First trimester biochemical screening for trisomy 21: the role of free beta hCG, alpha fetoprotein and pregnancy associated plasma protein A.

The potential efficacy of screening for trisomy 21 in the first trimester, using maternal serum markers alpha fetoprotein, free beta human chorionic gonadotropin, unconjugated oestriol and pregnancy associated plasma protein A, was studied in an unselected population of women between the seventh and fourteenth week of gestation. Using a combination of alpha fetoprotein and free beta human chorionic gonadotropin, 53% of affected pregnancies could be identified at a false positive rate of 5%. Unconjugated oestriol and pregnancy associated plasma protein A levels were lower in cases of trisomy 21, but their inclusion with other markers did not significantly improve detection rate. Monitoring the same pregnancies also in the second trimester showed that screening in the first trimester identified the same cases as in the second. We conclude that first trimester screening using free beta human chorionic gonadotropin and alpha fetoprotein, is a viable possibility and will lead to detection rates in excess of 50%. Prospective studies are needed to confirm these observations.