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 > or = 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.