The quad screen - also known as the quadruple marker test, the second trimester screen or simply the quad test - is a prenatal test that measures levels of four substances in pregnant womens blood: Alpha-fetoprotein (AFP), a protein made by the developing baby Human chorionic gonadotropin (HCG), a hormone made by the placenta Estriol, a hormone made by the placenta and the babys liver .The quad screen evaluates your chance of carrying a baby who has any of the following conditions: Down syndrome (trisomy 21), Trisomy 18, Abdominal wall defects, Trisomy 18. neural tube defects.