The Centers for Disease Control and Prevention, along with state health departments, is investigating 14 new reports of possible sexual transmission of Zika in the US, the agency announced Tuesday.

All of the cases involve the possible transmission of the virus to a female who hadn’t traveled from a male sexual partner who had recently traveled to an area where Zika was spreading through mosquitoes. While sexual transmission of the virus has been previously reported, the number of newly suspected cases suggests that sexual transmission may be more common than health experts previously expected.

“We were surprised that there was this number,” Dr. Anne Schuchat, the deputy director at the Centers for Disease Control and Prevention, said in an interview with the New York Times. “If a number of them pan out, that’s much more than I was expecting.”

So far, Zika infection has been confirmed in only two women among the 14 cases. Preliminary results suggest that four other women have been infected with the virus. The remaining eight cases are still awaiting results.

Several of the cases involve pregnant women, the agency noted. There’s no evidence at this time that women can transmit the virus to men.

While the CDC emphasized that mosquitoes are still considered the primary route of transmission, the agency has issued interim guidelines to prevent sexual transmission of the virus. The agency has also issued travel advisories for pregnant women, and women planning to become pregnant, who are considering visiting Zika-affected areas.