Understanding Your Genetic Testing Options: Screening

Screening tests, such as the alpha-fetoprotein measurement, help identify women who may be carrying a baby with certain birth defects. Screening tests are generally used in women who are at low risk of having a baby with problems. Women already known to be at increased risk of having an infant with some genetic problems, such as those over 35 years of age at delivery, should be offered diagnostic tests like amniocentesis and/or detailed ultrasound. These screening tests may also help a high-risk woman better understand her risk. Women are then better prepared to make a decision about invasive testing such as amniocentesis or chorionic villus sampling.

Although useful, screening tests are not 100-percent accurate. A negative test indicates you are in a low-risk category. It does not guarantee that your fetus is normal. An abnormal screening test requires further investigation but does not prove that your fetus has a birth defect. It merely places you in a high-risk category. Unfortunately, falsely positive screening tests are common.

Maternal Serum Alpha-fetoprotein (MSAFP)

What It Is:
Measuring MSAFP helps to identify women carrying a fetus with certain birth defects. MSAFP measures a protein that is made in the fetus and finds its way into the maternal blood stream. The amount of this protein normally present in the mother's blood depends on the length of the pregnancy (gestational age of the fetus), the number of babies you are carrying (twins, etc.), and individual variation.

Changes in the MSAFP level are associated with fetal and placental problems. A high MSAFP level indicates an increased risk for spina bifida or other defect that permits fetal internal tissue to come into contact with amniotic fluid. MSAFP screening can identify at least 85 percent of fetuses with spina bifida. A low MSAFP level suggests that Down syndrome is more likely to be present than if the MSAFP were normal. Currently this is not a very good way to screen for Down syndrome since the MSAFP alerts doctors to only a fraction of fetuses with this problem. Additional maternal blood tests are better.

This test is often part of the triple screen test that assesses whether further diagnostic testing may be needed. It is usually performed between the fourteenth and twenty-second week of pregnancy.

The Risks:
Since the MSAFP normally increases during pregnancy, mistakes in estimating your due date will confuse interpretation of the MSAFP results. The primary risk of this test is that your MSAFP test will be abnormal despite a normal fetus. This can cause unnecessary anxiety and additional testing. The risk of not having the screening tests is that you will not know about potentially serious problems with your fetus. MSAFP helps to detect only a few types of birth defects.