Image: Associated Press

A new study published in the New England Journal of Medicine suggests that Zika-infected women who are in their third trimester have virtually no chance of having children with microcephaly. Troublingly, the same study shows that women who exhibit no symptoms can still give birth to babies with brain abnormalities.

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The Zika virus has been at epidemic levels in South and Central America since the summer of last year, yet many questions remain unanswered. The disease has been linked to microcephaly and other birth defects in offspring, though it’s not clear why Zika affects some fetuses and not others. A new study conducted by scientists in Colombia—a country that has reported over 65,000 cases of Zika thus far—is helping to answer some of these questions, though they admit the results are still preliminary.

The researchers looked at 1,850 pregnant women who were infected with the Zika virus between August 9, 2015 and April 2, 2016. Their research shows that women who are infected with Zika during the third trimester (week 27 until the end of the pregnancy) have virtually no chance of giving birth to a baby with birth defects. When the researchers looked at a subgroup of the pregnant women who had given birth and had knowledge of the precise week of their Zika infection, more than 90 percent who were infected during the third trimester had given birth—yet not one of them gave birth to a baby with microcephaly or any other detectable brain abnormalities. That’s a relief.

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It’s important to point out, however, that many of the women involved in the larger study are still pregnant. A follow-up study in the coming months will paint a clearer picture of how Zika affects fetuses in the first and second trimesters. Given that the first trimester is critical for neuronal development, this is likely the time when fetuses are most vulnerable. But little is known about the disease’s impact during the middle part of the pregnancy.

Of significant concern, however, is the observation that asymptomatic mothers (i.e. mothers who were infected with Zika but displayed no obvious symptoms) can still give birth to babies with birth defects. Four of the babies looked at in the study had microcephaly, yet their mothers showed no signs of the disease.

This is a real problem because symptoms only appear in about one in five people infected with Zika. Symptoms include fever, rash, joint pain, and headache, and they typically disappear after about a week. But for most people, they have no idea they even have it.

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For those couples who are actively trying to get pregnant and may have had exposure to Zika, these CDC guidelines should answer any questions.

[New England Journal of Medicine]