The study in Cell Stem Cell showed that Zika infects a kind of neural stem cell that gives rise to the cerebral cortex, the brain's outer layer responsible for intellectual capabilities and higher mental functions.

Scientists are trying to understand whether and how the Zika virus might be causing birth defects.

Seventy-two of the women had their Zika infection confirmed by polymerase-chain reaction (PCR) testing, which can identify genetic material from the virus.

The study - which provides some of the strongest evidence that Zika virus causes microcephaly - found that almost one-third of women who had Zika infections during their pregnancy had an ultrasound that showed fetal abnormalities.

"The frequency was so high", said study co-author Karin Nielsen, a pediatric infectious disease specialist at the University of California at Los Angeles, who collaborated with scientists at Fiocruz, an institution in Brazil. "It seems like it can act on multiple fronts".

The Zika virus is usually spread and carried by mosquitos and is strongly related to the West Nile and yellow fever. According to the CDC, babies with microcephaly often have smaller head sizes and brains that might not have developed properly.

The second study, published in the New England Journal of Medicine, detailed the 88 cases followed at the Rio de Janeiro clinic between September 2015 and last month. The non-infected women all had normal ultrasounds.

Eight of the women in the study have delivered babies, including the two stillbirths and two who appeared healthy. That would be a more realistic experiment that would allow scientists to mimic the developing brain. They included calcification of the brain, placental insufficiency with low to no amniotic fluid, fetal growth restriction and central nervous system damage, including potential blindness.

Six of the women have given birth at the time of publication.

A doctor first suggested the possible link after noticing an increased number of babies had been born with the severe brain deformation in Zika-affected areas of Latin America. They also grew other cell types including pluripotent stem cells and immature neurons.

"Unfortunately, we still have many unanswered questions", said Dr. Christopher M. Zahn of the American College of Obstetricians and Gynecologists.

Zika, which is spreading rapidly in the Americas, is usually no more harmful than a bad cold or mild flu, but global anxiety about the mosquito-borne virus has been driven by its probable link to microcephaly and GBS.

The researchers were led in this endeavor by Arnaud Fontanet from France's Institut Pasteur, who believes their work is incredibly significant, since it reveals an existing connection between this virus and the serious neurological complications entailed by the Guillain-Barre syndrome.

The study involved using stem cells and infecting them with the virus and then analysing changes in gene expression caused by the virus.

There has been evidence about other mosquitoes linked to Zika.

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