The Food and Drug Administration warned expectant parents on Tuesday about the risk of false positive results from a booming line of prenatal blood tests that screen fetuses for rare genetic conditions.

The notice followed a New York Times examination in January that reported on the incidence of false positive results in some of these tests, known as noninvasive prenatal screening tests, or NIPTs. That article quoted women who received false positive screening results for extremely rare conditions; highlighted examples of misleading marketing by companies selling the tests; and described some reports of pregnancies that were terminated based on a screening without a confirmatory test.

The agency warned doctors not to diagnose genetic disorders based on these results alone and stressed the need to follow positive screenings with more reliable “diagnostic” testing. It said it was “aware of reports” that some women “have ended pregnancies based only on the results” of these genetic screening tests.

“While genetic noninvasive prenatal screening tests are widely used today, these tests have not been reviewed by the F.D.A. and may be making claims about their performance and use that are not based on sound science,” Dr. Jeff Shuren, the director of the agency’s Center for Devices and Radiological Health, said in a statement.

The tests are taken by over a third of pregnant women in the United States. But they fall into a category called lab-developed tests, which are not regulated or approved by the F.D.A. In its safety notice, the agency said it was working with Congress “on legislation to establish a modern regulatory framework for all tests,” including the category that covers these screenings.

The agency’s warning comes after numerous accounts, dating back years, of misinterpretations of the screenings as being definitive. But the criticism has intensified in recent months, with 97 Republican legislators sending a letter to the F.D.A. in January, prompted by the Times article.

“This is an area that has concerned the agency for a long time,” said Alberto Gutierrez, the former director of the F.D.A. office that oversees many medical tests.

The lack of regulation of lab-developed tests “has led to many cases where the laboratories have claimed more than they should based on the data that is available,” he added.

The warning does not come with any consequence for prenatal test manufacturers. But it urged them to submit their products for voluntary approval. A spokeswoman for the agency declined to say whether it was currently working with any manufacturers on this front.

In just over a decade, the tests have gone from laboratory experiments to a major industry, with companies like Labcorp and Quest Diagnostics entering the business, alongside many start-ups.

One large test maker, Natera, said that in 2020 it performed more than 400,000 screenings for one type of condition, called microdeletions — the equivalent of testing roughly 10 percent of pregnant women in America. On a conference call with investors in 2021, it revealed that 75 percent to 80 percent of its core prenatal testing business included microdeletion screenings.

Immediately after the F.D.A. warning was released on Tuesday, Natera’s stock dropped more than 7 percent, but it had regained much of its value by midmorning Wednesday.

“Natera fully supports the F.D.A.’s communication regarding NIPT and has long believed that educating patients and health care providers on the strengths and limitations is essential to patient care,” the company said in a statement.

Myriad, another test maker, said it would support an F.D.A. role in regulating lab-developed tests. Labcorp said that it appreciated the F.D.A.’s guidance on this subject, and that “screening tests should be followed with diagnostic testing when appropriate.”

Other large test makers, including Quest and BioReference Laboratories, did not immediately return requests for comment.

The Times examination included a review of 17 brochures from companies that sell prenatal screenings. At the time, 10 of the brochures did not mention that a false positive could happen. Mr. Gutierrez reviewed some of the materials and described them as “problematic” and “misleading.”

“Many laboratories that offer these tests claim the tests are ‘reliable’ and ‘highly accurate,’ offering ‘peace of mind’ for patients,” the agency wrote, quoting examples The Times published in January. “The F.D.A. is concerned that these claims may not be supported with sound scientific evidence.”

After reviewing the scientific literature, the agency said that while prenatal screening tests “generally perform well for ruling out disorders,” they are less reliable when delivering positive results. The most reliable positive results are for Down syndrome. For more rare genetic disorders, caused by small missing snippets of chromosomes known as microdeletions, the agency found that a positive screening result may be a false positive between 70 percent to 98 percent of the time.

Their findings align with the Times article, which identified five microdeletion screenings for which about 85 percent of the positive results are false positives.

“This is a strong response,” said Liz Richardson, who directs The Pew Charitable Trusts’ health care products project and has been an author of reports calling for regulation of prenatal testing. “It still does put a lot of the burden, in my opinion, on the consumers and the health care providers to know how to interpret these tests, when really the burden should be on the company to communicate in a truthful and non-misleading way,” she added.

“While this is a helpful first step, I think what’s needed is more comprehensive reform of F.D.A.’s oversight of these tests, as well as other laboratory-developed tests.”

A bill seeking to provide additional oversight is currently in Congress. The VALID Act is a bipartisan bill in both chambers that would give the F.D.A. the authority to regulate lab-developed tests in the same way that it regulates diagnostic tests. The bill is headed to committee this spring.

In a statement, Representative Chip Roy of Texas, one of the authors of the congressional letter and an opponent of abortion, applauded the F.D.A.’s notice as “another step towards protecting life.”

“Parents deserve complete information when making complicated medical decisions for their children — especially when these tests can be a matter of life or death,” he said.

Susan C. Beachy contributed research.