The distribution of nuchal translucency at 10–13 weeks of pregnancy

There is a need for a simple method of expressing nuchal translucency measurement in early pregnancy that will allow for gestational age and be useful in screening for Down's syndrome. To achieve this objective, we conducted a prospective study of 561 women with singleton pregnancies that were not affected by Down's syndrome at 10-13 weeks of gestation. Nuchal translucency measurements and crown rump length measurements were determined. Nuchal translucency measurement increased by about 17 per cent per week. Expressing the result as a multiple of the median (MOM) nuchal translucency for a given crown rump length allowed for this increase with gestational age and yielded a distribution of values that was approximately Gaussian. About 96 per cent of values lay between 0.5 and 2.0 MOM. The variance and therefore the false-positive rate of nuchal translucency were significantly reduced by recording several measurements and using the average: for example, the false-positive rate reduced from 8.3 per cent to 5.0 per cent if the average of six measurements were used instead of one--a potential 40 per cent reduction in the false-positive rate if the test were used in screening. Estimating the distribution of nuchal translucency in MOM values will assist in specifying the statistical parameters to be used in prenatal screening for Down's syndrome and the use of repeated nuchal translucency measurements is expected to have a useful effect on reducing the screening false-positive rate at a given MOM cut-off level.