The maternal serum alpha-fetoprotein (AFP) test is a screening test performed between week 15 and 20 of pregnancy (most often between week 16 and 18). Alpha-fetoprotein is produced by the fetal yolk sac, liver, and gastrointestinal tract. AFP increases and decreases at specific points in pregnancy. AFP testing is one part of the genetic screenings during prenatal doctor's visits. The AFP test is itself not diagnostic -- abnormal results may be indicative of the presence of certain congenital defects. Here’s a look at understanding how the AFP test is performed, what it measures, and how to understand the results.
How AFP Test Is Performed
Your doctor will take a sample of blood from you. The AFP levels in this sample are then measured and compared with average levels for women of the same gestational week, maternal age, and ethnicity. This test poses no risk to the fetus and only minor discomfort to the mother -- since the measured blood is maternal and the test doesn’t involve any sampling from the fetus or amniotic sac. The AFP test is usually run in conjunction with the human chorionic gonadotropin test and an unconjugated estriol test, as well as the inhibin A test (commonly referred to as the “quadruple”, or “quad screen”).
What the AFP Test Measures
Comparatively high levels of AFP indicate a higher chance of neural tube defects, which include anencephaly and spina bifida, or abdominal/esophageal issues such as gastroschisis, in which the intestines develop outside the abdominal wall. Comparatively low levels of AFP indicate a higher chance of chromosomal defects, such as trisomies (i.e. Trisomy 21, better known as Down Syndrome, and Trisomy 18, or Edwards Syndrome). Unexpectedly high or low levels can also signify a previously undetected multiple pregnancy or error in pregnancy dating.
Understanding AFP Results
The results of an AFP test report the chances of diseases and abnormalities for which it measures. Depending on the measured levels in the maternal blood sample, results are returned in a “1:x” format, where “X” represents the number chance of a certain abnormality. For example, 1:25 chance of Trisomy 18 means that there is a 4 percent chance the fetus has Down syndrome. A healthy 30-year-old mother typically has a 1:900 chance of conceiving a child with Down Syndrome.
There’s a common misconception the AFP test has a high rate of “false positives.” Because the AFP test is a screening tool, not a diagnostic test, the results only describe the chance for certain disorders, rather than a definitive diagnosis of a disease. If the test indicates a higher than average chance of certain abnormalities, the next step should be a diagnostic test such as a high-level ultrasound, amniocentesis, or chorionic villus sampling. If you receive abnormal results on an AFP test, talk to your doctor about the best course of action for your family and the risks involved in more invasive techniques.