SCREENING FOR DOWNS-SYNDROME BY FETAL NUCHAL TRANSLUCENCY MEASUREMENTIN A GENERAL OBSTETRIC POPULATION

Objective. To examine the effectiveness of nuchal translucency measure ment 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 perform ed for the usual indications and in cases of a nuchal translucency mea surement greater than or equal to 3 mm. Results. Down's syndrome was f ound in nine fetuses (0.6%). Screening by maternal age would have diag nosed six out of nine fetuses (67%) with trisomy 21 for an invasive te sting rate of 24%. Because the actual uptake of prenatal diagnosis for maternal age was 79%, only 44% of the Down's syndrome fetuses would h ave been detected prenatally by this screening method. A nuchal transl ucency 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 transl ucency thickness, corrected for the influence of gestation by 'delta-v alue', and maternal age performs differently according to the chosen c ut-off point for adjusted risk. A minimum risk of 1:100 would detect 7 8% of the Down's syndrome fetuses for a testing rate of 8.1%. By offer ing karyotyping to all women with a post-test risk of 1:300, the detec tion rate would increase to 100% with an invasive testing rate of 19.8 %. This is lower than the invasive testing rate of maternal age screen ing.Conclusions. These data suggest that nuchal translucency measureme nt is an effective screening method for trisomy 21 in an unselected ob stetric population.