Expecting a baby often provokes mixed emotions—wonder and amazement but also concern. Will the child be healthy? Happy? Find his or her spot in the world? Several prenatal blood tests are now available that attempt to ease some of the anxiety—at least about health. By analyzing trace amounts of fetal DNA in a pregnant woman's bloodstream, these tests (which go by such names as Harmony, MaterniT21 PLUS and verifi) can identify various genetic anomalies up to six months before birth. Whether or not parents to be take advantage of these laboratory measures is, of course, up to them. But results from screening tests can be misleading, and industry and federal regulators are not doing enough to ensure that people get all the information they need.

At present, the tests detect major abnormalities—such as three copies of the 21st, 18th or 13th chromosome, which lead, respectively, to Down, Edwards and Patau syndrome. These measures are a definite safety improvement over earlier procedures to check the genes of the unborn. Previously such chromosomal abnormalities could be detected prenatally only by invasive tests, such as amniocentesis, which carry a small risk of triggering a miscarriage.

The new screens were originally offered to women older than 35 years, who are at a higher risk of delivering babies with Down syndrome or other genetic maladies. But now companies are marketing such tests to women with low-risk pregnancies as well. Last November a market research firm reported that the tests accounted for $563.4 million worth of sales in 2014—a figure that is expected to quadruple by 2022.

Federal regulations have not caught up with the advancing technology, however. Under current rules, such gene screens are considered lab tests, which, unlike new drugs, do not have to show they offer clinically meaningful benefits. Instead manufacturers only need to demonstrate that their tests generate results within certain statistically acceptable limits of error.

This standard made more sense in the days when blood tests looked primarily for individual compounds, such as sugar molecules or hormones. Gene tests are different: they take a lot more interpretation and analysis to determine if a suspicious result indicates a true genetic aberration or merely a benign variation.

For one thing, the amount of fetal DNA found in the maternal bloodstream is minute and must be copied many times to generate enough material to test. The amplification process, among other things, may lead to double counting mistakes that give the false impression of an extra 21st chromosome, for example, where none exists. A second source of uncertainty stems from the fact that the new genetic tests are actually screening tests, which, by definition, cast a broad net that includes many more false-positive results than more specialized diagnostic procedures that are typically more accurate.

Ignoring this distinction can lead to serious problems. If a screening test on 1,000 people correctly identified 19 out of 20 true cases of a genetic problem, it would have what statisticians call a sensitivity rate of 95 percent. That sounds pretty good, but that same test might also yield 10 false positives—10 other people in the group for whom the test incorrectly suggested a problem. The sensitivity rate would still be 95 percent because the test caught most of the true positives. But just over 65 percent of all the positive results—19 out of 29—were actually correct.

It is this last ratio—what statisticians called the positive predictive value—that tells you how much faith you should really have in a particular test result. And yet most gene-screening companies do not provide the positive predictive values for their tests. Instead they tout their tests' sensitivity rate, which can mislead patients and even their physicians. Problems with prediction are why anyone who receives a positive result from a screening test should follow it up with a more precise diagnostic exam.

The U.S. Food and Drug Administration also needs to follow up on these tests. It should accelerate efforts to change the rules so that the makers of gene screens give more clinically relevant information, such as predictive values. The companies that offer noninvasive prenatal screening should do more to educate all of us about a test's potential drawbacks. And expectant parents should think carefully about whether they want to undergo these screenings in the first place, particularly because new blood tests that supposedly provide a glimpse into the entire fetal genome—including possible predispositions to heart disease, cancer or diabetes—are just around the corner.