The offspring of pregnant women infected with Zika virus during the first three months of pregnancy are at 1 percent to 13 percent risk of developing microcephaly, health officials in the United States estimated.
Previous calculations based on Brazilian data revealed that the range of microcephaly risk is at 1 percent up to 30 percent.
Now, researchers from the Centers for Disease Control and Prevention (CDC) calculated the range of microcephaly risk based on data from the Zika virus outbreak in French Polynesia in 2013 to 2014 and ongoing reports from the hard-hit Brazilian state Bahia.
Specifically, in French Polynesia, the risk of a baby developing Zika-related microcephaly was 1 percent. In Brazil, however, the risk is far higher.
CDC researchers, along with scientists from Harvard University, also discovered that the risk of babies developing microcephaly was low for pregnant women who contracted the infection beyond the first trimester.
Although little is known about the effects of Zika virus during pregnancy, data from French Polynesia and Bahia established a clear association between microcephaly risk and infection during the first trimester, they said.
Researchers said that if the risk of infection is same in other parts of the world where Zika has spread, many more microcephaly cases are likely to occur. They also warned that microcephaly may just be one of many adverse effects to fetal development that are linked with Zika.
Still, they said there are limitations to the report. Available data were actually very limited, especially in places such as Bahia, where total infection rates were unknown and cases of microcephaly are currently being evaluated and reported.
What's more, pregnant women infected with Zika are unlikely to show immediate and recognizable symptoms of the infection, especially during the early stages of pregnancy, when women are not aware they are with child.
The CDC advises the public, particularly pregnant women, to take precautions to avoid Zika infection. Healthcare systems should also prepare for an increased burden of pregnancy outcomes in the coming years, they said.
In the U.S., microcephaly is rare and affects 0.02 percent up to 0.12 percent of births. In Brazil, microcephaly cases have tragically increased to 5,235 as of March 2016.
The details of the new report are featured in the New England Journal of Medicine.
