Scientists have struggled with estimating the risk of the Zika virus to pregnant women in the past, but now they have a clearer picture.
They also found the risk for birth defects is greatest when a woman is infected during the first trimester.
And even if a woman has no symptoms of the virus, her baby can still be affected.
Scientists are still working on a vaccine.
In the meantime, health officials urge women who are pregnant to avoid Zika-affected areas.
Their male partners also need to take precautions.
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Scientists get clearer picture of Zika virus and pregnant women