The multiple marker screening (or triple screen or quad screen) is a simple blood test done between 15 and 20 weeks to detect the level of different substances in your blood: estriol, human chorionic gonadotropin (hCG), alpha-fetoprotein (AFP) and, if you're having a quad screen, inhibin-A.
Estriol is a hormone manufactured in combination by you, your placenta, and baby, while hCG and inhibin-A are hormones made solely by the placenta. AFP is a substance made by baby, and passes in a small amount from the placenta to your blood. Together, levels of these substances show baby's risk (or lack of) certain birth defects. (The only difference between the triple and quad screen is that the quad screen is more likely to identify whether a pregnancy is at risk for Down syndrome, and is less likely to give a false positive.) Abnormal results don't necessarily mean there's a problem, though—it's simply a signal that further testing (probably CVS or amniocentesis) is a good idea.
Make sure not to procrastinate: This test can only be accurately performed between 15 and 20 weeks of pregnancy, and all it requires is a blood drawing. The triple screen can also detect whether your pregnancy is further along than previously thought. And, if you’re beginning to suspect you might be eating for more than just two, this test will be able to tell you just how many babies you're growing inside.
American College of Obstetrics and Gynecologists. Your Pregnancy and Birth. 4th ed. Washington, DC: ACOG; 2005.