Nuchal scan

Overview
A nuchal scan is a sonographic prenatal screening scan (ultrasound) to help identify higher risks of Down syndrome in developing babies, particularly for older mothers who have higher risks of such pregnancies. The scan is carried out at 11-13 weeks pregnancy and assesses the amount of fluid behind the neck of the fetus - also known as 'the nuchal translucency'. Babies at risk of Down 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 foetal 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 and/or mental handicap. The most common genetic disorder is Down syndrome with the risk rising with maternal age from 1 in 1530 pregnancies at age 20, to 1 in 30 at age 44. Whilst the only way to know for sure whether or not the fetus has a chromosomal abnormality is by having an invasive test such as an amniocentesis or chorionic villus sampling, such tests carry a risk of causing a miscarriage variously estimated at between 1% and 0.06%, whether or not the fetus is normal or affected with Down. Most women, especially those with a low risk of having a Down-affected fetus, may wish to avoid the risk to the fetus and the discomfort of invasive testing.

The aim of the nuchal scan is to estimate the risk of the fetus having Down syndrome more accurately than calculating based on maternal age alone. Only those women with significantly higher risks than that predicted for their age group, or those with an estimated risk above that of the fetal loss rate associated with amniocentesis are advised to proceed to invasive testing.

Procedure
Nuchal scan is performed between the 11th and 13th week 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.

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 luncency 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. The blood test is used to measure the levels of hormones - primarily hCG and PAPP-A. In pregnancies affected by Down 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 Downs 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 (Downs 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 Downs syndrome to more than 95%. .

When screening is positive, amniocentesis testing is required to confirm the presence of a genetic abnormality. However this proceedure carries a 1% risk of miscarriage so prior screening with low false negative rates are needed to minimise the chance of miscarrying what is later proved to have been a genetically normal fetus.

Development of nuchal translucency
The translucent area measured (the nuchal translucency) is only usable to measure between 10 and 13 weeks’ 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.