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 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