Nuchal scan
Encyclopedia
A nuchal scan is a sonographic prenatal screening
scan (ultrasound
) to help identify higher risks of chromosomal defects including Down's syndrome in a fetus
, particularly for older women who have higher risks of such pregnancies. High thickness measurements are also associated with congenital heart defect
. The scan is carried out at 11–13.6 weeks pregnancy and assesses the amount of fluid behind the neck of the fetus - also known as the nuchal fold or the nuchal translucency. Fetuses at risk of Down's Syndrome tend to have a higher amount of fluid around the neck. The scan may also help confirm both the accuracy of the pregnancy dates and the fetal viability. Its high definition imaging may also detect other less common chromosomal abnormalities.
and midwives to estimate the risk of the fetus having Down's syndrome or other abnormalities more accurately than by maternal age alone.
21). The risk rises with maternal age from 1 in 1400 pregnancies below age 25, to 1 in 350 at age 35, to 1 in 100 at age 40.
The only way to be sure whether the fetus has a chromosomal abnormality is by having an invasive test such as an amniocentesis
or chorionic villus sampling
, but such tests carry a risk of causing a miscarriage
estimated variously as ranging between 1% or 0.06%. Based on maternal age, some countries offer invasive testing to women over 35; others to the oldest 5% of pregnant women. Most women, especially those with a low risk of having a Down-affected child, may wish to avoid the risk to the fetus and the discomfort of invasive testing.
Blood testing is also used to look for abnormal levels of fetal protein
or hormones. The results of all three factors may indicate a higher risk. If this is the case, the woman may be advised to have invasive tests.
Screening for Down's syndrome by a combination of maternal age and thickness of nuchal translucency in the fetus at 11–14 weeks of gestation was introduced in the 1990s. This method identifies about 75% of affected fetuses while screening about 5% of pregnancies. Natural fetal loss after positive diagnosis at 12 weeks is about 30%.
Normal thickness depends on the crown-rump length
(CRL) of the fetus. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality.
At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed, however, up to 13% of chromosomally normal fetuses present with a nuchal lucency of greater than 2.5mm, and thus for even greater accuracy of predicting risks, the outcome of the nuchal scan may be combined with the results of simultaneous maternal blood tests. In pregnancies affected by Down's syndrome there is a tendency for the levels of human chorionic gonadotropin
(hCG) to be increased and pregnancy-associated plasma protein A
(PAPP-A) to be decreased.
The advantage of nuchal scanning over the previous use of just biochemical blood profiling, is mainly the reduction in false positive rates.
Nuchal scanning alone detects 62% of all Down's Syndrome with a false positive rate of 5.0%, the combination with blood testing gives corresponding values of 73% and 4.7%.
In another study values of 79.6% and 2.7% for the combined screening were then improved with the addition of second trimester ultrasound scanning to 89.7% and 4.2% respectively.
A further study reported detection of 88% for trisomy 21 (Down syndrome) and 75% for trisomy 18 (Edwards syndrome
), with a 3.3% false-positive rate. Finally, using the additional ultrasound feature of an absent nasal bone can further increase detection rates for Down syndrome to more than 95%.
When screening is positive, Chorionic villus sampling
(CVS) or amniocentesis
testing is required to confirm the presence of a genetic abnormality. However this procedure carries a small risk of miscarriage so prior screening with low false positive rates are needed to minimize the chance of miscarrying.
is developing and the peripheral resistance of the placenta
is high. After 14 weeks the lymphatic system is likely to have developed sufficiently to drain away any excess fluid, and
changes to the placental circulation will result in a drop in peripheral resistance. So after this time any abnormalities causing fluid accumulation may seem to correct themselves and can thus go undetected by nuchal scanning.
The buildup in fluid is due to a blockage of fluid in the developing fetal lymphatic system
. Progressive increase in the width of the translucent area during the 11 to 14 week measurement period is thus indicative of congenital lymphedema
.
Prenatal diagnosis
Prenatal diagnosis or prenatal screening is testing for diseases or conditions in a fetus or embryo before it is born. The aim is to detect birth defects such as neural tube defects, Down syndrome, chromosome abnormalities, genetic diseases and other conditions, such as spina bifida, cleft palate,...
scan (ultrasound
Ultrasound
Ultrasound is cyclic sound pressure with a frequency greater than the upper limit of human hearing. Ultrasound is thus not separated from "normal" sound based on differences in physical properties, only the fact that humans cannot hear it. Although this limit varies from person to person, it is...
) to help identify higher risks of chromosomal defects including Down's syndrome in 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...
, particularly for older women who have higher risks of such pregnancies. High thickness measurements are also associated with congenital heart defect
Congenital heart defect
A congenital heart defect is a defect in the structure of the heart and great vessels which is present at birth. Many types of heart defects exist, most of which either obstruct blood flow in the heart or vessels near it, or cause blood to flow through the heart in an abnormal pattern. Other...
. The scan is carried out at 11–13.6 weeks pregnancy and assesses the amount of fluid behind the neck of the fetus - also known as the nuchal fold or the nuchal translucency. Fetuses at risk of Down's Syndrome tend to have a higher amount of fluid around the neck. The scan may also help confirm both the accuracy of the pregnancy dates and the fetal viability. Its high definition imaging may also detect other less common chromosomal abnormalities.
Indication
All women, whatever their age, have a small risk of delivering a baby with a physical or intellectual disability. The nuchal scan helps doctorsPhysician
A physician is a health care provider who practices the profession of medicine, which is concerned with promoting, maintaining or restoring human health through the study, diagnosis, and treatment of disease, injury and other physical and mental impairments...
and midwives to estimate the risk of the fetus having Down's syndrome or other abnormalities more accurately than by maternal age alone.
Down's Syndrome
The most common genetic disorder is Down's syndrome (trisomyTrisomy
A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...
21). The risk rises with maternal age from 1 in 1400 pregnancies below age 25, to 1 in 350 at age 35, to 1 in 100 at age 40.
The only way to be sure whether the fetus has a chromosomal abnormality is by having an invasive test such as an amniocentesis
Amniocentesis
Amniocentesis is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for...
or chorionic villus sampling
Chorionic villus sampling
Chorionic villus sampling , sometimes misspelled "chorionic villous sampling", is a form of prenatal diagnosis to determine chromosomal or genetic disorders in the fetus. It entails sampling of the chorionic villus and testing it...
, but such tests carry a risk of causing 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...
estimated variously as ranging between 1% or 0.06%. Based on maternal age, some countries offer invasive testing to women over 35; others to the oldest 5% of pregnant women. Most women, especially those with a low risk of having a Down-affected child, may wish to avoid the risk to the fetus and the discomfort of invasive testing.
Blood testing is also used to look for abnormal levels of fetal protein
Fetal protein
Fetal proteins are simply proteins present at highest levels at the fetus stage. Often related proteins assume similar roles after birth or in the embryo, in which case the fetal varieties are called fetal isoforms...
or hormones. The results of all three factors may indicate a higher risk. If this is the case, the woman may be advised to have invasive tests.
Screening for Down's syndrome by a combination of maternal age and thickness of nuchal translucency in the fetus at 11–14 weeks of gestation was introduced in the 1990s. This method identifies about 75% of affected fetuses while screening about 5% of pregnancies. Natural fetal loss after positive diagnosis at 12 weeks is about 30%.
Other chromosomal defects
Other chromosomal defects that cause a thicker nuchal translucency are- Turner syndromeTurner syndromeTurner 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...
- Trisomy 18
- Trisomy 13
- Triploidy
Other defects with normal karyotype
In fetuses with a normal number of chromosomes, a thicker nuchal translucency is associated with other fetal defects and genetic syndromes.Procedure
Nuchal scan is performed between 11 and 14 weeks of gestation, because the accuracy is best in this period. The scan is obtained with the fetus in sagittal section and a neutral position of the fetal head (neither hyperflexed nor extended, either of which can influence the nuchal translucency thickness). The fetal image is enlarged to fill 75% of the screen, and the maximum thickness is measured, from leading edge to leading edge. It is important to distinguish the nuchal lucency from the underlying amnionic membrane.Normal thickness depends on the crown-rump length
Crown-rump length
Crown-rump length is the measurement of the length of human embryos and fetuses from the top of the head to the bottom of the buttocks...
(CRL) of the fetus. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality.
Accuracy
Between 65 and 85% of trisomic fetuses will have a large nuchal thickness. Further, other, non-trisomic abnormalities may also demonstrate an enlarged nuchal transparency. This leaves the measurement of nuchal transparency as a potentially useful 1st trimester screening tool. Abnormal findings allow for early careful evaluation of chromosomes and possible structural defects on a targeted basis.At 12 weeks of gestational age, an "average" nuchal thickness of 2.18mm has been observed, however, up to 13% of chromosomally normal fetuses present with a nuchal lucency of greater than 2.5mm, and thus for even greater accuracy of predicting risks, the outcome of the nuchal scan may be combined with the results of simultaneous maternal blood tests. In pregnancies affected by Down's syndrome there is a tendency for the levels of human chorionic gonadotropin
Human chorionic gonadotropin
Human chorionic gonadotropin or human chorionic gonadotrophin is a glycoprotein hormone produced during pregnancy that is made by the developing embryo after conception and later by the syncytiotrophoblast .. Some tumors make this hormone; measured elevated levels when the patient is not...
(hCG) to be increased and pregnancy-associated plasma protein A
Pregnancy-associated plasma protein A
Pregnancy-associated plasma protein A, pappalysin 1, also known as PAPPA, is a protein used in screening tests for Down syndrome.This gene encodes a secreted metalloproteinase which cleaves insulin-like growth factor binding proteins . It is thought to be involved in local proliferative processes...
(PAPP-A) to be decreased.
The advantage of nuchal scanning over the previous use of just biochemical blood profiling, is mainly the reduction in false positive rates.
Nuchal scanning alone detects 62% of all Down's Syndrome with a false positive rate of 5.0%, the combination with blood testing gives corresponding values of 73% and 4.7%.
In another study values of 79.6% and 2.7% for the combined screening were then improved with the addition of second trimester ultrasound scanning to 89.7% and 4.2% respectively.
A further study reported detection of 88% for trisomy 21 (Down syndrome) and 75% for trisomy 18 (Edwards syndrome
Edwards syndrome
Trisomy 18 is a genetic disorder caused by the presence of all or part of an extra 18th chromosome. It is named after John H. Edwards, who first described the syndrome in 1960...
), with a 3.3% false-positive rate. Finally, using the additional ultrasound feature of an absent nasal bone can further increase detection rates for Down syndrome to more than 95%.
When screening is positive, Chorionic villus sampling
Chorionic villus sampling
Chorionic villus sampling , sometimes misspelled "chorionic villous sampling", is a form of prenatal diagnosis to determine chromosomal or genetic disorders in the fetus. It entails sampling of the chorionic villus and testing it...
(CVS) or amniocentesis
Amniocentesis
Amniocentesis is a medical procedure used in prenatal diagnosis of chromosomal abnormalities and fetal infections, in which a small amount of amniotic fluid, which contains fetal tissues, is sampled from the amnion or amniotic sac surrounding a developing fetus, and the fetal DNA is examined for...
testing is required to confirm the presence of a genetic abnormality. However this procedure carries a small risk of miscarriage so prior screening with low false positive rates are needed to minimize the chance of miscarrying.
Development of nuchal translucency
The translucent area measured (the nuchal translucency) is only useful to measure between 11 and 14 weeks of gestation, when the fetal lymphatic systemLymphatic system
The 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...
is developing and the peripheral resistance of 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...
is high. After 14 weeks the lymphatic system is likely to have developed sufficiently to drain away any excess fluid, and
changes to the placental circulation will result in a drop in peripheral resistance. So after this time any abnormalities causing fluid accumulation may seem to correct themselves and can thus go undetected by nuchal scanning.
The buildup in fluid is due to a blockage of fluid in the developing fetal lymphatic system
Lymphatic system
The 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...
. Progressive increase in the width of the translucent area during the 11 to 14 week measurement period is thus indicative of congenital lymphedema
Congenital lymphedema
Congenital lymphedema is a blockage of fluid in the developing fetal lymphatic system. It is diagnosed by a nuchal scan. Presence of this condition is also associated with congenital heart defect....
.
See also
- Prenatal diagnosisPrenatal diagnosisPrenatal diagnosis or prenatal screening is testing for diseases or conditions in a fetus or embryo before it is born. The aim is to detect birth defects such as neural tube defects, Down syndrome, chromosome abnormalities, genetic diseases and other conditions, such as spina bifida, cleft palate,...
which further discusses the reasons for prenatal screening and the ethics of such testing. - Congenital lymphedemaCongenital lymphedemaCongenital lymphedema is a blockage of fluid in the developing fetal lymphatic system. It is diagnosed by a nuchal scan. Presence of this condition is also associated with congenital heart defect....
- List of obstetric topics
- 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...