The quad screen is performed in the second trimester of pregnancy, typically between 15-20 weeks gestation. The quad screen analyzes four analytes in maternal blood: alpha-fetal protein (AFP), human chorionic gonadotropin (hCG), unconjugated estriol (μE3), and inhibin-A.

Using different factors including gestational age and maternal weight, the levels of each of these analytes is converted into a multiple of the median (MOM), with 1.0 MOM being the normal level. For example, if the AFP level was 0.5 MOM, then the level of AFP present in the woman's blood was half of the expected level.

The MOM values are used to calculate a risk for Down syndrome, trisomy 18 and open neural tube defects based in the pattern present.

If the risk for a pregnancy to have one of these conditions is elevated above a lab determined cut-off, then the test is considered a “screen positive” result, meaning that an increased risk for the condition to be present in the pregnancy was calculated by the quad screen.