The report, published online in JAMA late Tuesday, details what happened to 442 Zika-infected women who completed their pregnancies between Jan. 15 and Sept. 22. Twenty-six of those pregnancies, or 6 percent, resulted in one or more Zika-related birth defects. The birth defects primarily involved microcephaly with brain abnormalities.
But among women infected during the first trimester, 11 percent were reported to have fetuses or infants with birth defects, underscoring earlier research that Zika poses the greatest risk early in pregnancy.
CDC officials said the findings show that the rate of microcephaly and other fetal abnormalities related to Zika is similar among these babies born in the United States — whose mothers were infected during travel — to the estimated rate in Brazil, the epicenter of the Zika outbreak.
“This basically puts to rest speculation that Brazil was different in some way,” CDC Director Tom Frieden said in an interview Wednesday.
All the women had travel-associated infections, meaning they were infected outside the mainland United States through a mosquito bite or through sex with an infected traveler. Margaret Honein, chief of the birth defects branch at the CDC and an author of the report, noted the variety of countries where the women had been exposed. The countries were: Barbados, Belize, Brazil, Colombia, Dominican Republic, El Salvador, Guatemala, Haiti, Honduras, Mexico, Marshall Islands and Venezuela.
“The commonality here is Zika, not one geographic location,” she said.
Of the 26 infants or fetuses with birth defects, there were 18 with microcephaly, or about 4 percent of all completed pregnancies. That figure is substantially higher than the prevalence of microcephaly in the United States, which is typically 7 per 10,000 live births, or about .07 percent of live births.
Microcephaly is characterized by abnormally small heads and underdeveloped brains.
Officials said the findings support public health recommendations that pregnant women avoid traveling to areas where Zika is spreading.
One reason for extreme caution is that women often do not know they are infected with Zika, which typically causes few or no symptoms in adults. For the first time, the data in the study also show that there was virtually no difference in the rate of birth defects between women who had symptoms of Zika and those without symptoms.
“We really see no difference by symptom status,” Honein said.
Honein said the affected women, whose median age is 28, are “scattered across a large number of states.” States with the largest number of Zika cases include California, Texas, Georgia, Florida, Maryland, Delaware, Pennsylvania, New York and New Jersey, according to CDC data.
The report on birth defects among infected pregnant women in the United States is one of several studies released this week that provide sobering information about Zika’s devastating consequences in pregnancy.
Researchers in Brazil who have been following a group of Zika-infected women found that 42 percent of their infants were born with a wide range of complications, including microcephaly, brain lesions or calcium deposits indicating damage, lesions in the retina, deafness, and feeding difficulties, according to a study in the New England Journal of Medicine.
Although the researchers in Brazil and those in the United States used different case definitions, there are consistent findings, CDC's Honein said. In the Brazilian group, of the 20 women who completed their pregnancies and were exposed to Zika during their first trimester, two had babies with microcephaly, or about 10 percent.
A separate study released Tuesday by the CDC found that the virus can make thousands of copies of itself in the fetal brain and in the placentas of pregnant women, which may help explain how it causes these severe birth defects and pregnancy losses. Previous studies have shown traces of the virus, not the actual virus.
CDC scientists found that Zika virus RNA persisted in fetal brains and in placentas for more than seven months after the mothers contracted Zika. The researchers also found evidence of the virus replicating in an infant with microcephaly who died two months after birth.
The virus’s persistence could have implications for babies born with microcephaly and for apparently healthy infants whose mothers had Zika during their pregnancies, according to Julu Bhatnagar, who heads the molecular pathology team at CDC’s infectious disease pathology branch and the study’s lead author.
Given the severity of Zika-related birth defects and the preliminary estimates outlined in the report, officials said it's critically important for all pregnant women with possible exposure to Zika virus to be tested, and also for infants born to mothers with possible infection to be tested. Previous research has shown that Zika-related brain damage may not be apparent at birth.