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 women's 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 baby's liver
- Inhibin A, another hormone made by the placenta
Ideally, the quad screen is done between weeks 15 and 18 of pregnancy — during the second trimester. However, the procedure can be done up to week 22.
The quad screen is used to evaluate whether your pregnancy has an increased chance of being affected with certain conditions, such as Down syndrome or neural tube defects. If your risk is low, the quad screen can offer reassurance that there is a decreased chance for Down syndrome, trisomy 18, neural tube defects and abdominal wall defects.
If the quad screen indicates an increased chance of one of these conditions, you might consider additional screening or testing.