Alpha-fetoprotein (AFP) is a protein that is normally produced by the fetus' liver. AFP is present in the fluid around the fetus (amniotic fluid) and a small amount crosses the placenta and moves into the mother's blood stream. As the baby grows and produces more AFP, the amount in the mother's blood increases.
Between weeks 15 and 20 of a pregnancy, a maternal serum alpha-fetoprotein (MSAFP) screen may be offered. It's usually as part of a set of tests, which screen for genetic problems, called the quad screen. The AFP blood test determines how much AFP is in the mother's blood. The other tests measure the levels of other pregnancy hormones, including estriol, human chorionic gonadotropin (hCG), and inhibin A.
The quantity of AFP that is considered normal depends upon many variables, including age, weight, race, and week of pregnancy, therefore accurate dating is important. Insulin-dependent diabetes also influences AFP levels. Of those women whose tests show high or low levels of AFP, only two or three in 100 will have a child with a birth defect.
This test is offered to all pregnant women. You may choose to have this test if you want to know if your baby is at high risk for neural tube defects. You may want to take an MSAFP test before considering ultrasound, or amniocentesis.
An MSAFP screen involves a simple blood draw from the mother’s vein. Results are usually available in one to two weeks. Up to 10% of results are positive, meaning you have high- or low-AFP levels. With a positive AFP, additional tests will be suggested to help determine the cause.
Positive AFP levels are most often the result of a miscalculation in the age of the fetus (wrong dates) or due to multiple fetuses in the womb (twin or triplet pregnancy), with each producing AFP. In some instances, the reason for a positive AFP level remains unknown. Only a small number of positive results are related to a birth defect. Unusually high AFP levels are associated with an increased risk of a neural tube defect such as spina bifida or anencephaly, whereas unusually low levels (along with low levels of estriol and high levels of hCG) are related to an increased risk of a chromosomal abnormality such as Down syndrome. If further tests don’t show a chromosomal problem or an anatomic defect, an elevated AFP may predict problems with fetal growth later in pregnancy. This usually warrants follow-up ultrasounds and other fetal testing which your doctor will order.
It's important to remember that an MSAFP is a screening test only; it does not detect or diagnose birth defects. The majority of women who take this test receive normal, or negative, results.
Q: What are some of the benefits and risks of having this test?
A: A potential benefit is that the screening results may lead to the early detection of a developmental abnormality in the fetus. Early identification of any problem allows you and your health-care team crucial time to explore treatment options, ensuring the utmost safety of your pregnancy and delivery. With normal test results, you may benefit from the assurance that your baby does not appear to be at high risk for certain abnormalities. On the other hand, if the test reveals a positive AFP result, you risk experiencing anxiety as well as more invasive testing to determine the cause, even though most of the time the positive results do not indicate a birth defect.
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