In a move with enormous social implications in countries hardest hit by Zika, the World Health Organization is recommending that millions of people in areas where the mosquito-borne virus is spreading now consider delaying pregnancy to avoid risking a baby born with serious brain damage.

The virus has spread from Brazil, the epicenter of an epidemic, to nearly 50 countries, most of them in Latin America and the Caribbean. The WHO’s advice goes further than recommendations from the U.S. Centers for Disease Control and Prevention, which has said that the government’s job is to provide the science and information and that it is up to women, their partners and their doctors to make personal decisions about the timing of a pregnancy.

The WHO issued the guidance this week because of growing evidence that the virus is sexually transmitted far more often than previously thought. On Tuesday, the organization is convening its panel of Zika experts to give an update on the epidemic, and it will evaluate the risks tied to holding the Olympic Games in Rio de Janeiro in August.

To “prevent adverse pregnancy and fetal outcomes,” the WHO advises that “men and women of reproductive age, living in areas where local transmission of Zika virus is known to occur, be correctly informed and oriented to consider delaying pregnancy.”

Everything you ever wanted to know about the Zika virus and its spread across North and South America.

Global health experts said the recommendation was significant and has major social consequences.

“This is a huge deal for the WHO to come out with evidence-based recommendations and say, ‘Don’t get pregnant,’” said Rebecca Katz, a specialist in global health policy at Georgetown University. Not only does it confirm the risk to the fetus, she said, it also has huge implications for family planning and reproductive rights.

Most of the countries hardest hit by Zika are overwhelmingly Catholic, “where not all the population has equal access to care and equal access to contraception,” she said.

“It's really easy to say, ‘Don't have children,’” Katz said, “but that's not acknowledging reality.”  The U.N. agency's shift is likely to put pressure on governments and health ministries to address issues of contraception and abortion, she said.

The rapid spread of Zika  prompted some Latin American governments earlier this year to urge women to consider delaying pregnancy in an effort to prevent as many birth defects as possible.

In January, El Salvador took among the most aggressive stances, urging women to refrain from getting pregnant until 2018. The Central American nation saw its first suspected Zika cases in November and sent samples to the United States for testing.

Colombia, Jamaica and Honduras also have urged women to delay having babies.

In February, Pope Francis said that using artificial contraception may be morally acceptable in trying to avoid the virus, given the potential following infection for a pregnancy to end in miscarriage or an infant to be born with severe birth defects.

The pontiff’s comments came after he was asked whether abortion or birth control could be considered a “lesser evil” in the face of such risks. Even as he gave tacit approval to using contraception in such circumstances, he called the notion of aborting a fetus another matter entirely.

Microcephaly, a severe and rare condition characterized by abnormally small heads and underdeveloped brains, is seen as the greatest health risk to a fetus. But increasingly, studies are also linking Zika infections to many other health problems.

Pregnant women infected with the virus during their first trimester face as high as a 13 percent chance that their fetus will develop microcephaly, according to research published last month that analyzed data from one of the most affected areas in Brazil.

The U.S. mainland has yet to record a locally transmitted case of Zika, and federal health officials have repeatedly declined to suggest that American women should delay pregnancy because of the virus. The CDC is monitoring about 300 pregnant women who contracted Zika because of travel or an infected partner.

Widespread local transmission already is occurring in Puerto Rico, however, and officials expect that about 20 percent of the island’s 3.5 million residents could become infected.

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