Zika infections in Puerto Rico appear to be increasing rapidly, top U.S. health officials said on Friday, June 17, raising concerns for dozens if not hundreds of cases of microcephaly.

Puerto Rican health officials on Friday reported that 1,726 people in the U.S. commonwealth have been infected with Zika, including a total of 191 pregnant women. That is up from a total of 1,501 total infections and 182 infections in pregnant women for the previous week.

The numbers reflect the total number of confirmed Zika cases since the start of the outbreak late last year, a number that reflects largely symptomatic cases of Zika.

In new data released by the U.S. Centers for Disease Control and Prevention on Friday, health officials have also seen a sharp rise in the frequency in which Zika is detected in blood donations.

The numbers are based on the use of a test from Swiss drugmaker Roche Holding AG in use since April 3 to screen blood donations for Zika.

“This is a test that measures actual Zika virus in the blood. It’s highly sensitive and highly specific,” CDC Director Dr. Thomas Frieden told reporters in a conference call on Friday.

As of April 3, 68 out of 12,777 blood donations have tested positive for Zika, and the proportion of positive tests “has steadily increased,” Dr. Frieden said.

Testing in the most recent week ended June 11 showed 1.1 percent of donated blood was infected with Zika, the highest proportion seen yet in Puerto Rico, Dr. Frieden said.

Although the blood donor population is not representative of the general population, Dr. Frieden said the increasing prevalence of Zika in blood donations likely reflects increases in the population at large.

The Zika virus only remains present in the blood for about a week after symptoms appear, and diagnostic tests also look for signs of past infections. Dr. Frieden said analyzing Zika in blood donations provides a snapshot of current infections in the population at any given time, including asymptomatic cases.

Based on the findings, he estimates that as many as 2 percent of the population is infected with Zika each month.

The estimate suggests that over time, there is a substantially higher risk that a pregnant woman would become infected with Zika, putting her fetus at risk of birth defects.

That also means that in the coming months, “it’s possible that thousands of pregnant women in Puerto Rico could be infected in with Zika,” Dr. Frieden said, leading to “dozens or hundreds of infants being born with microcephaly in the coming year.”