ABSTRACT
Antenatal screening for Down syndrome has advanced over the last two decades. In the early 1980s screening was based on maternal age alone. By the end of the 1980s second trimester serum markers were added to maternal age, so that at a 5% falsepositive rate (FPR) over two-thirds of affected pregnancies could be identified compared with about one-third using age alone (1). By the 1990s tests using four serum markers in addition to maternal age were achieving detection rates (DRs) of about 75% for a 5% FPR (2) and first trimester screening markers, including nuchal translucency (NT) measurement and serum markers, were introduced. In 1999 the integration of screening markers across both trimesters into a single screening test improved screening performance compared with screening in either the first or second trimester alone. This integrated test merges first-and second-trimester markers into a single screening result, holding information collected in the first trimester (pregnancy-associated plasma protein A (PAPP-A) with or without NT) without immediate interpretation of the markers, until information from the second-trimester markers [alphafetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotrophin (hCG; either total or free b), and inhibin-A] was obtained. The integrated test could achieve DRs of about 85% for a 1% FPR (3,4). The test has now been introduced in a number of centers and found to be acceptable and effective in a multicenter international demonstration project.
