AFP and Down syndrome: The most widely used second trimester screening test is the quad test, which uses the mother's age, serum beta-human chorionic gonadotropin (βHCG), alpha-fetoprotein (AFP), unconjugated estriol (uE3), and inhibin A. In the combined screening protocol, the results of the first and second trimesters are integrated, and the sensitivity of Down syndrome diagnosis reaches 94–96%.