A new study has found that in the U.S., one out of every ten pregnant women with a confirmed Zika virus infection may go on to have a baby with serious birth defects.

The report, published Tuesday by the Centers for Disease Control and Prevention (CDC), analyzes data from the U.S. Zika Pregnancy Registry, a database that epidemiologists have been using to track pregnancy outcomes among Zika-affected women living in the U.S. It is the largest study so far of pregnancy outcomes in American women infected with Zika, and experts say its key findings are a cause for concern: not only is the risk of birth defects even higher than expected, but many doctors and families are skipping the tests needed to keep newborns safe. 

"Zika may seem like last year’s problem or an issue confined to Brazil and parts of the Caribbean," says CDC acting director Dr. Anne Schuchat, M.D. "[But] our findings reinforce that this is not the time to be complacent."

In 2016 alone, 1,297 pregnancies from 44 states (excluding Puerto Rico) were reported to the Registry. Some of the pregnant women were infected by mosquito bites they received in the U.S., but the vast majority of cases (including all that resulted in birth defects) were travel-related, meaning that the women contracted the virus while visiting a Zika-affected country, or through unprotected sex with someone who was infected abroad.  

The study authors focused on the 972 pregnancies that were “completed” by the end of 2016 and that had laboratory evidence of Zika virus infection. Of those 972, only 250 had a fully confirmed diagnosis (meaning their tests clearly showed evidence of the virus); 10% of those pregnancies (24 out of 250) resulted in fetuses or babies with serious birth defects. For women whose infections were confirmed during the first trimester, that percentage shot up to 15%.

Those numbers mark a 20-30-fold increase in birth defects compared with the baseline rate in the continental U.S. But experts say they may still be a gross underestimation of the actual risk: According to data from the registry, just one-fourth of the babies whose mothers showed some evidence of Zika infection during pregnancy received brain imaging tests after they were born.  

The CDC recommends that all such babies receive a head ultrasound or CT within their first year of life, to monitor for brain defects that may not be evident at birth. (The agency also recommends that all babies born to Zika-exposed mothers be tested for Zika themselves at birth; the current report found that at least one-third of doctors and families also skipped this test). 

For reasons that health officials have yet to determine, doctors and patients have largely ignored that advice. And because those scans weren't done in a majority of cases, it's impossible to say what portion of babies who initially appeared healthy eventually developed problems. 

The report does appear to solve at least one longstanding Zika mystery: The authors note that in their analysis, women who had symptoms of Zika virus infection (including fever, joint and muscle pain and nausea) and women who had no such symptoms, were equally likely to pass the virus on to their fetuses.  

The findings drive home the urgency of remaining vigilant about Zika prevention. The CDC urges women who are pregnant, or who are trying to become pregnant, to avoid traveling to Zika affected countries, to wear an EPA-registered insect repellent, and to take other basic precautions against mosquito bites, such as ridding their yards of standing water. For a more complete list of mosquito-bite prevention tips, see our 2017 Zika update here. And for the bug sprays that best protect against the Zika-carrying Aedes aegypti mosquito, visit our Insect Repellent Buying Guide