Pregnant women have many options for learning about the health of their unborn baby. AFP testing, also known by other names including multiple marker screening, assesses the chance for specific conditions in the baby. The first version of the test, available in the early 1980s, measured only one chemical, called AFP or alpha-fetoprotein. Made by the baby, AFP passes into the mother's bloodstream where it can be measured. As the test became more sophisticated, new chemicals were added, including human chorionic gonadotropin (hCG), estriol (uE3) and Inhibin A. Depending on the levels of the chemicals, the test can indicate whether a pregnancy is at increased risk for Down syndrome, neural tube defects, or trisomy 18. Results may also suggest increased risks for other conditions as well.
Because the levels of the chemicals can vary for many reasons, additional testing is necessary to determine whether there really is a problem with the baby. Melissa Trant, M.S., a prenatal genetic counselor, speaks with many clients about positive AFP test results. "It is critical that people understand the difference between a screening test and a diagnostic test," says Trant. "A screening test such as AFP only indicates the level of risk of a problem but cannot say whether a baby has Down syndrome or another condition."
When Is AFP Screening Performed?
Blood for AFP screening should be drawn between 15 and 20 weeks of pregnancy. Precise dating of the pregnancy is critical because the chemical values that are measured change during pregnancy. For example, a particular chemical level may be normal for a pregnancy at 16 weeks but abnormally high or low for a pregnancy at 19 weeks. Using ultrasound to measure certain parts of the baby is the most accurate way to date the pregnancy.
What Conditions Can AFP Screening Detect?
AFP screening is designed to detect at least three conditions: Down syndrome, trisomy 18 and neural tube defects. Down syndrome, caused by an extra copy of chromosome 21, causes mental retardation, characteristic facial features and an increased chance for other health problems including heart defects. Trisomy 18 is a severe condition caused by an extra chromosome 18. Most babies with trisomy 18 are miscarried or die in the newborn period, largely due to heart defects and brain abnormalities. Neural tube defects are openings in the baby's spine or brain (called spina bifida and anencephaly) that affect neurological functioning. AFP screening can also sometimes detect other conditions including Smith-Lemli-Opitz syndrome, which is a rare genetic condition that causes mental retardation and birth defects.
Benefits of AFP Screening
AFP screening can help a woman understand the chances for her baby to have certain problems without putting the pregnancy at risk for miscarriage. Because it is a simple blood test, there is no risk to the fetus. The test detects most cases of the conditions discussed previously. Detection rates for Down syndrome and trisomy increase with the mother's age. The detection rate for neural tube defects is approximately 80 percent for women of all ages.
Drawbacks of AFP Screening
As a screening test, AFP is not able to definitively tell whether a baby has a problem. In most cases, a positive result is a "false positive" and the baby is healthy. Anxiety can run high while the woman waits to find out whether follow-up tests indicate a problem or not. On the other hand, a woman may be falsely reassured by a negative AFP test result, incorrectly assuming it means that the baby is healthy. However, with appropriate pre-test counseling, a woman undergoing AFP screening should understand the meaning of a positive or negative result.
Follow-up Tests
Women who receive a positive AFP test result are offered ultrasound and amniocentesis. Ultrasound can confirm the dating of the pregnancy and check for any physical anomalies. While ultrasound cannot detect all birth defects, it can usually find major defects such as neural tube defects. Amniocentesis is an invasive test where a small sample of fluid is withdrawn from around the baby. Amniocentesis detects greater than 99 percent of chromosome abnormalities such as Down syndrome, but it carries a small risk of miscarriage.
References
- PregnancyToday: Pregnancy Checkups & Tests - AFP
- Melissa Trant, M.S., CGC; Genzyme Genetics; Los Angeles
- Prenatal Diagnosis; Major Fetal Abnormalities Associated With Positive Screening Tests for Smith-Lemli-Opitz Syndrome (SLOS); WY Craig et al; Feb. 2007
