby born within the normal range of weight for that gestational age is known as appropriate for gestational age (AGA). An abnormally slow growth rate results in the infant being small for gestational age
, and, on the other hand, an abnormally large growth rate results in the infant being large for gestational age
. A slow growth rate and preterm birth are the two factors that can cause a low birth weight
.
Gestation
Gestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
during pregnancy
Pregnancy
Pregnancy refers to the fertilization and development of one or more offspring, known as a fetus or embryo, in a woman's uterus. In a pregnancy, there can be multiple gestations, as in the case of twins or triplets...
, from fertilization until birth
Birth
Birth is the act or process of bearing or bringing forth offspring. The offspring is brought forth from the mother. The time of human birth is defined as the time at which the fetus comes out of the mother's womb into the world...
. Often, the terms fetal development, foetal development, or embryology
Embryology
Embryology is a science which is about the development of an embryo from the fertilization of the ovum to the fetus stage...
are used in a similar sense.
After fertilization the embryogenesis
Embryogenesis
Embryogenesis is the process by which the embryo is formed and develops, until it develops into a fetus.Embryogenesis starts with the fertilization of the ovum by sperm. The fertilized ovum is referred to as a zygote...
starts. In humans, when embryogenesis finishes, by the end of the 10th week of gestational age
Gestational age
Gestational age relates to the age of an embryo or fetus . There is some ambiguity in how it is defined:*In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age....
, the precursors of all the major organs of the body have been created. Therefore, the following period, the fetal period, is described both topically on one hand, i.e. by organ, and strictly chronologically on the other, by a list of major occurrences by weeks of gestational age.
Definitions of periods
- The perinatal period (from Greek peri, "about, around" and Latin nasci "to be born") is "around the time of birthChildbirthChildbirth is the culmination of a human pregnancy or gestation period with the birth of one or more newborn infants from a woman's uterus...
", specifically from 22 completed weeks (154 days) of gestationGestationGestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
(the time when birth weightBirth weightBirth weight is the body weight of a baby at its birth.There have been numerous studies that have attempted, with varying degrees of success, to show links between birth weight and later-life conditions, including diabetes, obesity, tobacco smoking and intelligence.-Determinants:There are...
is normally 500 g) to 7 completed days after birth. - The antepartum period (from Latin ante "before" and parere "to give birth") is literally equivalent to prenatal (from Latin pre- "before" and nasci "to be born"). Practically, however, antepartum usually refers to the period between the 24th/26th week of gestationGestationGestation is the carrying of an embryo or fetus inside a female viviparous animal. Mammals during pregnancy can have one or more gestations at the same time ....
al age until birth, for example in antepartum hemorrhage.
Fertilization
Semen
Semen is an organic fluid, also known as seminal fluid, that may contain spermatozoa. It is secreted by the gonads and other sexual organs of male or hermaphroditic animals and can fertilize female ova...
is deposited in the vagina
Vagina
The vagina is a fibromuscular tubular tract leading from the uterus to the exterior of the body in female placental mammals and marsupials, or to the cloaca in female birds, monotremes, and some reptiles. Female insects and other invertebrates also have a vagina, which is the terminal part of the...
, the spermatozoa travel through the cervix
Cervix
The cervix is the lower, narrow portion of the uterus where it joins with the top end of the vagina. It is cylindrical or conical in shape and protrudes through the upper anterior vaginal wall...
and body of the uterus and into the Fallopian tube
Fallopian tube
The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction...
s. Fertilization of the ovum
Ovum
An ovum is a haploid female reproductive cell or gamete. Both animals and embryophytes have ova. The term ovule is used for the young ovum of an animal, as well as the plant structure that carries the female gametophyte and egg cell and develops into a seed after fertilization...
(egg cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....
) usually takes place in the Fallopian tube. Many sperm must cooperate to penetrate the thick protective shell-like barrier that surrounds the ovum. The first sperm that penetrates fully into the egg donates its genetic material (DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
). The egg then polarizes
Developmental biology
Developmental biology is the study of the process by which organisms grow and develop. Modern developmental biology studies the genetic control of cell growth, differentiation and "morphogenesis", which is the process that gives rise to tissues, organs and anatomy.- Related fields of study...
, repelling any additional sperm. The resulting combination is called a zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
, a new and genetically unique human organism. The term "conception" refers variably to either fertilization or to formation of the conceptus
Conceptus
Conceptus denotes the embryo and its adnexa or associated membranes . The conceptus includes all structures that develop from the zygote, both embryonic and extraembryonic...
after uterine implantation, and this terminology is controversial.
Prior to fertilization, each ovum contains a complete human genome
Genome
In modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
, including a single X but no Y chromosome
Y chromosome
The Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
. Likewise, each spermatozoon contains a complete set of autosomes and a single sex chromosome, either X or Y. The resulting human zygote is similar to the majority of somatic cells because it contains two copies of the genome in a diploid set of chromosomes. One set of chromosomes came from the nucleus of the ovum and the second set from the nucleus of the sperm. If the spermatozoon contributes a Y chromosome then the zygote will develop as a male. Unlike the X chromosome
X chromosome
The X chromosome is one of the two sex-determining chromosomes in many animal species, including mammals and is common in both males and females. It is a part of the XY sex-determination system and X0 sex-determination system...
, the Y chromosome contains very little genetic information. However it does contain a gene, SRY
SRY
SRY is a sex-determining gene on the Y chromosome in the therians .This intronless gene encodes a transcription factor that is a member of the SOX gene family of DNA-binding proteins...
, which will switch on androgen production at a later stage, leading to the development of a male
Male
Male refers to the biological sex of an organism, or part of an organism, which produces small mobile gametes, called spermatozoa. Each spermatozoon can fuse with a larger female gamete or ovum, in the process of fertilization...
body type. In contrast, the mitochondrial genetic information of the zygote comes entirely from the mother via the ovum.
Embryonic period
The embryonic period in humans begins at fertilization (penetration of the egg by the sperm) and continues until the end of the 10th week of gestation (8th week by embryonic age).The embryo
Embryo
An embryo is a multicellular diploid eukaryote in its earliest stage of development, from the time of first cell division until birth, hatching, or germination...
spends the next few days traveling down the Fallopian tube
Fallopian tube
The Fallopian tubes, also known as oviducts, uterine tubes, and salpinges are two very fine tubes lined with ciliated epithelia, leading from the ovaries of female mammals into the uterus, via the utero-tubal junction...
. It starts out as a single cell zygote
Zygote
A zygote , or zygocyte, is the initial cell formed when two gamete cells are joined by means of sexual reproduction. In multicellular organisms, it is the earliest developmental stage of the embryo...
and then divides several times to form a ball of cells called a morula
Morula
A morula is an embryo at an early stage of embryonic development, consisting of cells in a solid ball contained within the zona pellucida....
. Further cellular division is accompanied by the formation of a small cavity between the cells. This stage is called a blastocyst
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...
. Up to this point there is no growth in the overall size of the embryo, as it is confined within a glycoprotein shell, known as the zona pellucida
Zona pellucida
The zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...
. Instead, each division produces successively smaller cells.
The blastocyst
Blastocyst
The blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...
reaches the uterus
Uterus
The uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
at roughly the fifth day after fertilization. It is here that lysis of the zona pellucida occurs. This process is analogous to zona hatching
Zona hatching
Zona hatching is a phenomenon occurring during prenatal development. Prior to this event, the predecessor of the embryo, in the form of a blastocyst, is surrounded by a glycoprotein sphere called the zona pellucida. To be able to perform implantation on the uterine wall, the blastocyst first needs...
, a term that refers to the emergence of the blastocyst from the zona pellucida, when incubated in vitro
In vitro
In vitro refers to studies in experimental biology that are conducted using components of an organism that have been isolated from their usual biological context in order to permit a more detailed or more convenient analysis than can be done with whole organisms. Colloquially, these experiments...
. This allows the trophectoderm cells of the blastocyst to come into contact with, and adhere to, the endometrial
Endometrium
-Function:The endometrium is the innermost glandular layer and functions as a lining for the uterus, preventing adhesions between the opposed walls of the myometrium, thereby maintaining the patency of the uterine cavity. During the menstrual cycle or estrous cycle, the endometrium grows to a...
cells of the uterus. The trophectoderm will eventually give rise to extra-embryonic structures, such as the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
and the membranes. The embryo becomes embedded in the endometrium in a process called implantation. In most successful pregnancies, the embryo implants 8 to 10 days after ovulation (Wilcox et al. 1999). The embryo, the extra-embryonic membranes, and the placenta are collectively referred to as a conceptus, or the "products of conception".
Rapid growth occurs and the embryo's main external features begin to take form. This process is called differentiation, which produces the varied cell types (such as blood cells, kidney cells, and nerve cells). A spontaneous abortion, or miscarriage
Miscarriage
Miscarriage or spontaneous abortion is the spontaneous end of a pregnancy at a stage where the embryo or fetus is incapable of surviving independently, generally defined in humans at prior to 20 weeks of gestation...
, in the first trimester of pregnancy is usually due to major genetic mistakes or abnormalities in the developing embryo. During this critical period (most of the first trimester), the developing embryo is also susceptible to toxic exposures, such as:
- AlcoholAlcoholIn chemistry, an alcohol is an organic compound in which the hydroxy functional group is bound to a carbon atom. In particular, this carbon center should be saturated, having single bonds to three other atoms....
, certain drugPsychoactive drugA psychoactive drug, psychopharmaceutical, or psychotropic is a chemical substance that crosses the blood–brain barrier and acts primarily upon the central nervous system where it affects brain function, resulting in changes in perception, mood, consciousness, cognition, and behavior...
s, and other toxins that cause birth defects, such as Fetal alcohol syndromeFetal alcohol syndromeFetal alcohol syndrome is a pattern of mental and physical defects that can develop in a fetus in association with high levels of alcohol consumption during pregnancy. Current research also implicates other lifestyle choices made by the prospective mother... - InfectionInfectionAn infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...
(such as rubellaRubellaRubella, commonly known as German measles, is a disease caused by the rubella virus. The name "rubella" is derived from the Latin, meaning little red. Rubella is also known as German measles because the disease was first described by German physicians in the mid-eighteenth century. This disease is...
or cytomegalovirusCytomegalovirusCytomegalovirus is a viral genus of the viral group known as Herpesviridae or herpesviruses. It is typically abbreviated as CMV: The species that infects humans is commonly known as human CMV or human herpesvirus-5 , and is the most studied of all cytomegaloviruses...
) - RadiationRadiationIn physics, radiation is a process in which energetic particles or energetic waves travel through a medium or space. There are two distinct types of radiation; ionizing and non-ionizing...
from x-rayX-rayX-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s or radiation therapyRadiation therapyRadiation therapy , radiation oncology, or radiotherapy , sometimes abbreviated to XRT or DXT, is the medical use of ionizing radiation, generally as part of cancer treatment to control malignant cells.Radiation therapy is commonly applied to the cancerous tumor because of its ability to control... - Nutritional deficiencies such as lack of folate which contributes to spina bifidaSpina bifidaSpina bifida is a developmental congenital disorder caused by the incomplete closing of the embryonic neural tube. Some vertebrae overlying the spinal cord are not fully formed and remain unfused and open. If the opening is large enough, this allows a portion of the spinal cord to protrude through...
Generally, if a structure pre-dates another structure in evolutionary terms, then it often appears earlier than the other in an embryo; this general observation is sometimes summarized by the phrase "ontogeny recapitulates phylogeny
Recapitulation theory
The theory of recapitulation, also called the biogenetic law or embryological parallelism—and often expressed as "ontogeny recapitulates phylogeny"—is a disproven hypothesis that in developing from embryo to adult, animals go through stages resembling or representing successive stages...
." For example, the backbone is a common structure among all vertebrates such as fish
Fish
Fish are a paraphyletic group of organisms that consist of all gill-bearing aquatic vertebrate animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish, as well as various extinct related groups...
, reptile
Reptile
Reptiles are members of a class of air-breathing, ectothermic vertebrates which are characterized by laying shelled eggs , and having skin covered in scales and/or scutes. They are tetrapods, either having four limbs or being descended from four-limbed ancestors...
s and mammal
Mammal
Mammals are members of a class of air-breathing vertebrate animals characterised by the possession of endothermy, hair, three middle ear bones, and mammary glands functional in mothers with young...
s, and the backbone also appears as one of the earliest structures laid out in all vertebrate embryos. The cerebrum in humans, which is the most sophisticated part of the brain
Human brain
The human brain has the same general structure as the brains of other mammals, but is over three times larger than the brain of a typical mammal with an equivalent body size. Estimates for the number of neurons in the human brain range from 80 to 120 billion...
, develops last. The concept of recapitulation is not absolute, but it is recognized as being partly applicable to development of the human embryo.
Gestational age vs. embryonic age
Gestational age is the time that has passed since the onset of the last menstruation, which generally or as standard occurs 2 weeks before the actual fertilization. Embryonic age, in contrast measures the actual age of the embryo or fetus from the time of fertilization. Nevertheless, menstruation has historically been the only means of estimating embryonal/fetal age, and is still the presumed measure if not else specified. However, the actual duration between last menstruation and fertilization may in fact differ from the standard 2 weeks by several days.Thus, the first week of embryonic age is already week three counting with gestational age.
Furthermore, the number of the week is one more than the actual age of the embryo/fetus. For example, the embryo is 0 whole weeks old during the 1st week after fertilization.
The following table summarizes the various expression systems during week number x of gestation.
| Week number |
Reached age (whole weeks) |
|
| Gestational | x | x-1 |
| Embryonic | x-2 | x-3 |
Week 1–2
Gestational age: 0 to 1 (whole) weeks old. 1–14 days from last menstruation.Embryonic age: -2 to -1 weeks old. (Week 1–2 of gestational age are theoretical extrapolations of embryonic age, since the fertilization hasn't actually occurred yet.)
Week 3
Gestational age: 2 (whole) weeks old. 15–21 days from last menstruation.Embryonic age: Week nr 1. 0 (whole) weeks old. 1–7 days from fertilization.
- Fertilization of the ovum to form a new human organism, the human zygote. (day 1 of fert.)
- The zygote undergoes mitotic cellular divisions, but does not increase in size. This mitosis is also known as cleavageCleavage (embryo)In embryology, cleavage is the division of cells in the early embryo. The zygotes of many species undergo rapid cell cycles with no significant growth, producing a cluster of cells the same size as the original zygote. The different cells derived from cleavage are called blastomeres and form a...
. A hollow cavity forms marking the blastocystBlastocystThe blastocyst is a structure formed in the early embryogenesis of mammals, after the formation of the morula. It is a specifically mammalian example of a blastula. It possesses an inner cell mass , or embryoblast, which subsequently forms the embryo, and an outer layer of cells, or trophoblast,...
stage. (day 1.5–3 of fert.) - The blastocyst contains only a thin rim of trophoblastTrophoblastTrophoblasts are cells forming the outer layer of a blastocyst, which provide nutrients to the embryo and develop into a large part of the placenta. They are formed during the first stage of pregnancy and are the first cells to differentiate from the fertilized egg...
cells and a clump of cells at one end known as the "embryonic pole" which include embryonic stem cells. - The embryo hatches from its protein shell (zona pellucidaZona pellucidaThe zona pellucida is a glycoprotein membrane surrounding the plasma membrane of an oocyte. It is a vital constitutive part of the latter, external but of essential importance to it...
) and performs implantation onto the endometrial lining of the mother's uterusUterusThe uterus or womb is a major female hormone-responsive reproductive sex organ of most mammals including humans. One end, the cervix, opens into the vagina, while the other is connected to one or both fallopian tubes, depending on the species...
. (day 5–6 of fert.) - If separation into identical twins occurs, 1/3 of the time it will happen before day 5.
Week 4
Gestational age: 3 weeks old. 22–28 days from last menstruation.Embryonic age: Week nr 2. 1 week old. 8–14 days from fertilization.
- Trophoblast cells surrounding the embryonic cells proliferate and invade deeper into the uterine lining. They will eventually form the placentaPlacentaThe placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...
and embryonic membranes. The blastocyst is fully implanted day 7–12 of fert. - Formation of the yolk sacYolk sacThe yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
. - The embryonic cells flatten into a diskEmbryonic diskThe floor of the amniotic cavity is formed by the embryonic disk composed of a layer of prismatic cells, the embryonic ectoderm, derived from the inner cell-mass and lying in apposition with the endoderm....
, two cells thick. - If separation into identical twins occurs, 2/3 of the time it will happen between days 5 and 9. If it happens after day 9, there is a significant risk of the twins being conjoinedConjoined twinsConjoined twins are identical twins whose bodies are joined in utero. A rare phenomenon, the occurrence is estimated to range from 1 in 50,000 births to 1 in 100,000 births, with a somewhat higher incidence in Southwest Asia and Africa. Approximately half are stillborn, and a smaller fraction of...
. - Primitive streakPrimitive streakThe primitive streak is a structure that forms during the early stages of avian, reptilian and mammalian embryonic development.-Introduction:...
develops. (day 13 of fert.) - Primary stem villi appear. (day 13 of fert.)
Week 5
Gestational age: 4 weeks old. 29–35 days from last menstruation.Embryonic age: Week nr 3. 2 weeks old. 15–21 days from fertilization.
- A notochordNotochordThe notochord is a flexible, rod-shaped body found in embryos of all chordates. It is composed of cells derived from the mesoderm and defines the primitive axis of the embryo. In some chordates, it persists throughout life as the main axial support of the body, while in most vertebrates it becomes...
forms in the center of the embryonic disk. (day 16 of fert.) - GastrulationGastrulationGastrulation is a phase early in the embryonic development of most animals, during which the single-layered blastula is reorganized into a trilaminar structure known as the gastrula. These three germ layers are known as the ectoderm, mesoderm, and endoderm.Gastrulation takes place after cleavage...
commences. (day 16of fert.) - A neural groove (future spinal cordSpinal cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system...
) forms over the notochord with a brainBrainThe brain is the center of the nervous system in all vertebrate and most invertebrate animals—only a few primitive invertebrates such as sponges, jellyfish, sea squirts and starfishes do not have one. It is located in the head, usually close to primary sensory apparatus such as vision, hearing,...
bulge at one end. Neuromeres appear. (day 18 of fert.) - SomiteSomiteA somite is a division of the body of an animal. In vertebrates this is mainly discernible in the embryo stage; in arthropods it is a characteristic of a hypothetical ancestor.- In vertebrates :...
s, the divisions of the future vertebra, form. (day 20 of fert.) - Primitive heart tube is forming. Vasculature begins to develop in embryonic disc. (day 20 of fert.)
Week 6
Gestational age: 5 weeks old. 36–42 days from last menstruation.Embryonic age: Week nr 4. 3 weeks old. 22–28 days from fertilization.
- The embryo measures 4 mm (1/8 inch) in length and begins to curve into a C shape.
- The heart bulges, further develops, and begins to beat in a regular rhythm. Septum primumSeptum primumIn the developing heart, the cavity of the primitive atrium becomes subdivided into right and left chambers by a septum, the septum primum, which grows downward into the cavity. The increasingly smaller gap below it is known as the ostium primum...
appear. - Branchial arches, grooves which will form structures of the faceFaceThe face is a central sense organ complex, for those animals that have one, normally on the ventral surface of the head, and can, depending on the definition in the human case, include the hair, forehead, eyebrow, eyelashes, eyes, nose, ears, cheeks, mouth, lips, philtrum, temple, teeth, skin, and...
and neckNeckThe neck is the part of the body, on many terrestrial or secondarily aquatic vertebrates, that distinguishes the head from the torso or trunk. The adjective signifying "of the neck" is cervical .-Boner anatomy: The cervical spine:The cervical portion of the human spine comprises seven boney...
, form. - The neural tubeNeural tubeIn the developing vertebrate, the neural tube is the embryo's precursor to the central nervous system, which comprises the brain and spinal cord...
closes. - The earEarThe ear is the organ that detects sound. It not only receives sound, but also aids in balance and body position. The ear is part of the auditory system....
s begin to form as otic pits. - ArmArmIn human anatomy, the arm is the part of the upper limb between the shoulder and the elbow joints. In other animals, the term arm can also be used for analogous structures, such as one of the paired forelimbs of a four-legged animal or the arms of cephalopods...
buds and a tail are visible. - Pulmonary primordium, the first traits of the lungLungThe lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
appear. - Hepatic plate, the first traits of the liverLiverThe liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
appear. - Buccopharyngeal membraneBuccopharyngeal membraneThe region where the crescentic masses of the ectoderm and endoderm come into direct contact with each other constitutes a thin membrane, the buccopharyngeal membrane , which forms a septum between the primitive mouth and pharynx...
ruptures. This is the future mouth. - Cystic diverticulum, which will become the gallbladderGallbladderIn vertebrates the gallbladder is a small organ that aids mainly in fat digestion and concentrates bile produced by the liver. In humans the loss of the gallbladder is usually easily tolerated....
, and dorsal pancreatic bud, which will become the pancreasPancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
appear. - Urorectal septumUrorectal septumThe entodermal cloaca is divided into a dorsal and a ventral part by means of a partition, the urorectal septum, which grows downward from the ridge separating the allantoic from the cloacal opening of the intestine and ultimately fuses with the cloacal membrane and divides it into an anal and a...
begins to form. Thus, the rectalRectumThe rectum is the final straight portion of the large intestine in some mammals, and the gut in others, terminating in the anus. The human rectum is about 12 cm long...
and urinary passageways become separated. - AnteriorAnterior hornThe term anterior horn may refer to either of two separate anatomical structures within the central nervous system:...
and posterior horns differentiate in the spinal cordSpinal cordThe spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the brain . The brain and spinal cord together make up the central nervous system... - SpleenSpleenThe spleen is an organ found in virtually all vertebrate animals with important roles in regard to red blood cells and the immune system. In humans, it is located in the left upper quadrant of the abdomen. It removes old red blood cells and holds a reserve of blood in case of hemorrhagic shock...
appears. - Ureteric budUreteric budThe ureteric bud is a protrusion from the mesonephric duct during the development of the urinary and reproductive organs. It later develops into the adult kidney, except for the nephrons, which, in contrast, originate from the metanephric blastema....
s appear.
Week 7
Gestational age: 6 weeks old. 43–49 days from last menstruation.Embryonic age: Week nr 5. 4 weeks old. 29–35 days from fertilization.
- The embryo measures 9 mm (1/4 inch) in length.
- Lens pits and optic cupOptic cupDuring embryonic development of the eye, the outer wall of the bulb of the optic vesicles becomes thickened and invaginated, and the bulb is thus converted into a cup, the optic cup , consisting of two strata of cells)...
s form the start of the developing eyeHuman eyeThe human eye is an organ which reacts to light for several purposes. As a conscious sense organ, the eye allows vision. Rod and cone cells in the retina allow conscious light perception and vision including color differentiation and the perception of depth...
. - Nasal pits form.
- The brain divides into 5 vesicleVesicle (biology)A vesicle is a bubble of liquid within another liquid, a supramolecular assembly made up of many different molecules. More technically, a vesicle is a small membrane-enclosed sack that can store or transport substances. Vesicles can form naturally because of the properties of lipid membranes , or...
s, including the early telencephalonTelencephalonThe cerebrum or telencephalon, together with the diencephalon, constitutes the forebrain. The cerebrum is the most anterior region of the vertebrate central nervous system. Telencephalon refers to the embryonic structure, from which the mature cerebrum develops...
. - LegLegŁęg may refer to the following places in Poland:*A former name for the town of Ełk *Part of the Czyżyny district of Kraków*Łęg, Pleszew County in Greater Poland Voivodeship...
buds form and handHandA hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
s form as flat paddles on the arms. - Rudimentary bloodBloodBlood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
moves through primitive vessels connecting to the yolk sacYolk sacThe yolk sac is a membranous sac attached to an embryo, providing early nourishment in the form of yolk in bony fishes, sharks, reptiles, birds, and primitive mammals...
and chorionic membranes. - The metanephros, precursor of the definitive kidney, starts to develop.
- The initial stomach differentiation begins.
Week 8
Gestational age: 7 weeks old. 50–56 days from last menstruation.Embryonic age: Week nr 6. 5 weeks old. 36–42 days from fertilization.
- The embryo measures 13 mm (1/2 inch) in length.
- LungLungThe lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...
s begin to form. - The brain continues to develop.
- Arms and legs have lengthened with footFootThe foot is an anatomical structure found in many vertebrates. It is the terminal portion of a limb which bears weight and allows locomotion. In many animals with feet, the foot is a separate organ at the terminal part of the leg made up of one or more segments or bones, generally including claws...
and handHandA hand is a prehensile, multi-fingered extremity located at the end of an arm or forelimb of primates such as humans, chimpanzees, monkeys, and lemurs...
areas distinguishable. - The hands and feet have digits, but may still be webbed.
- The gonadal ridgeGonadal ridgeIn embryology, the gonadal ridge is the precursor to the gonads. The gonadal ridge initially consists mainly of mesenchyme and cells of underlying mesonephric origin. Once oogonia enter this area they attempt to associate with these somatic cells...
begins to be perceptible. - The lymphatic systemLymphatic systemThe lymphoid system is the part of the immune system comprising a network of conduits called lymphatic vessels that carry a clear fluid called lymph unidirectionally toward the heart. Lymphoid tissue is found in many organs, particularly the lymph nodes, and in the lymphoid follicles associated...
begins to develop. - Main development of external genitalia starts.
Week 9
Gestational age: 8 weeks old. 57–63 days from last menstruation.Embryonic age: Week nr 7. 6 weeks old. 43–49 days from fertilization.
- The embryo measures 18 mm (3/4 inch) in length.
- Fetal heart tone (the sound of the heart beat) can be heard using dopplerDoppler fetal monitorInvented in 1958 by Dr. Edward H. Hon a Doppler fetal monitor or Doppler fetal heart rate monitor is a hand-held ultrasound transducer used to detect the heart beat of a fetus for prenatal care. It uses the Doppler effect to provide an audible simulation of the heart beat. Some models also...
. - NippleNippleIn its most general form, a nipple is a structure from which a fluid emanates. More specifically, it is the projection on the breasts or udder of a mammal by which breast milk is delivered to a mother's young. In this sense, it is often called a teat, especially when referring to non-humans, and...
s and hair follicleHair follicleA hair follicle is a skin organ that produces hair. Hair production occurs in phases, including a growth phase , and cessation phase , and a rest phase . Stem cells are principally responsible for the production of hair....
s begin to form. - Location of the elbows and toeToeToes are the digits of the foot of a tetrapod. Animal species such as cats that walk on their toes are described as being digitigrade. Humans, and other animals that walk on the soles of their feet, are described as being plantigrade; unguligrade animals are those that walk on hooves at the tips of...
s are visible. - Spontaneous limb movements may be detected by ultrasound.
- All essential organOrgan (anatomy)In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...
s have at least begun - The vitelline ductVitelline ductIn the human embryo, the vitelline duct, also known as the omphalomesenteric duct, is a long narrow tube that joins the yolk sac to the midgut lumen of the developing fetus...
normally closes
Fetal period
The fetal period begins at the end of the 10th week of gestation (8th week of development). Since the precursors of all the major organs are created by this time, the fetal period is described both by organ and by a list of changes by weeks of gestational age.Because the precursors of the organs are formed, fetus also is not as sensitive to damage from environmental exposures as the embryo. Instead, toxic exposures often cause physiological abnormalities or minor congenital malformation.
Changes by organ
Each organ has its own development.- Development of circulatory system
- Heart developmentHeart developmentThe heart is the first functional organ in a vertebrate embryo. There are 5 stages to heart development.- Specification of cardiac precursor cells :...
- Heart development
- Development of digestive system
- Tooth developmentTooth developmentTooth development or odontogenesis is the complex process by which teeth form from embryonic cells, grow, and erupt into the mouth. Although many diverse species have teeth, non-human tooth development is largely the same as in humans...
- Tooth development
- Development of endocrine system
- Development of integumentary system
- Development of lymphatic system
- Development of muscular system
- Development of nervous system
- Development of the urinary and reproductive system
- Development of the reproductive systemDevelopment of the reproductive systemThe development of the reproductive system is a part of prenatal development, and concerns the sex organs. It is a part of the stages of sexual differentiation...
- Development of the gonadsDevelopment of the gonadsThe prenatal development of the gonads is a part of the development of reproductive system and ultimately forms the testes in males and ovaries in females. They initially develop from the mesothelial layer of the peritoneum....
- Development of the gonads
- Development of the reproductive system
- Development of respiratory system
Changes by weeks of gestation
From the 8th week until birth (around 38 weeks), the developing organism is called a fetus. The fetus is not as sensitive to damage from environmental exposures as the embryo, and toxic exposures often cause physiological abnormalities or minor congenital malformation. All major structures are already formed in the fetus, but they continue to grow and develop.Weeks 10–12
Gestational age: 9–11 weeks old.Embryonic age: Weeks nr 8–10. 7–9 weeks old.
- Embryo measures 30–80 mm (1.2–3.2 inches) in length.
- Ventral and dorsal pancreatic budPancreatic budThe ventral and dorsal pancreatic buds are outgrowths of the duodenum during human embryogenesis. They join together to form the adult pancreas....
s fuse during the 8th week - IntestineIntestineIn human anatomy, the intestine is the segment of the alimentary canal extending from the pyloric sphincter of the stomach to the anus and, in humans and other mammals, consists of two segments, the small intestine and the large intestine...
s rotate. - Facial features continue to develop.
- The eyelidEyelidAn eyelid is a thin fold of skin that covers and protects an eye. With the exception of the prepuce and the labia minora, it has the thinnest skin of the whole body. The levator palpebrae superioris muscle retracts the eyelid to "open" the eye. This can be either voluntarily or involuntarily...
s are more developed. - The external features of the ear begin to take their final shape.
- The head comprises nearly half of the fetus' size.
- The face is well formed
- The eyelids close and will not reopen until about the 28th week.
- ToothToothTeeth are small, calcified, whitish structures found in the jaws of many vertebrates that are used to break down food. Some animals, particularly carnivores, also use teeth for hunting or for defensive purposes. The roots of teeth are embedded in the Mandible bone or the Maxillary bone and are...
buds, which will form the baby teeth, appear. - The limbLimb (anatomy)A limb is a jointed, or prehensile , appendage of the human or other animal body....
s are long and thin. - The fetus can make a fist with its fingers.
- Genitals appear well differentiated.
- Red blood cellRed blood cellRed blood cells are the most common type of blood cell and the vertebrate organism's principal means of delivering oxygen to the body tissues via the blood flow through the circulatory system...
s are produced in the liverLiverThe liver is a vital organ present in vertebrates and some other animals. It has a wide range of functions, including detoxification, protein synthesis, and production of biochemicals necessary for digestion...
.
Weeks 13 to 16
Gestational age: 12–15 weeks old.Embryonic age: Weeks nr 11–14. 10–13 weeks old.
- The fetus reaches a length of about 15 cm (6 inches).
- A fine hair called lanugoLanugoLanugo is fine, downy hair as a type of fur. It is often found in teratomas .-Fetal development:Lanugo grows on fetuses as a normal part of gestation, but is usually shed and replaced by vellus hair at about 33 to 36 weeks of gestational age...
develops on the head. - Fetal skinSkin-Dermis:The dermis is the layer of skin beneath the epidermis that consists of connective tissue and cushions the body from stress and strain. The dermis is tightly connected to the epidermis by a basement membrane. It also harbors many Mechanoreceptors that provide the sense of touch and heat...
is almost transparent. - More muscle tissue and bones have developed, and the bones become harder.
- The fetus makes active movements.
- Sucking motions are made with the mouth.
- MeconiumMeconiumMeconium is the earliest stools of an infant. Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water. Meconium is almost sterile, unlike later feces, is viscous and...
is made in the intestinal tract. - The liver and pancreasPancreasThe pancreas is a gland organ in the digestive and endocrine system of vertebrates. It is both an endocrine gland producing several important hormones, including insulin, glucagon, and somatostatin, as well as a digestive organ, secreting pancreatic juice containing digestive enzymes that assist...
produce fluid secretions. - From week 13, sex prediction by obstetric ultrasonographyObstetric ultrasonographyObstetric sonography is the application of medical ultrasonography to obstetrics, in which sonography is used to visualize the embryo or foetus in its mother's uterus...
is almost 100% accurate. - At week 15, main development of external genitalia is finished
Week 20
Gestational age: 18 weeks old.Embryonic age: Week nr 17. 16 weeks old.
- The fetus reaches a length of 20 cm (8 inches).
- LanugoLanugoLanugo is fine, downy hair as a type of fur. It is often found in teratomas .-Fetal development:Lanugo grows on fetuses as a normal part of gestation, but is usually shed and replaced by vellus hair at about 33 to 36 weeks of gestational age...
covers the entire body. - Eyebrows and eyelashes appear.
- Nails appear on fingers and toes.
- The fetus is more active with increased muscle development.
- "Quickening" usually occurs (the mother and others can feel the fetus moving).
- The fetal heartbeat can be heard with a stethoscopeStethoscopeThe stethoscope is an acoustic medical device for auscultation, or listening to the internal sounds of an animal body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins...
.
Week 23
Gestational age: 22 weeks old.Embryonic age: Week nr 21. 20 weeks old.
- The fetus reaches a length of 28 cm (11.2 inches).
- The fetus weighs about 925g.
- Eyebrows and eyelashes are well formed.
- All of the eye components are developed.
- The fetus has a hand and startle reflex.
- FootprintFootprintFootprints are the impressions or images left behind by a person walking. Hoofprints and pawprints are those left by animals with hooves or paws rather than feet, while "shoeprints" is the specific term for prints made by shoes...
s and fingerprintFingerprintA fingerprint in its narrow sense is an impression left by the friction ridges of a human finger. In a wider use of the term, fingerprints are the traces of an impression from the friction ridges of any part of a human hand. A print from the foot can also leave an impression of friction ridges...
s continue forming. - Alveoli (air sacs) are forming in lungs.
Week 27
Gestational age: 26 weeks old.Embryonic age: Week nr 25. 24 weeks old.
- The fetus reaches a length of 38 cm (15 inches).
- The fetus weighs about 1.2 kg (2 lb 11 oz).
- The brain develops rapidly.
- The nervous systemNervous systemThe nervous system is an organ system containing a network of specialized cells called neurons that coordinate the actions of an animal and transmit signals between different parts of its body. In most animals the nervous system consists of two parts, central and peripheral. The central nervous...
develops enough to control some body functions. - The eyelids open and close.
- The cochleae are now developed, though the myelin sheaths in neural portion of the auditory system will continue to develop until 18 months after birth.
- The respiratory system, while immature, has developed to the point where gas exchange is possible.
Week 31
Gestational age: 30 weeks old.Embryonic age: Week nr 29. 28 weeks old.
- The fetus reaches a length of about 38–43 cm (15–17 inches).
- The fetus weighs about 1.5 kg (3 lb 0 oz).
- The amount of body fat rapidly increases.
- Rhythmic breathing movements occur, but lungs are not fully mature.
- ThalamicThalamusThe thalamus is a midline paired symmetrical structure within the brains of vertebrates, including humans. It is situated between the cerebral cortex and midbrain, both in terms of location and neurological connections...
brain connections, which mediate sensory input, form. - Bones are fully developed, but are still soft and pliable.
- The fetus begins storing a lot of ironIronIron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, calciumCalciumCalcium is the chemical element with the symbol Ca and atomic number 20. It has an atomic mass of 40.078 amu. Calcium is a soft gray alkaline earth metal, and is the fifth-most-abundant element by mass in the Earth's crust...
and phosphorusPhosphorusPhosphorus is the chemical element that has the symbol P and atomic number 15. A multivalent nonmetal of the nitrogen group, phosphorus as a mineral is almost always present in its maximally oxidized state, as inorganic phosphate rocks...
.
Week 35
Gestational age: 34 weeks old.Embryonic age: Week nr 33. 32 weeks old.
- The fetus reaches a length of about 40–48 cm (16–19 inches).
- The fetus weighs about 2.5 to 3 kg (5 lb 12 oz to 6 lb 12 oz).
- Lanugo begins to disappear.
- Body fat increases.
- Fingernails reach the end of the fingertips.
- A baby born at 36 weeks has a high chance of survival, but may require medical interventions.
Weeks 36 to 39
Gestational age: 35–38 weeks old.Embryonic age: Weeks nr 34–37. 33–36 weeks old.
- The fetus is considered full-term at the end of the 37th week of gestational age.
- It may be 48 to 53 cm (19 to 21 inches) in length.
- The lanugo is gone except on the upper arms and shoulders.
- Fingernails extend beyond fingertips.
- Small breast buds are present on both sexes.
- Head hair is now coarse and thickest.
The development is continued postnatally with adaptation to extrauterine life
Adaptation to extrauterine life
At the end of pregnancy, the fetus must take the journey of childbirth to leave the reproductive female mother.Upon its entry to the air-breathing world, the newborn must begin to adjust to life outside the uterus.-Breathing and circulation:...
and child development stages
Child development stages
Child development stages describe theoretical milestones of child development. Many stage models of development have been proposed, used as working concepts and in some cases asserted as nativist theories....
.
Growth rate
The growth rate of an embryo and infant can be reflected as the weight per gestational ageGestational age
Gestational age relates to the age of an embryo or fetus . There is some ambiguity in how it is defined:*In embryology, gestational age is the time elapsed since conception. This interval is also termed fertilisation age....
, and is often given as the weight put in relation to what would be expected by the gestational age. A baby born within the normal range of weight for that gestational age is known as appropriate for gestational age (AGA). An abnormally slow growth rate results in the infant being small for gestational age
Small for gestational age
Small for gestational age babies are those who are smaller in size than normal for the baby's sex and gestational age, most commonly defined as a weight below the 10th percentile for the gestational age.-Terminology:...
, and, on the other hand, an abnormally large growth rate results in the infant being large for gestational age
Large for gestational age
Large for gestational age is an indication of high prenatal growth rate, often defined as a weight that lies above the 90th percentile for that gestational age...
. A slow growth rate and preterm birth are the two factors that can cause a low birth weight
Low birth weight
Low birth weight is defined as a birth weight of a liveborn infant of less than 2,500 g. regardless of gestational age-Causes:LBW is either the result of preterm birth or of the infant being small for gestational age , or a combination of...
.