Second trimester serum markers.

Prenatal screening for Down syndrome in the early second trimester with multiple maternal serum markers has been available for more than 15 years. The multiple marker combination with the highest screening performance currently available is alpha-fetoprotein (AFP), unconjugated estriol (uE3), human chorionic gonadotropin (hCG), and inhibin A, together with maternal age (so-called quad marker test). With this combination, a detection rate of 80% at a 5% false positive rate is achieved. Inhibin A, the newest addition to second trimester serum screening, is an alpha-beta subunit hormone of placental origin, and is measured using a monoclonal two-site ELISA validated for use in prenatal screening. Quality control parameters for inhibin A measurement are acceptable and are monitored through the proficiency testing program administered by the College of American Pathologists. Research into other possible second trimester screening markers has included studies on the maternal urine and serum levels of an hCG variant, hyperglosylated hCG (h-hCG; invasive trophoblast antigen). Recent data indicate that h-hCG is similar to hCG itself, although its measurement in maternal urine may improve the performance of the established serum marker combinations. With the introduction of first trimester screening markers and their use in an integrated first and second trimester marker approach to screening, and with the fact that many women do not seek prenatal care until the early second trimester, prenatal screening for Down syndrome using second trimester serum markers remains a major resource in obstetrical care.