Multiple marker screening comes in two varieties: the triple screen test and the quad screen test. As you might imagine, the triple screen test looks for three substances in the blood produced by the fetus and/or placenta: alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG) and estriol. The quad screen adds the substance inhibin A (Inh-A) to the mix.
By evaluating these substances in the blood at approximately the 15th to 20th week of pregnancy -- and combining them with factors such as the mother's age, weight and ethnicity -- doctors are looking for signs of either neural tube defects like anencephaly or spina bifida, or for markers that might indicated Down syndrome.
The tests are about 80 percent accurate in predicting neural tube defects. The triple screen is approximately 60 percent accurate in spotting Down syndrome; the quad screen correctly identifies the condition 75 percent of the time [source: Bell]. Because of these relatively low percentages, and the fact that the tests tend to deliver high false-positives, if a doctor spots something amiss, she will likely recommend further testing such as an amniocentesis (see sidebar) or a sonogram (if you haven't had one already). Sometimes, abnormal results from these tests can be explained by the fact that the pregnancy is actually further along then previously thought, which a sonogram would discover.