<P> 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 . </P> <P> 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 first trimester screening tool . Abnormal findings allow for early careful evaluation of chromosomes and possible structural defects on a targeted basis . </P> <P> How to define a normal or abnormal nuchal translucency measurement can be difficult . The use of a single millimeter cutoff (such as 2.5 or 3.0 mm) is inappropriate because nuchal translucency measurements normally increases with gestational age (by approximately 15% to 20% per gestational week from 10 to 13 weeks). At 12 weeks of gestational age, an "average" nuchal thickness of 2.18 mm has been observed; however, up to 13% of chromosomally normal fetuses present with a nuchal translucency of greater than 2.5 mm . 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 . </P> <P> The advantage of nuchal scanning over the previous use of just biochemical blood profiling is mainly the reduction in false positive rates . </P>

Normal range for nuchal fold at 12 weeks