A new report detailing the potential dangers that Zika poses to an unborn child reveals that the virus appears to be most harmful when transmitted from a pregnant mother to her fetus during the first trimester of pregnancy. The report, published Tuesday in the journal Radiology, found that transmission of Zika virus during the first trimester increases the likelihood of severe brain defects in the baby, including microcephaly.

“The first trimester is the time where infection seems to be riskiest for the pregnancy,” said study coauthor Dr. Deborah Levine, the director of Obstetric & Gynecologic Ultrasound at Beth Israel Deaconess Medical Center and a professor of radiology at Harvard Medical School in Boston, according to a news release. “From an imaging standpoint, the abnormalities in the brain are very severe when compared to other congenital infections.”

The researchers performed a retrospective review of imaging and autopsy findings associated with congenital Zika virus infection found in the Instituto de Pesquisa in Campina Grande state Paraiba (IPESQ) in Northeastern Brazil. Of the 438 pregnant patients referred to IPESQ due to rash during pregnancy or suspected abnormalities, imaging exams found that 17 fetuses had confirmed Zika infections while 28 had presumed Zika infections.

Imaging exams included fetal MRI, postnatal brain CT, postnatal brain MRI and longitudinal prenatal ultrasound.

In addition to microcephaly, a condition in which a child is born with an extremely small head due to an underdeveloped brain, the virus has been linked with eye defects, hearing impairment and stunted growth in babies. In the report, researchers discovered that fetuses exposed to Zika virus also suffered from a variety of brain abnormalities including gray and white matter volume loss, brainstem abnormalities, calcifications and ventriculomegaly, a condition in which the ventricles are enlarged.

In a news release, the researchers noted that the brain abnormalities seen in confirmed and presumed Zika groups were very similar, and that nearly all of the babies in each group had ventriculomegaly. While most fetuses had at least one exam indicating significantly small head circumference, three fetuses with severe ventriculomegaly registered normal head circumference. All of the babies exhibited reduced tissue volume in their brains, and all patients experienced varying abnormalities in cortical development.

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“The severity of the cortical malformation and associated tissue changes, and the localization of the calcifications at the grey-white matter junction were the most surprising findings in our research,” Dr. Fernanda Tovar-Moll, the report’s lead author and vice president of the D’Or Institute for Research and Education and a professor at the Federal University of Rio de Janeiro, said in the news release.

The researchers believe that the babies’ frequently collapsed skull shapes with overlapping sutures and redundant skin folds is due to a combination of the small brain as it develops and what was likely a larger head size that then decompresses due to ventriculomegaly or brain atrophy.

The researchers further found that 94 percent of the confirmed Zika group and 79 percent of the presumed Zika group had abnormalities of the corpus callosum, the nerve fiber bundle that allows communication between the left and right hemispheres of the brain.

Tovar-Moll said in the news release that researchers are continuing to delve deeper into the central nervous system changes induced by congenital Zika virus infection to better understand the spectrum of changes they have recorded.

“We are also interested in investigating how congenital Zika virus infection can interfere with not only prenatal, but also postnatal gray and white brain maturation,” she said.