Twin - AbsoluteAstronomy.com

A twin is one of two offspring

Offspring

In biology, offspring is the product of reproduction, of a new organism produced by one or more parents.Collective offspring may be known as a brood or progeny in a more general way...

produced in the same 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...

. Twins can either be monozygotic (in common parlance, "identical"), meaning that they develop from one 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...

that splits and forms two embryos, or dizygotic ("fraternal") because they develop from two separate eggs that are fertilized by two separate sperm

Sperm

The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

.

In contrast, a fetus

Fetus

A fetus is a developing mammal or other viviparous vertebrate after the embryonic stage and before birth.In humans, the fetal stage of prenatal development starts at the beginning of the 11th week in gestational age, which is the 9th week after fertilization.-Etymology and spelling variations:The...

which develops alone in the womb is called a singleton, and the general term for one offspring of a multiple birth

Multiple birth

A multiple birth occurs when more than one fetus is carried to term in a single pregnancy. Different names for multiple births are used, depending on the number of offspring. Common multiples are two and three, known as twins and triplets...

is multiple. It is theoretically possible for two singletons to be identical if all 23 chromosomes in both gamete

Gamete

A gamete is a cell that fuses with another cell during fertilization in organisms that reproduce sexually...

s from the mother and father were to be exact matches from one birth to the next. While this is statistically improbable (less than one in one billion-billion-billion chance) under natural conditions, a controlled pairing may someday be possible.

Statistics

The twin birth rate in the United States is almost 16 twin sets (or 32 twins) per 1,000 live births, while the Yoruba

Yoruba people

The Yoruba people are one of the largest ethnic groups in West Africa. The majority of the Yoruba speak the Yoruba language...

have the highest rate of twinning in the world, at 45-50 twin sets (or 90-100 twins) per 1,000 live births, possibly because of high consumption of a specific type of yam

Yam (vegetable)

Yam is the common name for some species in the genus Dioscorea . These are perennial herbaceous vines cultivated for the consumption of their starchy tubers in Africa, Asia, Latin America and Oceania...

containing a natural phytoestrogen which may stimulate the ovaries

Ovary

The ovary is an ovum-producing reproductive organ, often found in pairs as part of the vertebrate female reproductive system. Ovaries in anatomically female individuals are analogous to testes in anatomically male individuals, in that they are both gonads and endocrine glands.-Human anatomy:Ovaries...

to release an egg from each side.

The world's highest rates of twinning are found across Central Africa, where there are 18-30 twin sets (or 37-62 twins) per 1,000 live births. In Latin-America and South Asia and South-East Asia the lowest rates are found; only 6-9 twin sets per 1,000 live births. North-America and Europe have intermediate rates of 9–16 twin sets per 1,000 live births.

Due to the limited size of the mother's womb, multiple pregnancies are much less likely to carry to full term than single births, with twin pregnancies lasting only 37 weeks (3 weeks less than full term) on average.
Families with Twins have a higher chance of producing more twins than families without twins.

Zygosity

Zygosity refers to the similarity of alleles for a trait in an organism. If both alleles are the same, the organism is homozygous for the trait. If both alleles are different, the organism is heterozygous for that trait...

is the degree of identity in the 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....

of twins.
There are five common variations of twinning. The three most common variations are all dizygotic (fraternal):

Male–female twins are the most common result, 50 percent of dizygotic twins and the most common grouping of twins.
Female–female dizygotic twins (sometimes called "sororal twins")
Male–male dizygotic twins

The other two variations are monozygotic ("identical") twins:

Female–female monozygotic twins
Male–male monozygotic twins (least common)
Male-female "monozygotic" twins are possible, but exceedingly rare (but see below)

Among non-twin births, male singletons are slightly (about five percent) more common than female singletons. The rates for singletons vary slightly by country. For example, the sex ratio of birth in the US is 1.05 males/female, while it is 1.07 males/female in Italy. However, males are also more susceptible than females to death in utero, and since the death rate in utero is higher for twins, it leads to female twins being more common than male twins.

Fraternal/Sororal (dizygotic) twins

Fraternal or dizygotic (DZ) twins (also referred to as "non-identical twins", "dissimilar twins", "biovular twins", and, in cases of females, sororal twins) usually occur when two fertilized eggs are implanted in the uterus wall at the same time. When two eggs are independently fertilized by two different sperm

Sperm

cells, fraternal twins result. The two eggs, or ova, form two zygote

Zygote

s, hence the terms dizygotic and biovular.

Dizygotic twins, like any other siblings, have an extremely small chance of having the same chromosome profile. Like any other sibling

Sibling

Siblings are people who share at least one parent. A male sibling is called a brother; and a female sibling is called a sister. In most societies throughout the world, siblings usually grow up together and spend a good deal of their childhood socializing with one another...

s, dizygotic twins may look similar, particularly given that they are the same age. However, dizygotic twins may also look very different from each other. They may be of different sexes or the same sex. The same holds true for brothers and sisters from the same parents, meaning that fraternal and/or sororal twins are simply siblings who happen to be the same age.

Studies show that there is a genetic basis for dizygotic twinning. However, it is only their mother that has any effect on the chances of having such twins; there is no known mechanism for a father to cause the release of more than one 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...

. Dizygotic twinning ranges from six per thousand births in Japan (similar to the rate of monozygotic twins) to 14 and more per thousand in some African countries.

Dizygotic twins are also more common for older mothers, with twinning rates doubling in mothers over the age of 35. With the advent of technologies and techniques to assist women in getting pregnant, the rate of fraternals has increased markedly.

Monozygotic ("identical") twins

Monozygotic (MZ) or identical twins occur when a single egg

Ovum

is fertilized to form one zygote

Zygote

(hence, "monozygotic") which then divides into two separate 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...

Mechanism

Regarding spontaneous or natural monozygotic twinning, a recent theory posits that monozygotic twins are formed after 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,...

essentially collapses, splitting the progenitor cells (those that contain the body's fundamental genetic material) in half, leaving the same genetic material divided in two on opposite sides of the embryo. Eventually, two separate fetuses develop. Spontaneous division of the zygote into two embryos is not considered to be a hereditary trait, but rather a spontaneous or random event.

Monozygotic twins may also be created artificially by embryo splitting. It can be used as an expansion of IVF to increase the number of available embryos for embryo transfer

Embryo transfer

Embryo transfer refers to a step in the process of assisted reproduction in which embryos are placed into the uterus of a female with the intent to establish a pregnancy...

Incidence

Monozygotic twinning occurs in birthing at a rate of about three in every 1000 deliveries worldwide.

The likelihood of a single fertilization resulting in monozygotic twins is uniformly distributed in all populations around the world. This is in marked contrast to dizygotic twinning, which ranges from about six per thousand births in Japan (almost similar to the rate of identical twins, which is around 4–5) to 15 and more per thousand in some parts of India and up to over 20 in some Central African countries. The exact cause for the splitting of a zygote or embryo is unknown.

In-vitro fertilization techniques are more likely to create dizygotic twins. Only about three pairs of twins per 1,000 deliveries occur as a result of natural conception, while for IVF deliveries, there are nearly 21 pairs of twins for every 1,000.

Genetic and epigenetic similarity

Monozygotic twins are genetically nearly identical and they are always the same sex unless there has been a mutation during development. The children of monozygotic twins test genetically as half-siblings (or full siblings, if monozygotic twin sisters reproduce with monozygotic twin brothers), rather than first cousins. On rare occasions, monozygotic twins may express different phenotype

Phenotype

A phenotype is an organism's observable characteristics or traits: such as its morphology, development, biochemical or physiological properties, behavior, and products of behavior...

s, normally due to an environmental factor or the deactivation of different X chromosomes in female monozygotic twins, and in some extremely rare cases, due to aneuploidy

Aneuploidy

Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...

, twins may express different sexual phenotypes, normally from an XXY Klinefelter's syndrome

Klinefelter's syndrome

Klinefelter syndrome, 46/47, XXY, or XXY syndrome is a condition in which human males have an extra X chromosome. While females have an XX chromosomal makeup, and males an XY, affected individuals have at least two X chromosomes and at least one Y chromosome...

zygote splitting unevenly.

Even with nearly identical 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...

, epigenetic modification

Epigenetics

In biology, and specifically genetics, epigenetics is the study of heritable changes in gene expression or cellular phenotype caused by mechanisms other than changes in the underlying DNA sequence – hence the name epi- -genetics...

, caused by differing environmental influences throughout their lives, affects which genes are switched on or off. A study of 80 pairs of human monozygotic twins ranging in age from three to 74 showed that the youngest twins have relatively few epigenetic differences. The number of epigenetic differences increases with age. Fifty-year-old twins had over three times the epigenetic difference of three-year-old twins. Twins who had spent their lives apart (such as those adopted by two different sets of parents at birth) had the greatest difference. However, certain characteristics become more alike as twins age, such as IQ and personality. This phenomenon illustrates the influence of genetics in many aspects of human characteristics and behavior.

Phenotype similarity

Contrary to common opinion, monozygotic twins are not always of the same phenotypical sex. There have been described cases where monozygocity resulted in 46,XO (i.e. female with Turner syndrome

Turner syndrome

Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...

) and 46,XY (i.e. male). This is thought to be due to unequal distribution of zygotic protoplasm. However, as a rule, traits and physical appearances of MZ twins are very similar.

Monozygotic twins do generally look alike, although they do not have the same fingerprint

Fingerprint

A 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 (which are environmental as well as genetic). As they mature, monozygotic twins often look less alike or more alike because of lifestyle choices or external influences.

Semi-identical twins

Half-identical or semi-identical twins (also referred to as "half twins") are the result of a very rare form of twinning in which the twins inherit exactly the same genes from their mother but different genes from their father. Although examples of half-identical twins have been found, the exact mechanism of their conception is not well-understood, but could theoretically occur in polar body twinning where sperm cells fertilize both the ovum

Ovum

and the second polar body

Polar body

A polar body is a cell structure found inside an ovum. Both animal and plant ova possess it. It is also known as a polar cell.Asymmetrical cell division leads to the production of polar bodies during oogenesis. To conserve nutrients, the majority of cytoplasm is segregated into the secondary...

.

This situation is not the same as the common form of fraternal twinning, in which two genetically different ova are fertilized by two genetically different sperm. In this case, the ova are genetically identical.

Types

There are two mechanisms by which this might happen:

Polar twins (or "polar body twins"), where two sperm fertilize an 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...

, one of the two fertilizing a polar body
Polar body
A polar body is a cell structure found inside an ovum. Both animal and plant ova possess it. It is also known as a polar cell.Asymmetrical cell division leads to the production of polar bodies during oogenesis. To conserve nutrients, the majority of cytoplasm is segregated into the secondary...

; or where an 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...

splits into identical copies, one containing a polar body, prior to fertilization, allowing it to be fertilized by two different sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

.
Sesquizygotic twins, where two sperm
Sperm
The term sperm is derived from the Greek word sperma and refers to the male reproductive cells. In the types of sexual reproduction known as anisogamy and oogamy, there is a marked difference in the size of the gametes with the smaller one being termed the "male" or sperm cell...

fertilize the one ovum, forming a triploid, and then splitting.

A 1981 study of a deceased triploid XXX

Triple X syndrome

Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X chromosome in each cell of a human female. The condition always produces females, with an XX pair of chromosomes, as well as an additional chromosome, resulting in the formation of XXX. A mosaic form...

twin fetus without a heart

Parasitic twin

A parasitic twin is the result of the processes that produce vanishing twins and conjoined twins, and may represent a continuum between the two...

showed that although its fetal development suggested that it was an identical twin, as it shared a placenta with its healthy twin, tests revealed that it was likely a polar body twin. The authors were unable to predict whether a healthy fetus could result from a polar body twinning. In 2003 a study argued that many cases of triploidity arise from semi-identical twinning. In 2007, a study reported a case of a pair of living twins, one a hermaphrodite

Hermaphrodite

In biology, a hermaphrodite is an organism that has reproductive organs normally associated with both male and female sexes.Many taxonomic groups of animals do not have separate sexes. In these groups, hermaphroditism is a normal condition, enabling a form of sexual reproduction in which both...

and one a phenotypical male. The twins were both found to be chimera

Chimera (genetics)

A chimera or chimaera is a single organism that is composed of two or more different populations of genetically distinct cells that originated from different zygotes involved in sexual reproduction. If the different cells have emerged from the same zygote, the organism is called a mosaic...

s and to share all of their maternal DNA but only half of their father's DNA. The exact mechanism of fertilization could not be determined but the study stated that it was unlikely to be a case of polar body twinning.

Degree of separation

The degree of separation of the twins in utero depends on if and when they split into two zygotes. Dizygotic twins were always two zygotes. Monozygotic twins split into two zygotes at some time very early in the pregnancy. The timing of this separation determines the chorionicity and amniocity (the number of sacs) of the pregnancy. Dichorionic twins either never divided (i.e.: were dizygotic) or they divided within the first 4 days. Monoamnionic twins divide after the first week.

In very rare cases, twins become conjoined twins

Conjoined twins

Conjoined 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...

. Furthermore, there can be various degrees of shared environment of twins in the womb, potentially leading to pregnancy complications.

It is a common misconception that two 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...

s means twins are dizygotic. But if monozygotic twins separate early enough, the arrangement of sacs and placentas in utero is indistinguishable from dizygotic twins.

Type	Description	Day
Dichorionic-Diamniotic	Normally, twins have two separate (di- being a numerical prefix Numerical prefix Number prefixes are prefixes derived from numbers or numerals. In English and other European languages, they are used to coin numerous series of words, such as unicycle – bicycle – tricycle, dyad – triad – decade, biped – quadruped, September – October – November – December, decimal – hexadecimal,... for two) chorion Chorion The chorion is one of the membranes that exist during pregnancy between the developing fetus and mother. It is formed by extraembryonic mesoderm and the two layers of trophoblast and surrounds the embryo and other membranes... s and amniotic sac Amniotic sac The amniotic sac is the sac in which the fetus develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the Chorion,... s, termed Dichorionic-Diamniotic or "DiDi". It occurs in almost all cases of dizygotic twins (except in very rare cases of fusion between their blastocysts ), in 99.7% of all pregnancies, and in 18–36% (or around 25%) of monozygotic (identical) twins. DiDi twins have the lowest mortality risk at about 9 percent, although that is still significantly higher than that of singletons.	Dichorionic-Diamniotic twins form when splitting takes place by the third day after fertilization.
Monochorionic-Diamniotic Monochorionic twins Monochorionic twins are monozygotic twins that share the same placenta. If the placenta is shared by more than two twins , these are monochorionic multiples. Monochorionic twins occur in 0.3% of all pregnancies. 75% of monozygotic twin pregnancies are monochorionic; the remaining 25% are...	Monochorionic twins share the same 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... . Monochorionic twins generally have two amniotic sac Amniotic sac The amniotic sac is the sac in which the fetus develops in amniotes. It is a tough but thin transparent pair of membranes, which hold a developing embryo until shortly before birth. The inner membrane, the amnion, contains the amniotic fluid and the fetus. The outer membrane, the Chorion,... s (called Monochorionic-Diamniotic "MoDi"), which occurs in 60–70% of the pregnancies with monozygotic twins. Monochorionic-Diamniotic twins are almost always monozygotic, with a few exceptions where the blastocysts have fused.	Days 4-8
Monochorionic-Monoamniotic Monoamniotic twins Monoamniotic twins are identical twins that share same amniotic sac within their mother’s uterus. Monoamniotic twins are always identical, and always monochorionic as well , and are sometimes termed Monoamniotic-Monochorionic twins. They also share the placenta, but have two separate umbilical cords...	When the division of the developing zygote into 2 embryos occurs, 99% of the time it is within 8 days of fertilization Sometimes, monochorionic twins also share the same amnion Amnion The amnion is a membrane building the amniotic sac that surrounds and protects an embryo. It is developed in reptiles, birds, and mammals, which are hence called “Amniota”; but not in amphibians and fish , which are consequently termed “Anamniota”. The primary role of this is the protection of the... . This situation occurs in 1–2% of monozygotic twin pregnancies. Monoamniotic twins are always monozygotic. The survival rate for monoamniotic twins is somewhere between 50% to 60%. Consequently, if twins are monoamniotic that means that the two babies will be sharing a placenta and as a result, due to the small capacity of sharing a sac, the umbilical cord has an increased chance of being tangled around the babies. Because of this, there is an increased chance that the newborns may be miscarried or suffer from cerebral palsy Cerebral palsy Cerebral palsy is an umbrella term encompassing a group of non-progressive, non-contagious motor conditions that cause physical disability in human development, chiefly in the various areas of body movement.... due to the lack of oxygen.	Monoamniotic twins occur when the split takes place after the ninth day after fertilization.
Conjoined twins Conjoined twins Conjoined 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...	When the division of the developing 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... into 2 embryos occurs, 99% of the time it is within 8 days of fertilization. Mortality is highest for conjoined twins due to the many complications resulting from shared organs.	If the division of the zygote occurs later than the 12 days then conjoined twins Conjoined twins Conjoined 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... are usually the result.

Demographics

A recent study has found that insulin-like growth factor

Insulin-like growth factor

The insulin-like growth factors are proteins with high sequence similarity to insulin. IGFs are part of a complex system that cells use to communicate with their physiologic environment...

present in dairy products may increase the chance of dizygotic twinning. Specifically, the study found that vegan

Veganism

Veganism is the practice of eliminating the use of animal products. Ethical vegans reject the commodity status of animals and the use of animal products for any purpose, while dietary vegans or strict vegetarians eliminate them from their diet only...

mothers (who exclude dairy from their diets) are one-fifth as likely to have twins as vegetarian or omnivore mothers, and concluded that "Genotypes favoring elevated IGF and diets including dairy products, especially in areas where growth hormone

Growth hormone

Growth hormone is a peptide hormone that stimulates growth, cell reproduction and regeneration in humans and other animals. Growth hormone is a 191-amino acid, single-chain polypeptide that is synthesized, stored, and secreted by the somatotroph cells within the lateral wings of the anterior...

is given to cattle, appear to enhance the chances of multiple pregnancies due to ovarian stimulation."

From 1980–97, the number of twin births in the United States rose 52%. This rise can at least partly be attributed to the increasing popularity of fertility drugs

Fertility medication

Fertility medication are drugs which enhance reproductive fertility. For women, fertility medication is used to stimulate follicle development of the ovary...

like Clomid and procedures such as in vitro fertilization, which result in multiple births more frequently than unassisted fertilizations do. It may also be linked to the increase of growth hormone

Growth hormone

s in food.

Ethnicity

About 1 in 90 human births (1.1%) results from a twin pregnancy. The rate of dizygotic twinning varies greatly among ethnic group

Ethnic group

An ethnic group is a group of people whose members identify with each other, through a common heritage, often consisting of a common language, a common culture and/or an ideology that stresses common ancestry or endogamy...

s, ranging as high as about 45 per 1000 births for the Yoruba

Yoruba people

The Yoruba people are one of the largest ethnic groups in West Africa. The majority of the Yoruba speak the Yoruba language...

to 10% for Linha São Pedro, a tiny Brazilian settlement which belongs to the city of Cândido Godói

Cândido Godói

Cândido Godói is a municipality of 6,641 inhabitants in the state Rio Grande do Sul, Brazil near the Argentinian border, famous for the high number of twins born there...

. In Cândido Godói, one in five pregnancies have resulted in twins. The Argentine historian Jorge Camarasa has put forward a theory that experiments of the Nazi doctor Josef Mengele

Josef Mengele

Josef Rudolf Mengele , also known as the Angel of Death was a German SS officer and a physician in the Nazi concentration camp Auschwitz-Birkenau. He earned doctorates in anthropology from Munich University and in medicine from Frankfurt University...

could be responsible for the high ratio of twins in the area. His theory was rejected by Brazilian scientists who had studied twins living in Linha São Pedro; they suggested genetic factors within that community as a more likely explanation. A high twinning rate has also been observed in other places of the world, including Igbo-Ora

Igbo-Ora

Igbo-Ora is a town in Oyo State, south-western Nigeria, situated north of Lagos. As of 2001, the population of the town was approximately 60,000 people...

in Nigeria

Nigeria

Nigeria , officially the Federal Republic of Nigeria, is a federal constitutional republic comprising 36 states and its Federal Capital Territory, Abuja. The country is located in West Africa and shares land borders with the Republic of Benin in the west, Chad and Cameroon in the east, and Niger in...

and Kodinji

Kodinji

Kodinhi is a village in Malappuram district in Kerala, India. The village is situated close to the town of Tirurangadi and, as of 2008, is home to around 2,000 families....

in India

India

India , officially the Republic of India , is a country in South Asia. It is the seventh-largest country by geographical area, the second-most populous country with over 1.2 billion people, and the most populous democracy in the world...

.

The widespread use of fertility drugs causing hyperovulation (stimulated release of multiple eggs by the mother) has caused what some call an "epidemic of multiple birth

Multiple birth

s". In 2001, for the first time ever in the US

United States

The United States of America is a federal constitutional republic comprising fifty states and a federal district...

, the twinning rate exceeded 3% of all births. Nevertheless, the rate of monozygotic twins remains at about 1 in 333 across the globe.

In a study on the maternity records of 5750 Hausa

Hausa people

The Hausa are one of the largest ethnic groups in West Africa. They are a Sahelian people chiefly located in northern Nigeria and southeastern Niger, but having significant numbers living in regions of Cameroon, Ghana, Cote d'Ivoire, Chad and Sudan...

women living in the Savannah zone of Nigeria

Nigeria

, there were 40 twins and 2 triplets per 1000 births. Twenty-six percent of twins were monozygous. The incidence of multiple births, which was about five times higher than that observed in any western population, was significantly lower than that of other ethnic groups, who live in the hot and humid climate of the southern part of the country. The incidence of multiple births was related to maternal age but did not bear any association to the climate or prevalence of malaria

Malaria

Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

Predisposing factors

The predisposing factors of monozygotic twinning are unknown.

Dizygotic twin pregnancies are slightly more likely when the following factors are present in the woman:

She is of West Africa
West Africa
West Africa or Western Africa is the westernmost region of the African continent. Geopolitically, the UN definition of Western Africa includes the following 16 countries and an area of approximately 5 million square km:-Flags of West Africa:...

n descent (especially Yoruba
Yoruba people
The Yoruba people are one of the largest ethnic groups in West Africa. The majority of the Yoruba speak the Yoruba language...

)
She is between the age of 30 and 40 years
She is greater than average height and weight
She has had several previous pregnancies.

Women undergoing certain fertility treatments may have a greater chance of dizygotic multiple births. This can vary depending on what types of fertility treatments are used. With in vitro fertilization (IVF), this is primarily due to the insertion of multiple embryos into the uterus. Some other treatments such as the drug Clomid can stimulate a woman to release multiple eggs, increasing the likelihood of multiples.

Delivery interval

A 15-year German study of 8,220 vaginally delivered twins (that is, 4,110 pregnancies) in Hesse

Hesse

Hesse or Hessia is both a cultural region of Germany and the name of an individual German state.* The cultural region of Hesse includes both the State of Hesse and the area known as Rhenish Hesse in the neighbouring Rhineland-Palatinate state...

yielded a mean delivery time interval of 13.5 minutes. The delivery interval between the twins was measured as follows:

Within 15 minutes: 75.8%
16-30 minutes: 16.4%
31-45 minutes: 4.3%
46-60 minutes: 1.7%
Over 60 minutes: 1.8% (72 instances)

The study stated that the occurrence of complications "was found to be more likely with increasing twin-to-twin delivery time interval" and suggested that the interval be kept short, though it noted that the study did not examine causes of complications and did not control for factors such as the level of experience of the obstetrician, the wish of the women giving birth, or the "management strategies" of the procedure of delivering the second twin.

Vanishing twins

Researchers suspect that as many as 1 in 8 pregnancies start out as multiples, but only a single fetus is brought to full term, because the other has died very early in the pregnancy and has not been detected or recorded. Early obstetric ultrasonography

Obstetric ultrasonography

Obstetric 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...

exams sometimes reveal an "extra" fetus, which fails to develop and instead disintegrates and vanishes. This is known as vanishing twin

Vanishing twin

A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed by the mother or twin....

syndrome.

Conjoined twins

Conjoined twins (or the deprecated

Deprecation

In the process of authoring computer software, its standards or documentation, deprecation is a status applied to software features to indicate that they should be avoided, typically because they have been superseded...

term "Siamese twins") are monozygotic twins whose bodies are joined together during pregnancy. This occurs where the single zygote of MZ twins fails to separate completely, and the zygote starts to split after day 12 following fertilization. This condition occurs in about 1 in 50,000 human pregnancies.
Most conjoined twins are now evaluated for surgery to attempt to separate them into separate functional bodies. The degree of difficulty rises if a vital organ or structure is shared between twins, such as the brain

Brain

The 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,...

, heart

Heart

The heart is a myogenic muscular organ found in all animals with a circulatory system , that is responsible for pumping blood throughout the blood vessels by repeated, rhythmic contractions...

or liver

Liver

The 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...

Chimerism

A chimera is an ordinary person or animal except that some of their parts actually came from their twin or from the mother. A chimera may arise either from monozygotic twin fetuses (where it would be impossible to detect), or from dizygotic fetuses, which can be identified by chromosomal comparisons from various parts of the body. The number of cells derived from each fetus can vary from one part of the body to another, and often leads to characteristic mosaicism skin coloration in human chimeras. A chimera may be intersex

Intersex

Intersex, in humans and other animals, is the presence of intermediate or atypical combinations of physical features that usually distinguish female from male...

, composed of cells from a male twin and a female twin.

Parasitic twins

Sometimes one twin fetus will fail to develop completely and continue to cause problems for its surviving twin. One fetus acts as a parasite towards the other.
Sometimes the parasitic twin becomes an almost indistinguishable part of the other, and sometimes this needs to be medically dealt with.

Partial molar twins

A very rare type of parasitic twinning is one where a single viable twin is endangered when the other zygote becomes cancerous, or molar. This means that the molar zygote's cellular division continues unchecked, resulting in a cancerous growth that overtakes the viable fetus. Typically, this results when one twin has either triploidy or complete paternal uniparental disomy

Uniparental disomy

Uniparental disomy occurs when a person receives two copies of a chromosome, or part of a chromosome, from one parent and no copies from the other parent.. UPD could involve isodisomy or heterodisomy ....

, resulting in little or no fetus and a cancerous, overgrown placenta, resembling a bunch of grapes.

Miscarried twin

Occasionally, a woman will suffer a 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...

early in pregnancy, yet the pregnancy will continue; one twin was miscarried but the other was able to be carried to term. This occurrence is similar to the vanishing twin

Vanishing twin

A vanishing twin, also known as fetal resorption, is a fetus in a multi-gestation pregnancy which dies in utero and is then partially or completely reabsorbed by the mother or twin....

syndrome, but typically occurs later than the vanishing twin syndrome.

Low birth weight

Twins typically suffer from the lower birth weight

Birth weight

Birth 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...

s and greater likelihood of prematurity

Premature birth

In humans preterm birth refers to the birth of a baby of less than 37 weeks gestational age. The cause for preterm birth is in many situations elusive and unknown; many factors appear to be associated with the development of preterm birth, making the reduction of preterm birth a challenging...

that is more commonly associated with the higher multiple pregnancies.

Twin-to-twin transfusion syndrome

Monozygotic twins who share a placenta can develop twin-to-twin transfusion syndrome. This condition means that blood from one twin is being diverted into the other twin. One twin, the 'donor' twin, is small and anemic, the other, the 'recipient' twin, is large and polycythemic. The lives of both twins are endangered by this condition.

Human twin studies

Twin studies are utilized in an attempt to determine how much of a particular trait is attributable to either genetics or environmental influence. These studies compare monozygotic and dizygotic twins for medical, genetic

Genetics

Genetics , a discipline of biology, is the science of genes, heredity, and variation in living organisms....

, or psychological

Psychology

Psychology is the study of the mind and behavior. Its immediate goal is to understand individuals and groups by both establishing general principles and researching specific cases. For many, the ultimate goal of psychology is to benefit society...

characteristics to try to isolate genetic influence from epigenetic and environment

Natural environment

The natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....

al influence. Twins that have been separated early in life and raised in separate households are especially sought-after for these studies, which have been used widely in the exploration of human nature

Human nature

Human nature refers to the distinguishing characteristics, including ways of thinking, feeling and acting, that humans tend to have naturally....

. However, the utility and accuracy of these twin studies has been called into question and remains controversial. Classical twin studies have are now being supplemented with molecular genetic studies which identify individual genes.

Unusual twinnings

Among dizygotic twins, in rare cases, the eggs are fertilized at different times with two or more acts of sexual intercourse, either within one menstrual cycle (superfecundation

Superfecundation

Superfecundation is the fertilization of two or more ova from the same cycle by sperm from separate acts of sexual intercourse. The term superfecundation is derived from fecund, meaning the ability to produce offspring....

) or, even more rarely, later on in the pregnancy (superfetation

Superfetation

Superfetation is the simultaneous occurrence of more than one stage of developing embryo in the same animal. In mammals it manifests as the formation of a fetus from a different menstrual cycle while another embryo is already present in the uterus...

). This can lead to the possibility of a woman carrying fraternal twins with different fathers (that is, half-siblings). This phenomenon is known as heteropaternal superfecundation. One 1992 study estimates that the frequency of heteropaternal superfecundation among dizygotic twins whose parents were involved in paternity suits was approximately 2.4%; see the references section, below, for more details.

Dizygotic twins from biracial couples can sometimes be mixed twins

Mixed twins

Mixed twins are fraternal twins born to multiracial families which differ in skin color and other traits considered to be racial features.From a biological point of view, the differences in these fraternal or dizygotic twins from two biracial parents are not surprising. In humans, a relatively...

, which exhibit differing ethnic and racial features. One such pairing was born in Germany in 2008 to a white father from Germany and a black mother from Ghana.

Heterotopic pregnancy

Heterotopic pregnancy

A heterotopic pregnancy is a rare complication of pregnancy in which both extra-uterine and intrauterine gestation occur simultaneously....

is an exceedingly rare type of dizygotic twinning in which one twin implants in the uterus as normal and the other remains in the fallopian tube as an ectopic pregnancy

Ectopic pregnancy

An ectopic pregnancy, or eccysis , is a complication of pregnancy in which the embryo implants outside the uterine cavity. With rare exceptions, ectopic pregnancies are not viable. Furthermore, they are dangerous for the parent, since internal haemorrhage is a life threatening complication...

. Ectopic pregnancies must be resolved because they can be life-threatening to the mother. However, in most cases, the intrauterine pregnancy can be salvaged.

Among monozygotic twins, in extremely rare cases, twins have been born with different sexes (one male, one female). The probability of this is so vanishingly small (only 3 documented cases) that multiples having different sexes is universally accepted as a sound basis for a clinical determination that in utero multiples are not monozygotic. When monozygotic twins are born with different sexes it is because of chromosomal birth defects. In this case, although the twins did come from the same egg, it is incorrect to refer to them as genetically identical, since they have different karyotype

Karyotype

A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...

Semi-identical twins

Monozygotic twins can develop differently, due to different genes being activated. More unusual are "semi-identical twins". These "half-identical twins" are hypothesized to occur when an unfertilized egg cleaves into two identical attached ova and which are viable for fertilization. Both cloned ova are then fertilized by different sperm and the coalesced eggs undergo further cell duplications developing as a chimeric blastomere

Blastomere

A blastomere is a type of cell produced by division of the egg after fertilization.- References :* "Blastomere." Stedman's Medical Dictionary, 27th ed. . ISBN 0-683-40007-X...

. If this blastomere then undergoes a twinning event, two embryos will be formed, each of which have different paternal genes and identical maternal genes.

This results in a set of twins with identical genes from the mother's side, but different genes from the father's side. Cells in each fetus carry genes from either sperm, resulting in chimera

Chimera (genetics)

s. This form had been speculated until only recently being recorded in western medicine.

Animal twins

Twins are common in many animal species, such as cat

Cat

The cat , also known as the domestic cat or housecat to distinguish it from other felids and felines, is a small, usually furry, domesticated, carnivorous mammal that is valued by humans for its companionship and for its ability to hunt vermin and household pests...

s, sheep, ferret

Ferret

The ferret is a domesticated mammal of the type Mustela putorius furo. Ferrets are sexually dimorphic predators with males being substantially larger than females. They typically have brown, black, white, or mixed fur...

s and deer

Deer

Deer are the ruminant mammals forming the family Cervidae. Species in the Cervidae family include white-tailed deer, elk, moose, red deer, reindeer, fallow deer, roe deer and chital. Male deer of all species and female reindeer grow and shed new antlers each year...

. The incidence of twinning among cattle

Cattle

Cattle are the most common type of large domesticated ungulates. They are a prominent modern member of the subfamily Bovinae, are the most widespread species of the genus Bos, and are most commonly classified collectively as Bos primigenius...

is about 1–4%, and research is under way to improve the odds of twinning, which can be more profitable for the breeder if complications can be sidestepped or managed. A female calf that is the twin of a bull becomes partially masculinized and is known as a freemartin

Freemartin

A freemartin or free-martin is an infertile female mammal which has masculinized behavior and non-functioning ovaries. Genetically and externally the animal is female, but it is sterilized in the womb by hormones from a male twin, becoming an infertile partial intersex...

. The nine-banded armadillo (Dasypus novemcinctus
Nine-banded Armadillo
The nine-banded armadillo , or the nine-banded, long-nosed armadillo, is a species of armadillo found in North, Central, and South America, making it the most widespread of the armadillos...

) has identical twins (usually four babies) as its regular reproduction and not as exceptional cases.

Statistics

Zygosity

Fraternal/Sororal (dizygotic) twins

Monozygotic ("identical") twins

Mechanism

Incidence

Genetic and epigenetic similarity

Phenotype similarity

Semi-identical twins

Types

Degree of separation

Demographics

Ethnicity

Predisposing factors

Delivery interval

Vanishing twins

Conjoined twins

Chimerism

Parasitic twins

Partial molar twins

Miscarried twin

Low birth weight

Twin-to-twin transfusion syndrome

Human twin studies

Unusual twinnings

Semi-identical twins

Animal twins

See also

Further reading