Maternal serum screening is typically performed in the first and second trimesters of pregnancy. The first-trimester screening usually occurs between 11 and 14 weeks of gestation and includes measurements of pregnancy-associated plasma protein-A (PAPP-A) and human chorionic gonadotropin (hCG). The second-trimester screening, also known as the quadruple test, is done between 15 and 20 weeks and measures alpha-fetoprotein (AFP), hCG, estriol, and inhibin-A.
