WASHINGTON — The Centers for Disease Control and Prevention has issued detailed and sweeping guidelines for preventing sexual transmission of the Zika virus, action that reveals medical experts’ evolving understanding of the scope and severity of health risks posed by the mosquito-borne pathogen.
Among the guidelines for couples trying to have babies: If a woman has received a Zika diagnosis or has symptoms of Zika after possible exposure, the CDC recommends that she wait at least eight weeks after her symptoms first appear before trying to become pregnant.
Likewise, if a man has been given a Zika diagnosis 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 the extra months added to minimize risk.
The CDC released the new guidance in three reports. The US agency and the World Health Organization had already issued warnings about the dangers of Zika infection — particularly brain damage — for fetal development and the importance of couples’ using condoms or abstaining from sex.
But with sexual transmission turning out to be more common than initially thought, US 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.’’
As of this week, 19 of the 273 confirmed Zika cases on the US mainland involve pregnant women. Six of those involved sexual transmission, the CDC said.
The recommendations issued Friday cover a much broader range of scenarios than the agency outlined last month.
Whereas earlier guidance was aimed at ‘‘protecting women who were already pregnant,’’ Jamieson said, the latest directions focus on the time 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.
With men and women who had possible exposure from recent travel or sexual contact but who remain free of symptoms, for example, health 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 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 do not want to get pregnant but have no access to effective birth control, putting them at risk for unintended pregnancies and Zika infection, researchers said.
