Your Pregnancy After 35 : Tests for You and Your Baby (part 5) - Alpha-Fetoprotein Testing, Multiple-Marker Tests

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Alpha-Fetoprotein Testing

The alpha-fetoprotein (AFP) test is a blood test done on the mother-to-be. As your baby grows inside you, it produces alpha-fetoprotein in its liver. Some alpha-fetoprotein crosses fetal membranes and enters your circulation. It’s possible to measure AFP by drawing your blood; too much or too little alpha-fetoprotein in your blood can be a sign of fetal problems.

Measurement of the amount of alpha-fetoprotein in your blood can help your healthcare provider predict problems in your baby, such as spina bifida or Down syndrome. However, AFP detects only about 25% of Down syndrome cases. If Down syndrome may be indicated, additional detailed diagnostic tests will probably be ordered.

The AFP test is not done on all pregnant women, although it is required in some states. It is not used routinely in Canada. AFP is often used with other tests as part of a multiple-marker test—a triple-screen test or a quad-screen test; see the discussions below. If the test isn’t offered to you, ask about it. There is relatively little risk, and it tells your doctor how your fetus is growing and developing.

The AFP test is usually performed between 16 and 18 weeks of pregnancy. Timing is important and must be correlated to the gestational age of your pregnancy and to your weight. One important use of the test is to help a woman decide whether to have amniocentesis.

If AFP detects a possible problem, more definitive testing may be ordered. An elevated alpha-fetoprotein level can indicate problems with the fetus, such as spina bifida or anencephaly. An association has been found between a low level of alpha-fetoprotein and Down syndrome. AFP can detect the following:

neural-tube defects

severe kidney or liver disease

esophageal or intestinal blockage

Down syndrome

urinary obstruction

osteogenesis imperfecta (fragility of the baby’s bones)

If your alpha-fetoprotein level is abnormal, your doctor may choose to do a higher-level ultrasound, amniocentesis or CVS to look for suspected problems. This ultrasound may also help determine how far along in pregnancy you are.

One problem with the AFP test is a high number of false-positive results; that is, the results say there is a problem when there isn’t one. If 1000 women take the AFP test, 40 test results come back as “abnormal.” Of those 40, only one or two women actually have a problem.

If you have an AFP test and your test result is abnormal, don’t panic. You’ll take another test and have an ultrasound. Results from these second tests should give you a clearer answer. Be sure you understand what “false-positive” and “false-negative” test results mean. Ask your healthcare provider to explain what each result can mean to you.

Multiple-Marker Tests

Tests that go beyond alpha-fetoprotein testing are available to help your healthcare provider determine if your child might have Down syndrome and to rule out other problems in your pregnancy. They are called multiple-marker tests and include the triple-screen test and the quad-screen test.

The Triple-Screen Test

The triple-screen test helps identify problems using three blood components—alpha-fetoprotein, 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

The quad-screen 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 standard 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%.

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