Prenatal testing: Birth defects often come as a surprise.

Why Are We So Surprised By Major Birth Defects?

Why Are We So Surprised By Major Birth Defects?

Health and medicine explained.
Oct. 19 2011 11:33 AM

A Womb Without a View

Major birth defects come as a surprise for most parents, but they don’t have to.

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At this point, almost every obstetrician in the country who manages high-risk pregnancies thinks ultrasound screenings are a good idea—so long as they they’re done in a high-quality, high-volume center. (A good center is key since the doctors’ and technicians’ skills vary a lot. Just this month, I saw a pregnant patient who’d been assured her baby was fine, yet a week before birth our ultrasound detected clear signs of a major heart defect, missing stomach, and a malformed brain, among other problems.) Among large, developed Western nations, only the United States, the Netherlands, and Spain fail to recommend complete fetal ultrasounds for all pregnant women. (Germany and France, which have the highest detection rates for major defects, recommend a complete scan every trimester.)

Without comprehensive prenatal ultrasound, women are at the mercy of conventional “risk-based” screening, in the form of a blood test that provides information on three (and only three) potential problems: spina bifida, Down syndrome, and Edwards syndrome. By measuring the levels of estriol, alpha-fetoprotein, and several other substances in a pregnant woman, the test assigns a certain probability to each defect. The lab report reads like a Vegas betting line. For example, a woman of a given age might have a baseline 1-in-476 chance of having a baby with Down syndrome before she even takes the test, and then be told that her true risk, determined from her blood sample, is 1 in 51. (In 2007, ACOG added an ultrasound measurement of the fetus’s neck to the standard test, but continued the practice of reporting proportions.)


That’s a problem because many patients find these statistics utterly baffling. In 1999, researchers found that one-half of all patients can’t make sense of them; for example, many think a 1-in-200 risk of a birth defect is more favorable than a 1-in-400. Perhaps as a result, few women with elevated risks choose to have amniocentesis, the follow-up procedure that would give a more definitive result.

Amniocentesis carries its own risks: It causes miscarriages at a rate that falls between 1 in 300 and 1 in 1,600. How should one probability be weighed against another? Many patients aren’t sure. Interestingly, no regulatory authority tracks doctors’ complication rates with amniocentesis, and ACOG does not set a minimum number of procedures for each doctor per year (meaning that your doctor may not do them often enough to stay sharp).

Given all these concerns, what should expectant women do? No test can catch every problem, of course. Here’s the bottom line: Until sophisticated new blood tests or high-quality scans become widely available—for example, one that provides the same information as amniocentesis but without the risks of miscarriage—the best resource to help navigate prenatal testing is a genetic counselor. These professionals, typically on staff at large birth centers, help women make sense of their options for prenatal testing—and remind them that the usual blood tests cover only a few, relatively uncommon problems. (They also may guide couples of certain high-risk populations, like Ashkenazi Jews, to more specialized testing.) For now, women should also consider a comprehensive fetal ultrasound in the second trimester at a high-volume, tertiary-care center. Otherwise, they may be turning a blind eye to their baby’s health.