Prenatal detection of trisomy 21: combined experience of two British hospitals

A retrospective study was performed to determine the detection rate of trisomy 21 in two British hospitals using a combination of: (1) second trimester serum screening with maternal age, alphaFP and hCG; (2) karyotyping for raised maternal age and high background risk of aneuploidy; and (3) second trimester fetal anomaly ultrasonography at 18-22 week gestation. 36-410 women with a median age of 27 years were studied. Trisomy 21 detected by the combination of methods in both hospitals was compared with the actual number of pregnancies affected by trisomy 21, to determine the detection rate. Serum screening as the backbone of the service detected 31/48 (65%) trisomy 21 affected pregnancies. Karyotyping for maternal age and previous aneuploidy detected eight trisomy 21 affected pregnancies, and second trimester ultrasound a further six, giving a total detection rate of 45/56 (80%). Thus, the detection rate of trisomy 21 in our population is 65% by serum screening alone. This is similar to demonstration projects, but the addition of second trimester ultrasonography and karyotyping for maternal age and prior risk, contributes further to improve the overall sensitivity to 80%. The invasive procedure rate was 4.8% of all women.