Women infected during the first trimester of pregnancy had an even greater risk of defects, about 15%, according to the CDC.
These estimates are higher than what health officials have previously reported and highlight the serious risk for birth defects posed by Zika infection during pregnancy.
For the study, the researchers looked at data from nearly 1,000 pregnant women in 44 states who had some evidence of a Zika infection. Most were exposed during travel abroad.
Of the 972 completed pregnancies, 250 women had confirmed Zika infections. Of those, 24 pregnancies – about 10% – resulted in a fetus or baby with congenital abnormalities.
Screening for Zika is difficult because many infected people are asymptomatic or have symptoms that could be confused with other ailments, and the longer the time lag after infection, the more complicated diagnosis becomes.
Even more disturbing, the risk for birth defects is likely higher than the study suggests, because 75% of the babies born to mothers with some evidence of Zika infection did not receive the recommended brain imaging after birth. The link between Zika and birth defects was first identified because the infection can cause microcephaly, or an abnormally small head.
Subsequent research indicated that babies may appear healthy at birth, with a normal head size, but have underlying brain abnormalities. Of 895 live births, only 221 babies received any kind of neuroimaging.
“It’s really key for these babies to have a head ultrasound or CT scan to look for abnormalities that may not be apparent at birth,” said Margaret Honein, chief of the birth defects branch at the CDC and lead author of the report. “Knowing that not all these babies are receiving brain imaging, this may be significantly underestimating the complete number of infants with birth defects.”