Alpha-fetoprotein screening is a blood test that measures the level of alpha-fetoprotein (AFP) in the mother’s blood during pregnancy. AFP is a protein normally produced by the fetal liver. It is present in the fluid surrounding the fetus (amniotic fluid), and it crosses the placenta into the mother's blood. The AFP blood test is called maternal serum AFP (MSAFP). AFP can also be measured in the amniotic fluid (AFAFP).
Abnormal levels of AFP may signal the following:
- open neural tube defects (ONTD), such as spina bifida
- Down syndrome
- other chromosomal abnormalities
- defects in the abdominal wall of the fetus
- twins or multiple births, since more than one fetus could be 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 type of AFP test vary slightly, most alpha-fetoprotein tests follow this process:
- blood is drawn from a vein between the 15th and 20th weeks of pregnancy
- the blood sample is sent off for laboratory analysis
- results are usually available within one to two weeks, sometimes 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 it 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, which indicate a problem when the fetus is actually healthy, or false-negative results, which indicate a normal result when the fetus actually does have a health problem.
The purpose of this screening test is to identify pregnant women who are at increased risk of having a baby with a birth defect. Positive results will signal the need for additional prenatal testing. This is a relatively inexpensive screening test that can be offered to all pregnant women in order to identify those who may benefit from additional, non-standard testing.