The inhibin A test is done to measure the amount of this hormone in a pregnant woman's blood to see if the baby may have Down syndrome. Inhibin A is made by the placenta during pregnancy. The level of inhibin A in the blood is used in a maternal serum quadruple screening test. Generally done between 15 and 20 weeks, this test checks the levels of four substances in a pregnant woman's blood. The quad screen checks alpha-fetoprotein (AFP), human chorionic gonadotropin (hCG), a type of estrogen (unconjugated estriol, or uE3), and the hormone inhibin A. The levels of these substances-along with a woman's age and other factors-help the doctor estimate the chance that the baby may have certain problems or birth defects.