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 pregnan cies that were not affected by Down's syndrome at 10-13 weeks of gesta tion. Nuchal translucency measurements and crown rump length measureme nts were determined. Nuchal translucency measurement increased by abou t 17 per cent per week. Expressing the result as a multiple of the med ian (MOM) nuchal translucency for a given crown rump length allowed fo r this increase with gestational age and yielded a distribution of val ues that was approximately Gaussian. About 96 per cent of values lay b etween 0.5 and 2.0 MOM. The variance and therefore the false-positive rate of nuchal translucency were significantly reduced by recording se veral measurements and using the average: for example, the false-posit ive rate reduced from 8.3 per cent to 5.0 per cent if the average of s ix measurements were used instead of one-a potential 40 per cent reduc tion in the false-positive rate if the test were used in screening. Es timating the distribution of nuchal translucency in MOM values will as sist in specifying the statistical parameters to be used in prenatal s creening for Down's syndrome and the use of repeated nuchal translucen cy measurements is expected to have a useful effect on reducing the sc reening false-positive rate at a given MOM cut-off level. (C) 1998 Joh n Wiley & Sons, Ltd.