"When I found out one of them had microcephaly, the ground fell out from beneath me," the 25-year-old said as she sat on the sofa of her home in the city of Santos. "You always hope that they will be born well, thinking that it could be a mistake by the doctor or in the ultrasound."

Her son Lucas, who she holds in her arms, was born healthy in November. His twin sister Laura, whose head is visibly much smaller, requires regular treatment by a team of neurologists and physiotherapists in nearby Sao Paulo.

With two young children already, Oliveira - who suffered the symptoms of Zika early in pregnancy - knew that life would be difficult with a child with developmental problems. Her husband, the family's only breadwinner, brings home just over 2,000 reais ($566) a month.