Health authorities in the United States said they were investigating 14 new reports of the Zika virus possibly being transmitted by sex, including to pregnant women. If confirmed, the unexpectedly high number would have major implications for controlling the virus, which is usually spread by mosquito bites.

Scientists had believed sexual transmission of Zika to be extremely rare. Only a few cases have ever been documented. But if all the women in the cases the Centers for Disease Control and Prevention is examining test positive for the virus — as two women already have, and four others have done in preliminary lab tests — officials believe there is no way other than sex that they could have contracted it.

The specter of so many cases — all in the continental United States — brings fresh complexity to the medical mystery of Zika. The virus is suspected to cause birth defects and a rare condition of temporary paralysis.

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

In all the cases the CDC 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 CDC reported the potential cases in an alert to health care providers on Tuesday.