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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.In 2011, Sequenom announced the launch of Materni T21, a non-invasive blood test with a high level of accuracy in detecting Down syndrome (and a handful of other chromosomal abnormalities).As of 2015, there are five commercial versions of this screen (called cell-free fetal DNA screening) available in the United States.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 amniotic membrane.
Natural fetal loss after positive diagnosis at 12 weeks is about 30%.
The nuchal scan helps physicians estimate the risk of the fetus having Down syndrome or other abnormalities more accurately than by maternal age alone.
Until recently, the only reliable ways to determine if the fetus has a chromosomal abnormality was to have an invasive test such as amniocentesis or chorionic villus sampling, but such tests carry a risk of causing a miscarriage estimated variously as ranging between 1% Most women, especially those with a low risk of having a child with Down syndrome, may wish to avoid the risk to the fetus and the discomfort of invasive testing.
Nuchal scan (NT procedure) 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).
If this is the case, the woman may be advised to have a more reliable screen such as cell-free fetal DNA screening or an invasive diagnostic test (such as chorionic villus sampling or amniocentesis).