Federal health officials said Tuesday as many as 14 people in the U.S. may have contracted the Zika virus through sex, a surprising development that makes researchers rethink the virus’s usual routes of transmission.

Several of those women are pregnant, which puts their babies at risk for a potentially life-threatening birth defect that scientists believe is caused by the virus.

{mosads}The Centers for Disease Control and Prevention (CDC) disclosed the latest data on Tuesday amid intensifying concerns over the virus on Capitol Hill.

The Atlanta-based agency also doubled down on its warnings for women who are or may become pregnant, telling them to avoid unprotected sex if they have a male sex partner possibly exposed to the virus. Some women may want to avoid sex altogether to make certain they are not at risk, the agency said.

The still-evolving science of the Zika virus underscores uncertainty among even top government researchers.

“These new reports suggest sexual transmission may be a more likely means of transmission for Zika virus than previously considered,” the CDC wrote in a statement Tuesday.

The CDC is amplifying its warning at the same time the Obama administration is battling for nearly $2 billion in emergency funding to further study the virus and halt its spread.

“We do think this is a critical situation,” Dr. Anne Schuchat, the principal deputy director for the CDC, told The Hill when asked about the funding on Tuesday.

Schuchat said the CDC is still investigating whether the 14 cases, reported by state health departments, are linked to sex.

“We are concerned, and that’s one of the reasons we released the health advisory, even though we aren’t finished with the investigations,” she said, adding that she believes the CDC’s guidance will help prevent a larger number of sexually transmitted Zika virus cases.

“We hope that we won’t see more of this, and that’s one of the reasons we put out the guidance,” she said. “We believe these episodes occurred before our guidance went out.”

While the disease is not known to have harmful impacts on adults, it has been linked to hundreds, and possibly thousands, of birth defects born to infected women in Central and Latin American countries. In most adults, the Zika virus has symptoms that are more mild than dengue fever.

The CDC launched a study of the Zika virus’s correlation to a condition called microcephaly, which causes babies to be born with abnormally small heads, in Brazil this week.

The disease, which has perplexed medical researchers across the globe, has spread rapidly, mostly through mosquitoes.  

—Updated at 4:12 p.m.