If nuchal translucency screening is combined with first-trimester serum screening the need for fetal karyotyping decreases

Background. This case-control study was performed to evaluate whether adding first-trimester maternal serum testing to nuchal translucency measurement would improve the antenatal detection of Down's syndrome and decrease the number of women offered fetal karyotyping. Methods. In the Swedish Nuchal Translucency Trial 39,572 pregnant women were randomized to a routine scan at 12-14 gestational weeks including nuchal translucency screening for Down's syndrome, or a routine scan at 16-18 gestational weeks. From the early scan group 47 pregnancies with Down's syndrome were identified and for each case three controls were chosen. Of 189 women asked to participate, 31 cases and 108 controls with a singleton pregnancy and frozen sample from 8-14 gestational weeks available for analysis accepted participation. Maternal sera were analyzed for free beta human chorionic gonadotrophin and pregnancy-associated plasma protein A. The risk for Down's syndrome was calculated using combinations of maternal age, crown-rump length, nuchal translucency, and biochemistry. A risk >= 1/250 was considered increased and an indication for fetal karyotyping. Results. Risk calculated on the basis of maternal age alone would have identified 21 of the 31 Down's syndrome cases by karyotyping 61 of the 139 fetuses. Maternal age and nuchal translucency would have identified 29 cases by karyotyping 51 fetuses. Maternal age, nuchal translucency, and biochemistry would also have identified 29 cases by karyotyping 37 fetuses. Conclusions. By adding first trimester biochemistry to nuchal translucency measurement the detection rate of fetuses with Down's syndrome seems to remain unchanged whereas the antenatal risk group to be offered fetal karyotyping decreases.