The level of hCG in the blood is often used as part of a screening for birth defects in a maternal serum triple or quadruple screening test . These tests are usually done between 15 and 20 weeks of pregnancy to check the levels of three or four substances in a pregnant woman's blood. The triple screen checks hCG, alpha-fetoprotein (AFP), and a type of estrogen (unconjugated estriol, or uE3). The quad screen checks these substances and the level of the hormone inhibin A. The levels of these substances-along with a woman's age and other factors-help the doctor figure out the chance that the baby may have certain problems or birth defects.