The maternal alpha-fetoprotein (AFP) test is a blood test done on you to determine abnormalities in your baby. Measurement of the amount of alpha-fetoprotein in your blood can help your doctor predict problems, such as Down syndrome and spina bifida.
At this time, it is not performed on all pregnant women. However, some states require it, such as California and New York. If the test is not offered to you, discuss it with your doctor.
The AFP test is usually performed between 16 and 20 weeks of pregnancy, and test results must be correlated with the mother-to-be's age and weight, and the gestational age of the fetus. If AFP detects a problem, additional testing is usually ordered.
The test is designed to detect babies with
|severe kidney disease|
|severe liver disease|
|esophageal or intestinal blockage|
|osteogenesis imperfecta (fragility of the baby's bones)|
High Error Rate
The test is not specific enough to be relied on by itself. For example, if 1,000 women are tested, 40 tests will come back abnormal. Of those 40 tests, only one or two actually have a problem. If you have an AFP test and your test result is abnormal, don't panic! Another AFP test will be done to correlate results, and an ultrasound will be performed.
AFP detects neural-tube defects, which are abnormalities in the bone surrounding the spinal cord, in the brain stem or in the brain itself. One of the most common neural-tube defects is spina bifida— an absence of vertebral arches, which allows the spinal membrane to protrude. Another abnormality is anencephaly;the brain develops only a rudimentary brain stem.
Triple-Screen and Quad-Screen Tests
These tests go beyond alpha-fetoprotein testing to help your healthcare provider determine if your child might have Down syndrome and to rule out other problems.
The triple-screen test helps identify problems using three blood components: alpha-fetoprotein, a pregnancy hormone called human chorionic gonadotropin (HCG) and a form of estrogen produced by the placenta called unconjugated estriol. Abnormal levels of these three blood chemicals can indicate Down syndrome. For older mothers, the detection rate is higher than 60%, with a false-positive rate of nearly 25%. Abnormal results of a triple-screen test are usually double-checked with ultrasound and amniocentesis.
The quad-screen test is used rarely at present. Only a few hospitals in the United States have laboratories equipped to conduct this test. The test is similar to the triple-screen but adds a fourth measurement—the blood level of inhibin-A, a chemical produced by the ovaries and the placenta. This fourth measurement raises the sensitivity of the triple-screen test by 20% in determining if a fetus has Down syndrome. The quad-screen test identifies almost 80% of fetuses with Down syndrome. It has a false-positive rate of 5%.