What is an alpha-fetoprotein screening (AFP)?
Alpha-fetoprotein screening is a blood test that checks the level of alpha-fetoprotein in the mothers' blood during pregnancy. AFP is a protein normally made by the baby’s liver. It is found in the fluid surrounding the baby in the womb (amniotic fluid). AFP goes through 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.
Too much or too little AFP may be a sign of:
Open neural tube defects (ONTD), such as spina bifida
Other chromosomal abnormalities
Defects in the abdominal wall of the baby
Twins (more than one baby is making the protein)
A miscalculated due date, as the levels change during pregnancy
You may have an AFP test as one part of a two-, three-, or four-part screening. This is sometimes called a multiple marker screen. The other parts of the test may check for:
hCG (Human chorionic gonadotropin hormone). This is a hormone made by the placenta.
Estriol. This hormone is made by the placenta.
Inhibin-A. This is a hormone made by the placenta.
Abnormal test results for AFP and other markers may show a need for more testing. An ultrasound can usually confirm the dates of the pregnancy. It is also used to look at the baby’s spine and other body parts for defects. An amniocentesis may be needed for accurate diagnosis.
How is an alpha-fetoprotein test done?
In most cases, an alpha-fetoprotein test is done this way:
Blood is usually taken from a vein between the 15th and 20th weeks of pregnancy.
The blood sample is then sent off to be checked at a lab.
Results of the tests are usually ready in 1 to 2 weeks or less, depending on the lab.
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.
A multiple marker screening test is not diagnostic. This means it is not 100% accurate. It is only a screening test to see who might need more tests for their pregnancy. There can be false-positive results. These results show a problem when the baby is actually healthy. False negative results show that everything is OK when the baby actually does have a health problem.
The purpose of this screening test is to find out which women have a higher risk of having a baby with a birth defect. It is also used to find the women who need additional testing during pregnancy. Without the AFP test, some women would not be given additional testing.
March 21, 2017
Prenatal Screening and Diagnosis of Neural Tube Defects. UpToDate
Burd, Irina, MD, PhD,Dozier, Tennille, RN, BSN, RDMS