Medical Content Created by the Faculty of the
Harvard Medical School
Your doctor needs to weigh you on the day or days you have your blood drawn and ask when your last period began or what your expected due date is. The analysis of the results will take into account your weight and stage of pregnancy to determine whether the levels are normal.
For first trimester screening, your blood is drawn between the 9th and 13th weeks of pregnancy. The blood is tested for two protein and hormone levels: human chorionic gonadotropin (hCG) and pregnancy-associated plasma protein-A (PAPP-A). At the time of these tests, an ultrasound of the fetus is done to measure the thickness of the tissue at the back of the fetus's neck.
For second trimester screening, your blood is drawn between your 15th and 20th weeks of pregnancy. The blood is tested for four protein and hormone levels: maternal serum alpha fetoprotein (MSAFP), unconjugated estriol (uE3), inhibin A, and human chorionic gonadotropin (hCG).