Likewise, if a man has been diagnosed with Zika or has symptoms of the illness, he should wait at least six months from those first signs before having unprotected sex, according to the public-health agency. That longer waiting period reflects the length of time the virus has been found in semen — 62 days — with additional months added to minimize risk.

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The CDC released the new guidance in three reports. It and the World Health Organization had already issued warnings about the dangers of Zika infection for fetal development —particularly brain damage — and the importance of couples using condoms or abstaining. But with sexual transmission turning out to be more common than initially thought, U.S. health officials became much more specific.

“I want to emphasize that this is not an exact science,” said Denise Jamieson, a clinical obstetrician who is part of the team leading the CDC’s Zika response. “We have so little data to base this on. We’re doing the best we can.”

Jamieson, who has worked in public health for 19 years, said she has “never seen anything move this fast. Almost every day there is some new piece of information.”

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As of this week, 19 of the 273 confirmed Zika cases on the U.S. mainland involve pregnant women. Six of those cases were transmitted sexually, the CDC said.

The recommendations issued Friday cover a much broader range of scenarios than what the agency described last month after a Dallas resident was infected by having sex with someone who had contracted the disease while traveling in Venezuela. Whereas earlier guidance was aimed at “protecting women who were already pregnant,” Jamieson said, the latest directive focuses on the period around conception.

They include updated information for health-care professionals counseling patients about pregnancy planning and the timing of pregnancy after possible exposure, as well as information about how long men and women should consider using condoms or not having sex.

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With men and women who had possible exposure from recent travel or sexual contact but who remain free of symptoms, for example, providers should advise patients to wait at least eight weeks after that potential exposure before trying to get pregnant.

In making the recommendations, the CDC said it considered the longest known risk period and then multiplied that by three.

One of the new reports highlights the potential risk facing women of childbearing age in Puerto Rico, where the virus is spreading more rapidly than anywhere else in the United States. An estimated 138,000 women on the island don’t want to get pregnant but have no access to effective birth control, putting them at risk for unintended pregnancies and Zika infection, researchers said.

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About two-thirds of pregnancies in the territory are unintended. In 2014, there were about 34,000 births.

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Puerto Rico has at least 350 confirmed Zika cases, with at least 40 involving pregnant women. CDC officials have warned that thousands of pregnant women could get infected with the mosquito-borne pathogen there, but the report is the first time they have quantified that risk.

CDC officials declined to recommend — as some countries have done — that men and women living in Zika-affected regions postpone pregnancy, saying it is a complex and personal decision. Jamieson said the Department of Health and Human Services is working on finding additional resources to support increased access there to the most effective forms of contraception. That includes working with community health centers and private organizations, she said.

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Mosquitoes remain the primary way Zika is spread, and preventing bites is the best way to avoid infection. Typical symptoms include rash, red eyes and joint pain, although 80 percent of individuals who are infected never experience any problems.

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Global health officials say circumstantial evidence now overwhelmingly links Zika infections during pregnancy to a range of birth defects. Brazil, the epicenter of the epidemic in the Americas, is expected to have more than 2,500 cases of microcephaly based on current data, the head of the World Health Organization said earlier this week. The rare congenital condition causes babies to be born with abnormally small heads and often underdeveloped brains.