First trimester screening for aneuploidy: serum biochemical markers.

The article reviews screening for Down syndrome in the first trimester (8-13 gestational weeks) with maternal serum analytes. In the first trimester, 2 serum markers stand out: pregnancy-associated plasma protein-A, a large glycoprotein tetramer, and free beta-human chorionic gonadotropin (beta-hCG), 1 of the 2 subunits of the glycoprotein hormone hCG. Some data indicate that hCG itself may be as effective as free beta-hCG in the first trimester. Maternal serum levels of pregnancy-associated plasma protein-A are low and free beta-hCG are high (consensus multiple of the medians, 0.4 and 1.8, respectively) in Down syndrome pregnancy. The consensus estimate of screening performance by using pregnancy-associated plasma protein-A and free beta-hCG in combination with maternal age is 60% detection rate at a 5% false positive rate. This is similar to the screening performance of second trimester double markers, but not as good as the screening performance of second trimester triple or quad markers. For this reason, first trimester screening with serum markers alone cannot be recommended except in cases in which second trimester screening cannot be done.