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Routine prenatal screening tests can understandably cause expectant parents a good amount of worry. But it’s reassuring to know that despite the possibility of scary results, the vast majority of pregnancies end in the birth of a healthy baby.
The screening process usually begins with a test known as the expanded alphafetoprotein (AFP). A blood test given between 15 and 20 weeks of pregnancy, the AFP screens for neural-tube defects, such as spina bifida, and genetic abnormalities, such as Down syndrome, by measuring three hormones: alphafetoprotein, human chorionic gonadotropin and estriol. In the majority of cases, the test indicates that the baby does not have any of the conditions being screened for.
However, the AFP does occasionally issue a false positive even if all is perfectly well: As many as 5 percent of women—or 1 in 20—have a positive AFP test, says John Williams III, M.D., a perinatologist and director of reproductive genetics at Cedars-Sinai Medical Center in Los Angeles. Of those, fewer than 1 in 350 actually have something to worry about. (There are two main reasons for false positives: the mother is of “advanced maternal age” or conception occurred on a different date than was calculated.)
If your results are positive, your doctor will likely refer you to a center that specializes in high-risk pregnancies. Some states require that patients with positive AFP results be referred to a high-risk pregnancy specialist, or perinatologist. If you do not get a referral to a specialist, you may want to request one.
In the specialist’s office, a genetic counselor will meet with you and your partner, take a family history and review your AFP results to assess your risks. He will explain what the numbers do and don’t mean and discuss further testing, as well as your options in case an abnormality is detected.
The next step is to have a perinatologist conduct an ultrasound to confirm the fetus’s age. “If a woman is off on [the date of her last period] by two weeks either way, that really changes the outcomes,” Williams says. A full 30 percent of women referred to him after receiving a positive AFP test find their results fall in the normal range once an ultrasound determines the correct fetal age.
During the ultrasound, the perinatologist will also look for any physical abnormalities to help rule out chromosomal or other birth defects. If he sees something that suggests a problem—or if he cannot rule one out—he may recommend amniocentesis, which usually can be done at the same appointment.