Story highlights Researchers are studying why some babies born to Zika-infected moms develop microcephaly

They hope to find a genetic explanation for why one twin has microcephaly and the other does not

Santos, Brazil (CNN) Jacqueline Silva de Oliveira sits on the edge of her bed, holding her 6-month-old son, Lucas. He squirms in her arms before he finally screams out, hungry and demanding milk.

His twin sister, Laura, barely notices, just a slight nod and a twitch of her eyes. Half his size, she is quiet, asleep on the other end of the bed, as she often is. When she wakes, even her cries seem to struggle from her throat. She can't breastfeed. She can barely hold up her small head. She has microcephaly.

Jacqueline Silva de Oliveira with her twins, Lucas and Laura.

"We try to live in the moment with her," de Oliveira says of Laura. "Because we worry, what if she needs a wheelchair? How will she move around in this house? It's really complicated to think about the future."

It was only three months after she gave birth that de Oliveira found out that she'd had that Zika virus while pregnant. Her husband had Zika before her, with strong symptoms, and eventually she developed a rash.

"I thought it was allergies," she said. "And it only lasted a day, so I didn't even get tested or anything." She went to the doctor, thinking she had the dengue fever common in that area, and the doctor recommended that she go to the hospital. But it was very far away. "I didn't go because I was already feeling really bad," she said.

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