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Maternal Serum Screening vs. Diagnostic Tests

For several years The American College of Obstetricians and Gynecologists has recommended that all pregnant women be offered maternal serum screening tests for three common birth defects: Down syndrome, trisomy 18 and open neural tube defects. In 2007 the recommendation was changed to say that all pregnant women also be offered diagnostic testing for these disorders.

So you may choose to have a maternal serum screening test, a diagnostic test, a combination of the tests, or no testing at all.

Here are the most important differences between the two kinds of tests:

Maternal Serum Screening Test

A maternal serum screening test:

• tells you the chance that your baby could have one of these birth defects.
• leads to the detection of about 80-90% of babies with these birth defects, depending on the particular test your doctor uses and how far along you are in your pregnancy. That means that about 10-20% of these birth defects are missed by screening.
• sometimes shows a high risk even when the baby does not have one of these problems.
• does not tell you any information about other birth defects.
• does not cause miscarriages.

The American College of Obstetricians and Gynecologists recommends that women with a “high risk” maternal serum screening result be offered diagnostic testing. Women with a “low risk” result usually choose not to have diagnostic testing.

Diagnostic Test

Diagnostic tests are performed on samples obtained by amniocentesis or chorionic villi sampling (CVS). These tests:

• tell you with greater than 99% accuracy whether your baby is affected or is not affected with Down syndrome or trisomy 18.
• tell you with greater than 90% accuracy whether your baby is affected with an open neural tube defect.
• routinely look for certain other chromosome abnormalities, which happen less frequently than the most common disorders but, like them, cause serious mental or physical problems or both.
• can be combined with other specific diagnostic tests, if a couple has additional risks for a specific inherited disorder.
• are estimated to cause miscarriage in 1 in 300 to 1 in 500 pregnancies (up to 1 in 100 if the early diagnostic test of CVS is done).

Remember, not even diagnostic testing can show all birth defects or genetic diseases. Even when all the results of diagnostic testing are normal, the pregnancy still has a 3-5% risk for problems that cannot be detected before birth.