Getting a False Positive on a Screening Test

Reviewed by Dr. Elisa Ross in June 2010

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A positive prenatal test result doesn't always mean there's something wrong with your baby. In fact, false positives are more common than you think.

During pregnancy, you're poked and prodded to provide blood, urine, and ultrasound images, all in hopes of finding definitive answers about your body and your baby. Yet many tests you'll experience during pregnancy aren't yes or no tests—they can't necessarily determine that you'll have a healthy baby or that something isn't right. Instead, many prenatal tests are looking for whether your baby is at a greater risk of having certain problems based on the results. Learning more about prenatal testing can relieve anxiety, especially if you are one of the many women who get a false positive.

Understanding Tests—Diagnostic vs. Screening

"The most important preliminary point to understand with all prenatal tests is the difference between a screening test and a diagnostic test," explains Erica Lyon, author of The Big Book of Birth and the founder and director of Realbirth, a childbirth education center in New York City. "A screening test shows perhaps a possible increased chance of something existing whereas a diagnostic test determines if something does, in fact, exist."

The Quad Screen

The quad screen is perhaps one of the most misunderstood of the prenatal tests. The triple screen (also known by several other names including the multiple marker screening and maternal serum screening) is a blood test administered between 15 to 20 weeks gestational age. It looks at the levels of four pregnancy hormones—hCG (human chorionic gonadotrophin), estriol, inhibin-A, and AFP (alpha-fetoprotein)—in your bloodstream; hormone levels outside of the normal range may indicate a problem.

"The [quad] screen is a screening test," says Lyon. "The most important thing to understand specifically about the screen is that it has an inordinately high false-positive rate, meaning it often shows an increased chance, when in fact, absolutely everything is all right."

Of those who take the test, five to seven percent will come back with a positive result, explains Dr. Mary E. Norton, director of the Prenatal Diagnostic Center at the University of California, San Francisco Medical Center. The quad screen assesses of the level of risk for having a child with Down syndrome, another chromosomal problem, or spina bifida and other neural tube defects.

"The thought process is that while [the quad screen] may not be very accurate, its primary value is that in a non-invasive way, since its only a blood test, it is somewhat helpful in screening for the rare case of a young woman who wouldn't necessarily get more diagnostic testing, for example an amnio," explains Lyon.