“We were surprised that there was this number,” Dr. Anne Schuchat, the deputy director at the C.D.C., said in an interview. “If a number of them pan out, that’s much more than I was expecting.”

In all the cases the C.D.C. is examining, women in the continental United States had sex with men who had traveled to countries where the virus is circulating, and developed symptoms associated with the virus within about two weeks of their male partners’ symptoms.

Officials at the C.D.C. reported the potential cases in an alert to health care providers on Tuesday.

The agency did not say exactly how many of the women were pregnant, but it reiterated its recommendation that people returning from Zika-infected areas use condoms or abstain from sex for the duration of their partner’s pregnancy. The alert said there was no evidence that women could transmit Zika virus to their sex partners, but added that more research was needed to be sure.

This country has become a laboratory of sorts to test the sexual transmission of Zika, as scientists race to understand the disease. Transmission by mosquitoes is not yet happening in the continental United States because it is still winter, so health officials say they believe that any infection of an American resident who has not traveled to a place where Zika is circulating has probably been contracted through sex.