Screening for Down's syndrome by fetal nuchal translucency measurement in a general obstetric population

Objective. To examine the effectiveness of nuchal translucency measurement in the detection of trisomy 21 in a low-risk population. Design. Prospective cohort study. Subjects. A total of 1473 women with viable singleton pregnancies between 10 and 14 weeks' gestation attending an antenatal clinic for routine obstetric care. Methods. The fetal nuchal translucency was measured in all women. Fetal karyotyping was performed for the usual indications and in cases of a nuchal translucency measurement ≥ 3 mm. Results. Down's syndrome was found in nine fetuses (0.6%). Screening by maternal age would have diagnosed six out of nine fetuses (67%) with trisomy 21 for an invasive testing rate of 24%. Because the actual uptake of prenatal diagnosis for maternal age was 79%, only 44% of the Down's syndrome fetuses would have been detected prenatally by this screening method. A nuchal translucency of 3 mm or more identified 67% of the fetuses with trisomy 21, for an invasive testing rate of 2.2%. The combination of nuchal translucency thickness, corrected for the influence of gestation by 'delta-value', and maternal age performs differently according to the chosen cut-off point for adjusted risk. A minimum risk of 1:100 would detect 78% of the Down's syndrome fetuses for a testing rate of 8.1%. By offering karyotyping to all women with a post-test risk of 1:300, the detection rate would increase to 100% with an invasive testing rate of 19.8%. This is lower than the invasive testing rate of maternal age screening. Conclusions. These data suggest that nuchal translucency measurement is an effective screening method for trisomy 21 in an unselected obstetric population.