U.S. health officials on Monday updated their Zika virus guidelines, saying that pregnant women could contract Zika from a sex partner of either gender.

The virus can cause serious birth defects including microcephaly, where babies are born with abnormally small heads and brains.

The new update follows news last week of the first recorded female-to-male transmission of the virus during sex.

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While mosquitoes are by far the most common form of transmission, cases of sexual transmission can occur, either male-to-female, female-to-male or female-to-female.

For that reason, experts at the U.S. Centers for Disease Control and Prevention say the new guidelines "include the possibility of sexual transmission from an infected woman."

"CDC recommends that all pregnant women with sex partners (male or female), who live in or traveled to an area with Zika, use condoms during sex or abstain from sex for the remainder of their pregnancy," the agency said.

"Sex includes vaginal, anal and oral sex, and may also include the sharing of sex toys," the CDC clarified.

These precautions now include either straight or lesbian couples where one partner could pass the virus on to her pregnant partner.

Any pregnant woman who suspects that she may have been exposed to Zika -- either through a mosquito bite or sexual contact with an infected person -- should also be tested for the virus, the agency stressed.

The CDC also pointed out that "new information has indicated that some infected pregnant women can have evidence of Zika virus in their blood for longer than the previously recommended seven-day window."

Because of that new data, the agency now recommends that the time frame for blood testing for Zika be lengthened to 14 days.

The vast majority of cases of Zika infection and associated microcephaly have occurred in Latin America, especially Brazil, where thousands of cases have been reported.

However, Zika may be making inroads into the United States. Late last week, Florida health officials said they were investigating a second possible case of locally transmitted Zika infection.

The first possible case of local infection in the continental United States was reported last Tuesday by the Florida health department. That case involved a woman in Miami-Dade County, while the newer, second case involved a resident of Broward County, north of Miami.

Florida health officials said they are capturing and testing mosquitoes in the neighborhoods where the two unidentified patients live. Meanwhile, Gov. Rick Scott has asked for assistance from the U.S. Centers for Disease Control and Prevention, The Miami Herald reported.

The CDC said it has provided $2 million for Zika preparedness and another $5.6 million was just allotted, the newspaper reported.

There have been more than 1,400 confirmed Zika cases in the United States, but so far all of them have been contracted through travel abroad -- either by a mosquito bite or by sexual intercourse with someone who had traveled to a Zika-infected area.

Brazil has been the epicenter of the Zika epidemic to this point. Infections have also been reported in other Latin American and Caribbean nations.

CDC officials have said repeatedly they expect to see cases of local transmission of the Zika virus this summer in southern states with warm, humid climates such as Florida, Louisiana and Texas. The virus is typically transmitted through the bite of Aedes mosquitoes.

The CDC has reported 14 cases of sexually transmitted infections. These infections are thought to have occurred because the patients' partners had traveled to countries where Zika is circulating, the CDC said.

Typically, the Zika virus doesn't cause serious illness. Only about 20 percent of patients notice symptoms.

But the virus also has been linked to a rare paralyzing condition called Guillain-Barre syndrome.

The CDC advises pregnant women not to travel to an area where Zika transmission is ongoing, and to use insect repellent and wear long pants and long-sleeved shirts if they are in those areas.