T

he first study to follow Zika-infected women throughout their pregnancies found that a large percentage of them are carrying fetuses with birth defects, a finding likely to deepen concern about the current outbreak.

Ultrasounds showed signs not only of microcephaly, a condition in which babies are born with abnormally small heads, but also a number of other defects, many of which have never been recorded in conjunction with Zika virus before.

Other fetuses died just weeks before their due date, although there were no signs of congenital abnormalities.

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The data were collected just weeks ago, and were rushed into publication by the New England Journal of Medicine on Friday, given the urgency of the Zika outbreak in Latin America and the Caribbean. Scientists still lack basic information about Zika, including the rate at which women infected during pregnancy deliver newborns with birth defects.

The researchers in the new study followed 88 women in different stages of pregnancy in Rio de Janeiro.

Of the 72 who tested positive for Zika, only 42 had ultrasounds performed, but their results were alarming: 12 of their fetuses showed signs of birth defects, which is 29 percent. No such defects were observed in the ultrasounds of Zika-negative mothers.

The virus often leads to relatively mild symptoms, such as joint pain, fevers, or rashes; most people don’t even notice they are infected. But there is strengthening evidence that the virus can have terrible consequences for developing fetuses.

Brazil, which saw a surge of Zika cases starting late last year, has reported a spike in cases of microcephaly. On Friday, Colombia, which has also seen Zika spread, reported its first case of Zika-linked microcephaly.

The Rio de Janeiro study, however, is the first to follow women through pregnancy to look at the possible effects of Zika.

“We saw a very high percentage of abnormal fetal ultrasounds, and we saw a significant number of central nervous system findings, which is surprising to us,” said Dr. Karin Nielsen-Saines, a pediatric infectious disease specialist at UCLA, and one of the lead authors of the study, along with a team from the Fundacao Oswaldo Cruz in Brazil.

Nielsen-Saines was also surprised — and alarmed — to see problems such as fetal death arising in the third trimester of pregnancy, as those risks were thought to be highest in earlier stages of pregnancy. “These babies were just a few weeks from being born,” she said. “That’s unusual. Syphilis can cause that, but that had not been reported with Zika.”

Dr. Catherine Spong, acting director of the National Institute of Child Health and Human Development, warned that the sample size is small, and that Zika did not necessarily cause all these problems. “Stillbirth has many, many different causes, including maternal age, genetic factors,” she said.

Yet she agreed that these results are very important and concerning.

To Nielsen-Saines, this study confirms that pregnant women with Zika should be seen frequently by obstetrician-gynecologists for ultrasounds. “This alerts people that pregnancy in women affected with Zika are high-risk pregnancies,” she said.

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