Federal health authorities said for the first time Friday how long couples who have been exposed to the Zika virus should wait before trying to get pregnant.
The officials, at the Centers for Disease Control and Prevention, said that they had decided to recommend a waiting period based on the latest information about the science of the virus. The Zika virus has been linked to a surge of birth defects in Brazil, and to cases of Guillain-Barré syndrome, in which a person's immune system attacks part of the nervous system, leading to some paralysis.
Women who have had symptoms of the virus or tested positive for it should wait at least eight weeks after their symptoms first appeared before trying to get pregnant, the agency said. Officials recommended that men who had symptoms should wait six months before having unprotected sex. The virus has been known to live longer in semen.
"We're learning more every day and evidence of a link between Zika and a spectrum of birth outcomes is becoming stronger and stronger," said Dr. Denise J. Jamieson, one of the leaders of the pregnancy and birth defects team, which is part of the CDC's Zika Virus response team.
She added: "We've become more concerned" about the period around the time of conception. "For people who either have the Zika disease or who travel to an area with active Zika transmission, we are now recommending they wait a period of time before trying to get pregnant."
For people who have traveled to Zika-infected areas, but had no active signs of the disease, the CDC recommended a shorter waiting period — eight weeks for men and women before trying to get pregnant "in order to minimize risk."
Jamieson said the CDC based the waiting periods on the longest known risk period and "allowed three times the longest period" out of an abundance of caution.
The situation is trickiest for men and women who live in areas where Zika is circulating. CDC officials recommended that doctors talk with their patients about the risks of the virus, but stopped short of recommending that women delay pregnancy. (Several countries outside the United States, including Brazil, Ecuador, Colombia and Jamaica, have in recent months at least suggested that women postpone getting pregnant for indefinite or varying periods of time. El Salvador has recommended that women wait until 2018.)
"These are very complex, deeply personal decisions," the CDC said in a statement.
For men with pregnant partners who travel to a Zika-infected area, the recommendation remains the same: Use condoms for the duration of the pregnancy. Condoms should be used for vaginal, anal and oral sex.
The agency said the waiting periods for men also applied to how to avoid sexual transmission of Zika to their partners after they had traveled to Zika-infected areas, a phenomenon that is more common than scientists once thought. Men with symptoms should consider using condoms or not having sex for at least six months. For men without symptoms, the period was eight weeks.
The agency also estimated that about 138,000 women in Puerto Rico are not using birth control that is effective, and may be at risk of unintended pregnancy.