The quadruple marker test is a second-trimester prenatal screening that measures four substances in a pregnant woman’s blood: alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), estriol, and inhibin-A. This test assesses the risk of chromosomal abnormalities like Down syndrome (trisomy 21), trisomy 18, and neural tube or abdominal wall defects in the developing baby.