The nuchal scan helps physicians estimate the chance of the fetus having Down syndrome or other abnormalities more accurately than by maternal age alone.

In 2011, Sequenom announced the launch of MaterniT21, a non-invasive blood test with a high level of accuracy in detecting Down syndrome (and a handful of other chromosomal abnormalities).

Screening for Down 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.

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).

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 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.

[11] Nuchal scanning alone detects 62% of all Down syndrome (sensitivity) with a false positive rate of 5.0%; the combination with blood testing gives corresponding values of 73% and 4.7%.

[15] 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.

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.