Leticia de Araujo holds her one-month-old daughter in February 2016 in Rio de Janeiro. The baby was born with microcephaly after being exposed to the Zika virus during her mother's pregnancy. (Antonio Lacerda/European Pressphoto Agency)

Pregnancies of women in the United States infected with the Zika virus are about 20 times more likely to result in babies with certain birth defects, compared with the prevalence of these birth defects before the Zika epidemic swept through the Americas, according to a report released Thursday.

Researchers at the Centers for Disease Control and Prevention are trying to determine how common these birth defects, such as microcephaly, brain abnormalities, eye defects and central nervous system problems, were in the years before the Zika outbreak. Although a Zika infection during pregnancy is linked to a distinct pattern of birth defects, those abnormalities are not unique to Zika. Genetic factors and other viral infections may also cause these birth defects, although in many cases the causes are unknown, experts say.

The CDC report provides a baseline reference to interpret the impact of Zika on the prevalence of these birth defects in the United States. The new data underscore the dangers of Zika infection during pregnancy and the advice that pregnant women should avoid travel to Zika-affected areas, especially as warmer weather returns. If pregnant women must travel or live there, they should take measures to prevent mosquito bites or sexual transmission of the virus, which can be passed through semen.

The magnitude of birth defect risks also underlines the critical importance of early and accurate testing for pregnant women to give them the full range of options, health-care providers say. In Washington, D.C., the District's public-health lab has produced erroneous or inconclusive results for nine, and perhaps more, pregnant women who were tested for Zika between July and December of last year. The lab reported that the women had tested negative for Zika, when they may in fact have been infected. The mistakes, made public Feb. 16, have prompted officials to redo more than 400 tests, including those for nearly 300 pregnant women.

The findings “give you an idea of how large the risk may be,” said Janet Cragan, a medical officer at CDC’s birth defects branch who led the research.

Researchers analyzed data on birth defects from three surveillance programs in the United States. The researchers identified 747 infants and fetuses with one or more of these defects from data collected by programs in Massachusetts, North Carolina and Atlanta. The prevalence was about 3 for every 1,000 births between 2013 and 2014. The defects included the entire range of abnormalities associated with Zika, from microcephaly, characterized by abnormally small heads and often underdeveloped brains, to vision problems and joints with limited range of motion, such as clubfoot.

Brain abnormalities or microcephaly accounted for the largest number and highest prevalence, followed by neural tube defects, such as spina bifida, the report said.

After Zika spread throughout the Americas, the CDC began tracking the number of pregnant women in the United States who became infected and what happened to their pregnancies. Data from the U.S. Zika pregnancy registry identified 26 infants and fetuses with these birth defects among 442 completed pregnancies during a nine-month period in 2016. The proportion of infants with these birth defects was about 6 percent, or nearly 60 out of every 1,000 completed pregnancies with Zika infections. This proportion is about 20 times higher than the prevalence of such birth defects during the pre-Zika years.

Of the 26 infants with birth defects, 22 had a brain abnormality or microcephaly, a proportion that is about 33 times higher than the prevalence among pregnancies in the years before the Zika epidemic, according to the report.

Margaret Honein, chief of the birth defects branch at the CDC and an author of the report, said it's too early to give a precise estimate for the additional risk of birth defects from a Zika infection.

“I don't have that level of precision, but there is strong evidence of a major increase in risk,” Honein said. She said the findings demonstrate the importance of having monitoring systems that collect data on birth defects.

The state data come from birth defects surveillance programs that identify diagnoses among infants and fetuses mostly through review of administrative records. The state data for this report was collected from states where health department staff members routinely go to hospitals and other institutions that provide care for pregnant women and review the records of infants and fetuses.

CDC has awarded more than $27 million to 50 jurisdictions to establish or bolster surveillance systems that rapidly collect information about Zika-related birth defects.

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