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abies exposed to the Zika virus as fetuses are more than 50 times more likely to be born with abnormally small heads and underdeveloped brains — a condition called microcephaly — as other babies, according to a new study.

The authors of the study, published Thursday in Lancet Infectious Diseases, cautioned that the finding was an interim result and research is still going on, “so the magnitude needs to be treated with some caution.” But outside researchers said this type of research, known as a case-control study, is more likely to avoid the biases that can affect other types of studies. This was the first case-control study looking at the association between microcephaly and Zika.

Even if the strength of the association decreases as more cases of microcephaly are included, the study still starkly demonstrates the risk Zika poses to developing fetuses.

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“Their results highlight the striking magnitude of the association between microcephaly and laboratory-confirmed Zika virus infection,” the Brazilian scientist Patricia Brasil and Dr. Karin Nielsen-Saines, a pediatric infectious diseases expert at UCLA, wrote in an editorial also published Thursday.

Researchers studied 32 babies born with microcephaly between January and May in hospitals in Recife, a Brazilian city in the region hardest hit by the microcephaly epidemic. They compared those newborns to 62 babies with normal-sized heads who were born at the same number of weeks of gestation and whose mothers lived in the same region.

The scientists found that 41 percent of the babies born with microcephaly were infected with Zika. None of the other babies tested positive for the virus. Through a statistical analysis, the authors concluded that babies exposed to Zika in utero are more than 50 times more likely to have microcephaly.

The researchers said they published their preliminary findings without waiting to finish the larger study — which will look at 200 babies with microcephaly and 400 control babies — because the results were so powerful.

The interim research also made clear how widely and quickly Zika can spread when it enters a new population.

Eighty percent of the mothers of babies born with microcephaly tested positive for Zika infections — as did 64 percent of mothers of the control babies.

The authors said they hope the study demonstrates how crucial Zika prevention efforts are.

“The association is so strong and you have such a high frequency of infection even among mothers in the control group,” said Celina Maria Turchi Martelli, a researcher with Brazil’s Oswaldo Cruz Foundation and an author of the study.

The study, which came at the request of Brazilian health officials, will also investigate why northeastern Brazil has seen so many cases of microcephaly, even compared to other areas where the virus has spread. Health officials have speculated that genetic or environmental factors, or prior dengue infections, can increase the risk of Zika leading to microcephaly.

The Zika virus, which has swept through much of Latin America and the Caribbean, is primarily spread by mosquitoes but can also be spread through sex. It typically causes no or only minor symptoms.

But if it infects fetuses, Zika is now known to cause a wide array of birth defects, and some studies have suggested that the virus can continue to cause damage to the central nervous system after a baby is born.

The researchers also conducted brain scans on 27 of the 32 babies born with microcephaly (of the other five, three died before the scan could be done, one was stillborn, and one was in intensive care). Of the babies who were scanned, 11 had some form of visible brain abnormality, which the authors say was a smaller percentage than other reports have found.

Martelli said it was unclear why fewer babies in this study had visible brain abnormalities, but said the results nonetheless showed the range of clinical symptoms Zika can cause in newborns, ranging from visible calcifications in the brain to epilepsy.

In the editorial, Brasil and Nielsen-Saines wrote that the study raised the question of how microcephaly is defined. There are two broad categories of microcephaly: disproportionate, in which the head is abnormally small for the infant’s body, and proportionate, in which the entire newborn is unusually small for the number of weeks of gestation.

Both need to be tracked as a result of Zika, which has also been associated with restricting the growth of fetuses overall, Nielsen-Saines said in an interview.

If the baby has proportionate microcephaly — and is just born small — then it’s possible that the brain development could catch up, although “a lot of questions still need to be answered,” Nielsen-Saines said.

But, she added, “If there’s microcephaly because there’s structures in the brains that are missing, then it’s a very different story.”

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