Alpha-fetoprotein (AFP)

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 also called MSAFP (maternal serum AFP).

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--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.

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.

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?