Story highlights A new blood screening test shows a rise in Zika cases in Puerto Rico

Pregnant women are at high risk for delivering "dozens or hundreds" of babies with microcephaly

(CNN) Puerto Rico could see dozens, if not hundreds, of babies with microcephaly in the coming months, Centers for Disease Control Director Dr. Tom Frieden said Friday while announcing the results of blood screenings underway in the U.S. territory.

"Based on the best info available, Zika is increasing rapidly in Puerto Rico," Frieden said. "The importance of this is that thousands of pregnant women could become infected, which could lead to dozens or hundreds of babies born with microcephaly."

Microcephaly is a birth defect in which the baby is born with a small head and brain, which often leads to serious developmental delays or even death.

Though Frieden has sounded the alarm about projected cases of microcephaly in Puerto Rico in the past, the current warning is based on data coming in from a newly developed nucleic acid blood screening test, authorized by the Food and Drug Administration under an investigational new drug application.

The test, which has been in place since April 3, is "very sensitive, able to identify even a few copies of virus per milliliter of blood," and has shown no false positives to date, said Dr. Matthew Kuehnert, director of the CDC's Office of Blood, Organ and Other Tissue Safety.

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