What is an alpha-fetoprotein screening (AFP)?
Alpha-fetoprotein screening is a blood test that measures the level of alpha-fetoprotein in the mothers' blood during pregnancy. AFP is a protein normally produced by the fetal liver and is present in the fluid surrounding the fetus (amniotic fluid), and crosses the placenta into the mother's blood. The AFP blood test is called MSAFP (maternal serum AFP). The AFP can also be measured in the amniotic fluid, called AFAFP.
Abnormal levels of AFP may signal the following:
Open neural tube defects (ONTD), such as spina bifida
Other chromosomal abnormalities
Defects in the abdominal wall of the fetus
Twins — more than one fetus is making the protein
A miscalculated due date, as the levels vary throughout pregnancy
AFP screening may be included as one part of a two-, three-, or four-part screening, often called a multiple marker screen. The other parts may include the following:
hCG — human chorionic gonadotropin hormone (a hormone produced by the placenta)
Estriol — a hormone produced by the placenta.
Inhibin-A — a hormone produced by the placenta.
Abnormal test results of AFP and other markers may indicate the need for additional testing. Usually an ultrasound is performed to confirm the dates of the pregnancy and to look at the fetal spine and other body parts for defects. An amniocentesis may be needed for accurate diagnosis.
How is an alpha-fetoprotein test performed?
Although the specific details of each procedure vary slightly, generally, an alpha-fetoprotein test follows this process:
Blood is usually drawn from a vein between the 15th and 20th weeks of pregnancy.
The blood sample is then sent off for laboratory analysis.
Results are usually available within one to two weeks or less, depending on the laboratory.
What are the risks and benefits of alpha-fetoprotein screening?
There are no risks of having the actual test performed other than the usual risks of a blood test. Abnormal test results of AFP and other markers may indicate the need for additional testing. Usually an ultrasound is performed to confirm the dates of the pregnancy and to examine the fetal spine and other body parts for defects. An amniocentesis may be needed for accurate diagnosis.
Multiple marker screening is not diagnostic. This means it is not 100 percent accurate, and is only a screening test to determine who in the population should be offered additional testing for their pregnancy. There can be false-positive results--indicating a problem when the fetus is actually healthy, or false negative results--indicating a normal result when the fetus actually does have a health problem.
The purpose of this screening test is to identify those women in the population who are at increased risk of having a baby with a birth defect, whose pregnancies need additional testing, who otherwise would not have been offered this additional fetal testing.