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Alpha-Fetoprotein (AFP) Screening Test

Reviewed by James Brann, M.D.

AFP Test Screens are Screening Tools, Not Diagnostic Tests

Learn about alpha fetoprotein screening in pregnancy.

At some point between pregnancy week 16 and 18, your doctor or healthcare provider will ask you to take a second trimester screening test called the alpha-fetoprotein (AFP) test. This is a routine blood test that examines your risk of having a baby with genetic abnormalities. The AFP test screens for problems, such as neural tube defects, abdominal wall defects, and chromosomal abnormalities in your baby.

Some states require you to undergo this screening test. However, this test is especially recommended for older mothers (35 and up), women who abused drugs or harmful medications during pregnancy, and women with a history of birth defects in their family.

The alpha-fetoprotein screening test examines your blood for its level of alpha-fetoprotein – which is a protein produced by the developing baby's liver and yolk sac. A small amount of the AFP enters the mother's bloodstream. Abnormal levels of this protein can indicate that you're pregnant with a baby that has birth defects.

AFP screening is often performed along with other blood tests. When your blood sample is sent off to the lab for analysis, your AFP levels and the levels of other hormones – inhibin-A, human chorionic gonadotropin, and estriol – will be checked. For this reason, the AFP test is part of the "triple screen," or the multiple marker screening test.

If the results of the multiple marker screening test come back abnormal, this means you're at higher risk of carrying a baby with problems. Try not to freak out if you get an abnormal result. The AFP is just a screening test. You will need to undergo diagnostic testing, such as an amniocentesis, to know for sure that your baby has a genetic or chromosomal defect.

Although the AFP blood test is fairly precise, it is not always 100 percent accurate. There are certain factors that can mess up the results and give you inaccurate results. In order to get the most reliable result, the lab has to take the following into consideration:

  • Age of the Pregnancy: Your healthcare provider can perform the AFP blood test at any point between 15 and 20 weeks pregnant, but the results are most reliable when the test is given between 16 and 18 weeks of gestation.


  • Woman's Weight: The pregnant woman's weight can dilute the level of AFP in her blood. If maternal weight is not measured accurately, it can distort the results.


  • Ethnicity and Race: If the mother's ethnicity is not correctly recorded, this can make the results unreliable. The level of alpha-fetoprotein is 10 percent higher in African-Americans, compared to Caucasians and other races.


  • Twin and Multiple Pregnancy: In order to get the most accurate result, the lab must know if you are carrying twins or multiples. Women carrying more than one baby have a higher level of AFP in their blood.


  • Diabetes: A pregnant woman with diabetes is at elevated risk for neutral tube defects. The level of AFP in the bloodstream is 15 percent lower than normal. The lab must know that the woman is diabetic, so they can lower the "normal AFP" level to get the most accurate result.


  • Fetal Death: In the sad event that the unborn baby has died in the womb, this will increase the level of AFP in the woman's bloodstream.
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