Pregnant women infected with the Zika virus during their first trimester face as high as a 13 per cent chance that their fetus will develop a severe and rare brain defect, according to research published today.

That condition, known as microcephaly, is characterised at birth by an abnormally small head and often incomplete brain development. Researchers at the Centres for Disease Control and Prevention and Harvard T.H. Chan School of Public Health identified the sharply higher risk after analysing data from one of the hardest hit areas in Brazil, the epicentre of the rapidly evolving Zika outbreak.

Typically, microcephaly occurs in .02 per cent to .12 per cent of all births. Even more common congenital conditions, such as Down syndrome, are often seen in less than 1 per cent of births. By contrast, the study published in the New England Journal of Medicine found the estimated risk for microcephaly with Zika infections in the first trimester of pregnancy ranged from 1 per cent to 13 per cent.

The analysis is the first to quantify such risk in pregnant women infected during the current outbreak, which has seen the mosquito-borne virus spread to more than 40 countries and territories in the Americas and beyond. The latest tests showed the same strain is now on the African archipelago of Cape Verde.


"It is an appreciable risk," said Michael Johansson, a CDC biologist and lead author of the study. "We need to do whatever we can to help women avoid Zika virus infections during pregnancy."

The study comes just weeks before the start of the northern summer and mosquito season across the United States. The CDC and local public health officials, particularly in the South and Southwest, are highly concerned about many communities' ability to track and prevent spread of the virus.

Researchers found a strong association between the risk of microcephaly and infection during the first trimester, but "a negligible association in the second and third trimesters". They were only able to provide a range of risk because of the significant uncertainty about the overall rate of Zika infection in the population studied and the accuracy of the microcephaly cases reported.

Much more research needs to be done about the effects of Zika at all stages of pregnancy, Johansson said, and other studies are now tracking hundreds of Brazilian women and babies.

No other countries where Zika is being transmitted locally have suffered the same spike in microcephaly cases - but that could change soon. "If the risk of infection and adverse outcomes is similar in the other geographic areas where Zika virus has since spread, many more cases of microcephaly and other adverse outcomes are likely to occur," the study warned.

It urged health care systems "to prepare for an increased burden of adverse pregnancy outcomes in the coming years".

The CDC is monitoring 279 pregnant women infected with Zika in the United States, while Brazil has recorded nearly 3600 pregnant women with Zika just since January. Since the outbreak began there last year, more than 1400 cases of microcephaly and other fetal neurological disorders have been confirmed.

Last month, CDC scientists confirmed that the virus causes microcephaly and other severe fetal abnormalities during pregnancy. But researchers didn't know the magnitude of that risk. Some infected women have given birth to healthy babies. An analysis of a 2013-2014 Zika outbreak in French Polynesia estimated the risk for microcephaly to be around 1 per cent for pregnant women infected in their first trimester.

The estimate from French Polynesia is from a single outbreak, however, with microcephaly cases identified after the fact.

Researchers in the current study analyzed the approximate number of suspected Zika cases reported in Bahia, Brazil from February 2015 to this past February and the number of reported and estimated microcephaly cases from January 2015 to February. They then calculated the estimated microcephaly risk by comparing estimated infection risk in each trimester to microcephaly incidence. The first trimester of pregnancy was linked with the highest risk, they found.