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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 thickness of soft tissues of the nape of neck of the fetus. There are two distinct measurements -- the nuchal translucency, which is measured earlier in pregnancy at the end of the first trimester, and for which there is a lower threshold for increased diameter, and the nuchal fold, which is measured towards the end of the second trimester. 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.
All women, whatever their age, have a small risk of delivering a baby with a physical or intellectual disability. The nuchal scan helps physicians estimate the risk of the fetus having Down's syndrome or other abnormalities more accurately than by maternal age alone.
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%.
Other chromosomal defects that cause a thicker nuchal translucency are
In fetuses with a normal number of chromosomes, a thicker nuchal translucency is associated with other fetal defects and genetic syndromes.
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 (CRL) of the fetus. Among those fetuses whose nuchal translucency exceeds the normal values, there is a relatively high risk of significant abnormality.
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 (hCG) to be increased and pregnancy-associated plasma protein A (PAPP-A) to be decreased.
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.
The translucent area measured (the nuchal translucency) is only useful to measure between 11 and 14 weeks of gestation, when the fetal lymphatic system 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.
At the end of the second trimester, the nuchal translucency can no longer be seen and instead the nuchal fold is measured. The fold is more focal and at the level of the posterior fossa. This measurement has a higher threshold of normal, although the implications of increased thickness are similar to those of translucency.
The nuchal scan first came into widespread use in 2003.