Nuchal translucency

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Nucal thickness at 11 weeks

Late in the first trimester, an echolucent area can be identified at the back of the neck of normal fetuses. Normally thin, it has been observed that an unusually thick translucency is sometimes associated with such abnormalities as trisomy 21 and other fetal malformations.

Between the 11th and end of the 13th week of gestation, the measurement of nuchal translucency 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 overall size of the fetus (CRL), but it should not exceed 3 mm at any gestational age. 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.

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 free ß-hCG and PAPP-A. In pregnancies affected by Downs syndrome there is a tendency for the levels of free ß-hCG to be increased and PAPP-A to be decreased. Over all detection rate for using the combined method is 85% (4.2% of these are advised to have an amniocentesis).