A pregnant woman who becomes ill from the Zika virus faces a 7 percent chance that her child will be born with birth defects, and that risk jumps to nearly 13 percent if she becomes ill during the first trimester, a new study conducted in French territories in the Americas has concluded.

The finding “emphasizes the serious global health threat to pregnant women and their infants posed by congenital (Zika virus) infection,” said Dr. Margaret Honein of the Centers for Disease Control and Prevention in Atlanta, writing in an editorial in the New England Journal of Medicine, where the study appears.

The estimates do not include less-obvious developmental problems that may surface later in life.

In addition, because the study only included women who fell ill, they don’t include the risk for women who may have been infected but didn’t experience symptoms, which happens in about 80 percent of cases.

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“Other studies showed earlier that the risk of births defects did not depend on the presence or the severity of (Zika)-related symptoms,” chief author Dr. Bruno Hoen of the University Medical Center of Guadeloupe told Reuters Health by email.

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“This was really well done and it really does nail down the rate that we’re seeing in this population,” said Dr. Carl Fichtenbaum, a professor in the division of infectious diseases at the University of Cincinnati College of Medicine in Ohio, who was not involved in the research.

“The (birth defect) rate might differ in other populations, depending on whether it’s your first Zika infection or your second,” he told Reuters Health by phone. “In populations where it’s just been introduced it might be more severe for the fetus, but I think time will tell.”

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The 527 surviving babies in the new study will be followed for at least two years.

“Only the longer-term follow-up of the children born to the women in the current study will help identify the full spectrum of (Zika)-related complications,” the Hoen team said.

The virus was linked to birth defects, particularly having an abnormally small head – a condition known as microcephaly – during a 2016-2017 outbreak. Zika vaccines are still being tested.

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Estimates of the rate of congenital neurologic defects in infected newborns have ranged from 6 percent to 42 percent. The Hoen team included women who were pregnant from March through November of 2016 when it was confirmed that they had been infected with Zika.

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Their study of 555 fetuses and infants found evidence of microcephaly in 5.8 percent. The rate of severe microcephaly was 1.6 percent.

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The earlier the infection occurred in pregnancy, the greater the risk.

Neurologic or eye problems were seen in 12.7 percent of the babies whose mothers were infected during the first trimester. The rates were 3.6 percent when mothers were infection during the second trimester and 5.3 percent during the third.

This is “some of the most compelling data to date that the risk of brain abnormalities, microcephaly, and eye anomalies extends to infections in every trimester of pregnancy,” said Honein in her editorial.

The French territories were French Guiana, Guadeloupe and Martinique.

A Zika registry in the United States has seen birth defects in 10 percent of women known to have been infected with the virus. When the infection occurred during the first trimester, the rate has been 15 percent.

But those studies “do not provide information on the estimated 80 percent of pregnant women with (Zika) infections who have no reported symptoms,” Honein writes. “Population level increases in (Zika)-associated birth defects are unlikely to be recognized without ongoing timely and comprehensive surveillance of birth defects that captures all affected fetuses and infants regardless of whether maternal (Zika) exposure or infection was identified.”

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Zika “should definitely be added to the list of infectious agents that can cause severe birth defects, as are rubella virus, cytomegalovirus, and others,” Hoen said in his email.

The study also emphasizes the urgent need to protect women who might become pregnant when they are traveling or living in areas with Zika-infected mosquitoes, to test women at risk for Zika when they become pregnant, and to develop an effective vaccine, he said.