Alpha fetoprotein is a protein produced by fetal cells that can be used to detect certain birth defects in pregnant women. A blood test is done between the 15th and 20th week of pregnancy to determine protein levels, which can indicate the likelihood of birth defects. However, abnormal protein levels can also occur in normal pregnancies, so the test is not always accurate.
Alpha fetoprotein is a type of protein produced in large quantities by fetal cells. By the time a child turns 1, alpha fetoprotein levels have dropped significantly. Levels of this protein are abnormally high in some types of abnormal pregnancies, so a test to determine protein levels is a useful diagnostic or screening tool, particularly when combined with other diagnostic blood tests.
The simplest alpha fetoprotein test is a blood test to detect levels of the protein in a pregnant woman’s blood. This blood test is done, along with tests for some hormones, between the 15th and 20th week of pregnancy. Blood test results, along with details such as the pregnant woman’s age and the presence or absence of pregnancy risk factors, provide information that is used to determine the likelihood that the fetus will be born with one or more birth defects.
This test can be used to estimate the risk of several conditions and birth defects. For example, combined with blood tests for the hormones estrogen, human chorionic gonadotropin, and inhibin A, the fetoprotein blood test can detect Down syndrome about 80 percent of the time. This test can also be used to detect neural tube defects, a type of abnormal development of the brain or spine.
Alpha fetoprotein levels begin to increase at approximately the 14th week of pregnancy and continue to increase through the third trimester. A month or two before birth, protein levels drop and then begin to decline. In a pregnant woman, normal protein levels can be up to 2.5 times what is considered normal for a woman who is not pregnant.
Abnormally high or low protein levels have several implications. One is that the baby’s estimated gestational age was miscalculated, possibly meaning the doctor needs to revise her estimated delivery date. High protein levels could also mean that the pregnant woman is having twins or triplets. An abnormally high level of protein may indicate that the fetus has a neural tube defect or an abdominal wall defect called an omphalocele, in which some of the abdominal organs are located outside the body. It could also mean that the fetus has died. An abnormally low level of protein can indicate that the fetus has Down syndrome.
Measuring alpha fetoprotein levels is not always an accurate test. It is possible for protein levels to be unusually high or low during a normal pregnancy. For this reason, the test is considered a starting point for diagnosis rather than an end point.
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