What to expect if your prenatal screening test comes back positive.
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.
Amnio: A Definitive Diagnosis
Performed between weeks 15 and 20, amniocentesis can conclusively diagnose neural-tube defects, Down syndrome and a host of other chromosomal defects. The procedure involves inserting a thin needle through the woman's abdomen and into the amniotic sac to withdraw a sample of fluid.
Because it carries a very slight (1 in 200) risk of procedure-related miscarriage, doctors usually don't perform amniocentesis unless risk factors or a patient's age warrant it. (Because of their increased risk of having a baby with chromosomal defects, pregnant women age 35 and older are routinely referred for amniocentesis.) The results of an amnio usually take 10 to 14 days. Thus, in many cases, a patient's mind can be put to ease by as early as 20 weeks.
"The hardest part is the waiting," says Scott Copabianco, M.D., an obstetrician-gynecologist in Mission Viejo, Calif. "I know; I've been there." His wife had a positive AFP test followed by amniocentesis with both of their children, who are now perfectly healthy 8- and 10-year-olds. He says the reason her numbers were high is still a mystery.
But Copabianco's experience, both professional and personal, helps him reassure his patients that the vast majority of the time, everything is just fine.
The chances of having a false-positive alphafetoprotein (AFP) test increase with age: from 2.9 percent at 25 to 14 percent at 35 and to 40 percent at 40.