The number of people infected with Zika in Puerto Rico is rising at an alarming rate, putting pregnant women at even greater risk of their babies suffering severe birth defects, a top U.S. public health official said Friday.

The latest data show that the most accurate, real-time indicator of Zika infection suggests that thousands of pregnant women there could contract the virus in the coming months. That, in turn, could lead to “dozens to hundreds of infants born with microcephaly in the coming year,” Tom Frieden, director of the Centers for Disease Control and Prevention, said during a media briefing.

The data come from a CDC report showing the percent of Puerto Ricans who are testing positive for Zika during blood donor screening. The numbers are the closest approximation for a representative sampling of infection rates on the island as a whole. “These numbers are increasing faster than we had anticipated,” Frieden said in a separate interview, referring to the blood-test results. He noted that Puerto Rico has not yet reached the typical peak of mosquito-fueled disease outbreaks, which occurs over the summer and into the fall.

About 1 percent of the island’s blood donors tested postive for Zika in the week ending June 11, the highest level since testing began in April, according to the agency’s report. The test measures whether someone is infected at that moment. A 1 percent positive rate translates to roughly a 2 percent infection rate each month, Frieden said.