A: A multiple marker screening, commonly called a triple screen or a quad screen, is a blood test that's usually offered to all pregnant women between 15 and 20 weeks. It's used to check your baby's risk of having problems with the brain and spinal cord (called neural-tube defects) and chromosomal abnormalities, such as Down syndrome.
It's important to remember that these tests only assess your baby's risk of having these conditions -- they can't diagnose a problem for sure. If you're older than 35, your baby's risk for such problems is higher, so your doctor may suggest skipping the triple or quad screen and going directly to amniocentesis (a more invasive procedure that involves extracting a small amount of amniotic fluid from the uterus), which is more accurate in diagnosing abnormalities.
The multiple marker screening measures the levels of three (in a triple screen) or four (in a quad screen) hormones and proteins in your blood, including AFP, hCG, estriol, and inhibin A. The results are plugged into a formula that also factors in your age, weight, race, and other issues; when taken all together, these figures can provide an idea of how likely it is that your baby could be born with one of several birth defects. The tests are about 70 to 80 percent accurate at detecting the risk of Down syndrome and certain neural-tube defects.
Should your tests yield abnormal results, don't panic. Of all the women who take a multiple marker screening, about 5 percent will have abnormal results -- but only 2 to 3 percent of that group will actually have a baby with abnormalities, according to the American College of Obstetricians and Gynecologists (ACOG). Most of the time, an abnormal result simply means that your baby's younger or older than your doctor thought (which is important, since levels of these substances depend on your specific stage of pregnancy).
For women with abnormal test results, the next step is usually a detailed ultrasound, which can determine the baby's age more accurately and reveal if you're carrying twins (another cause of abnormal results). If those causes are ruled out, then your doctor may recommend amniocentesis for further testing.