Zika Virus Could Infect 'Thousands' of Pregnant Women in Puerto Rico, CDC Chief Warns

The CDC is also seeing an increase of Zika-infected blood donations there.

Microcephaly is a birth defect characterized by an abnormally small head and brain, leading to significant developmental problems.

The CDC is also seeing increasing rates of Zika-infected blood in blood donation centers in Puerto Rico, Frieden said. However, since the blood is being screened, Frieden said there was almost no risk that donated blood would lead to further Zika infections.

"There is no known risk for transfusion because of this highly sensitive test that is being used," Frieden said.

Currently he said 1.1 percent of donated blood has been found to be infected with the Zika virus, but that he anticipates in the general population of Puerto Rico the rate of Zika infection is higher. As a result of these increasing infection rates a pregnant woman would have a "significant" risk of contracting the virus in Puerto Rico, Frieden said.

Since there is no ongoing transmission of the Zika virus in the continental United States, Frieden said that blood centers are screening potential donors by asking about their travel history rather than directly testing the blood. Blood donation centers in the U.S. do have access to the test used in Puerto Rico to identify Zika-tainted blood donation, but only one center is currently using the technology, Frieden said.

If the Zika virus starts to spread within the U.S., blood centers could quickly take measures to safeguard the blood supply from spreading the Zika virus, Frieden said.

While exactly how the virus affects the brain of a developing fetus remains unknown, officials are learning more about who is most at risk.

Frieden said that based on studies in other countries, early information indicates that pregnant women who have a Zika infection in their first trimester may have up to a 15 percent chance of having a child with microcephaly.

Preliminary results from a study that was published last week found women infected in their third trimester had little to no risk of having a child with microcephaly. However, officials caution that even children without microcephaly might have cognitive or developmental delays that have not yet been measured.