A new report from the Centers for Disease Control and Prevention lays out for the first time the risk for Zika-related birth defects for each trimester of pregnancy.
Researchers have consistently highlighted the first trimester as the most dangerous for infections involving the virus, which is spread primarily by mosquitoes and sex, but the CDC's analysis provides a detailed breakdown throughout pregnancy.
Among women with confirmed Zika infection during the first trimester, 8 percent — nearly 1 in 12 — had a baby or fetus with Zika-related birth defects. For infections in the second trimester, 5 percent were affected, and in the third trimester, 4 percent, according to the CDC.
The report represents the largest number of completed pregnancies with laboratory confirmation of Zika infection to date. It’s consistent with earlier research, but because it includes many more pregnant women than earlier analyses, the data are considered to be stronger.
The analysis is based on information from 2,549 women with possible Zika infection who completed their pregnancies in the U.S. territories from Jan. 1, 2016, to April 25, 2017. Of those pregnancies, 122 women had babies or fetuses with birth defects. They included microcephaly, the most high-profile abnormality associated with Zika infection, but also a host of other problems, such as eye damage, hearing loss and restricted movement of arms and legs.
Many of these babies will need special care and support for the rest of their lives.
Although the report does not provide a breakdown by territory, the largest number of cases were likely from Puerto Rico, the place in the United States hardest hit by Zika.
On Monday, Puerto Rico announced its Zika epidemic had ended because the number of new cases had dropped significantly since August. At the height of the epidemic, more than 8,000 cases were reported in a four-week period.
U.S. health officials and public health experts have expressed particular concern about the underreporting of birth defects in Puerto Rico, in particular the island's use of a more narrow definition for reporting birth defects.
But for the information in Thursday’s report, Puerto Rican authorities provided data that conformed to CDC’s case definition. The data on birth defects also includes cases from the U.S. Virgin Islands, the Federated States of Micronesia, American Samoa and the Marshall Islands.
Overall, about 5 percent of women in the U.S. territories with a confirmed Zika infection during pregnancy had a baby or fetus with birth defects. That’s consistent with an earlier CDC report about Zika-infected pregnant women in the 50 states and the District of Columbia.
Although the earlier report found what appeared to be a higher risk of birth defects from infection during the first trimester, the results actually were similar statistically because Thursday's report was based on many more women, “and the certainty is better,” said Anne Schuchat, CDC’s acting director.
“The bottom line is that there is no doubt that Zika infection during pregnancy, during any trimester, can lead to severe birth defects,” she stressed during a media briefing. She said she was pleased that the incidence of new Zika cases in Puerto Rico is low, yet “it’s hard to know exactly what will happen in a new mosquito season.”
CDC officials continue to urge pregnant women to take precautions and clinicians to conduct the recommended postnatal neuroimaging for babies born to mothers with suspected Zika infections, even if they appear healthy at birth.