Maternal Serum Screening for Birth Defects
What is the AFP screening test?
A screening test that can be done using a small sample of a pregnant woman's blood to identify pregnant women who may be at increased risk for having a baby with certain birth defects, such as an open neural tube defect (spina bifida), Down syndrome, and Trisomy 18.
How is the AFP screening test done?
A small amount of blood is drawn from a vein in the pregnant woman's arm. The test is done at 15 to 18 weeks of pregnancy in most cases as this produces the most accurate results. Four substances present in the blood sample will be measured. These substances are Alpha-fetoprotein (AFP), human chorionic gonadotropin (HCG), unconjugated estriol (uE3), and dimeric inhibin A (DIA.) The amount of each substance in the blood sample as well as number of weeks pregnant, height, weight, race, insulin dependant diabetes, single or twin pregnancy, maternal age and any significant family history are all taken into account to calculate the individual patient's specific risk.
Why is the AFP test called a screening test?
A screening test can help predict the risk of a problem. It is not a diagnostic test and cannot give you a definite "yes" or "no" answer.
What does an abnormal screening result mean?
Some women having an AFP screening test will be identified as being in the high-risk group even though they are carrying a normal fetus. This screening result does not always mean there are problems in the baby. For example, the due date might have been estimated incorrectly, or twins might be present instead of just one baby. Women in the high-risk group are offered diagnostic tests, which can give a definitive answer. A sonogram or amniocentesis are examples of further testing, which might be offered if the screening results are abnormal. No test is perfect. Not every abnormal result of a screening test will mean that your baby has a birth defect. Sometimes there is no reason for an abnormal screening test. Not every normal result of a screening test will mean that your baby does not have a birth defect. It is up to you whether to be tested. Some women find having the screening test is reassuring, and other women would rather not have the information. The results of the tests can help some women make decisions about their options.
Average test results for 1,000 women:
