Alpha-fetoprotein (AFP) tests in pregnancy
Early screening of fetal health
When you are pregnant, substances from your fetus mix with your own blood. One of these substances is alpha-fetoprotein. Alpha-fetoprotein (AFP) is produced by the fetus and can be detected in a blood sample taken from your arm. An AFP test is usually done between weeks 15 and 17 of pregnancy.
A high level of AFP can be an indication of a neural tube defect such as spina bifida, however, high levels of AFP can be present for other reasons.
- The level of AFP in your bloodstream increases substantially in the fourth to sixth months of pregnancy, so if you have a high result it may indicate that your pregnancy is further advanced than you realise and your due date needs to be calculated again.
- If you are carrying twins your AFP will also be high.
In both these cases an ultrasound should shed some light on the situation.
Understanding AFP tests
An AFP test is only a screening test. The most it can do is point to a possible problem. If the results of an AFP test point to a problem, other tests will be needed to confirm the results. Bear in mind that most AFP test results are normal. Even when they are not, the results of the follow-up tests most often are.
AFP test results
The results of your AFP blood test may show the presence of certain fetal substances in your blood and can alert your doctor or obstetrician to possible birth defects.
Most AFP results are negative (normal). This means the test results show no signs of the birth defects tested for. Sometimes results are positive (abnormal). Often, this is simply because:
- your due date is different than first thought; or
- you have twins.
Some abnormal results show that the fetus may have one of the following problems:
- neural tube defects (problems with the spine, such as spina bifida);
- defects in the wall of the abdomen; or
- genetic defects (physical or mental problems, such as Down syndrome).
AFP test results can sometimes be wrong. These are called false negatives or false positives. Be sure to ask your doctor or obstetrician any questions you have about your results.
If your test results are positive, you may be offered more tests. These additional tests can confirm whether there really is a problem or whether everything is normal. The tests include the following.
- Ultrasound: this uses sound waves to create an image of the fetus.
- Recalculated AFP: a second AFP test, used if ultrasound shows that your due date is different than first thought or that you have twins.
- Amniocentesis: this is a test of the fluid that surrounds the fetus in the womb.
If these tests show no cause for concern, your doctor or obstetrician may still want to watch your pregnancy carefully. That way, any problems can be spotted early.
Should you have an AFP test?
Having an AFP test is up to you and, like all screening tests, is optional. An AFP test can warn you about some fetal birth defects. Here are some more facts to help you decide if you want to have it.
- An AFP test causes no health risk to you or the fetus.
- An AFP test can only point to possible problems with the fetus.
- If the test points to a possible problem, other tests will be needed to confirm the AFP results.
- Even AFP test results that are normal cannot tell for sure that the fetus is healthy.
Other commonly used tests, such as the 18 to 20 week ultrasound scan, can frequently detect abnormalities hinted at by an AFP test. A first trimester prenatal test that involves a blood test (performed at 9 to 13 weeks), combined with a special ultrasound scan called a nuchal translucency scan (performed at 11 to 13 weeks) has largely replaced the AFP test as a screening test for Down’s syndrome.
The AFP test is just one of many you may have while pregnant. It is certainly not essential. Most women’s AFP test results are normal. Even if they are not, the chances are high that the fetus is healthy. Ask your doctor or obstetrician any questions you have about this or the other tests.
2. MayoClinic.com. Quad screen [updated 2008, Jul 2; accessed 2009, Jun 19]. Available at: http://www.mayoclinic.com/health/quad-screen/MY00127/