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MIAMI (CBSMiami) – A new CDC report says Zika can copy itself in the brains of unborn babies and the placentas of pregnant women.

The finding could explain how the virus causes devastating birth defects and miscarriages, even if the mother only experienced minor symptoms.

Most people infected with Zika never develop symptoms, but infection during pregnancy can cause devastating birth defects, including microcephaly, in which a baby’s skull is much smaller than expected because the brain hasn’t developed properly.

Microcephaly is diagnosed based on a measurement of the baby’s head circumference. It can be done during pregnancy using ultrasound, or after the baby is born. Doctors then compare the measurement to standard sizes of other kids, based on gender and age.

The study released Tuesday is the first to show the Zika virus replicates both before and after birth.

Researchers say this is more evidence that Zika is most-dangerous to unborn babies when mothers contract it in the earliest months of pregnancy.