Zika pregnancies 20 times more likely to lead to defects

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 to 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. 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, healthcare 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.