People living in areas where the Zika virus is circulating should consider delaying pregnancy to avoid having babies with birth defects, the World Health Organization has concluded.

The advice affects millions of couples in 46 countries across Latin America and the Caribbean where Zika transmission is occurring or expected. According to a recent study, more than five million babies are born each year in parts of the Western Hemisphere where the mosquitoes known to spread the virus are found.

At the moment, Puerto Rico, the United States Virgin Islands and American Samoa are the only parts of the United States with local transmission of the virus. But clusters of cases are expected to appear in Florida and along the Gulf Coast this summer.

The governments of five countries have issued similar advice, as has the health secretary of Puerto Rico, but the Centers for Disease Control and Prevention has decided against this approach on the grounds that government doctors should not intrude on personal decisions best made by women and their partners.

The virus, carried by the yellow fever mosquito, has been linked to abnormally small heads and brain damage in infants, a condition called microcephaly. In rare cases, the infection has caused paralysis and sometimes fatal complications in adults.

The W.H.O.’s new guidelines essentially acknowledge that, with no vaccine available and mosquito eradication efforts failing to stem the spread of the infection, delaying pregnancy may be the best way that women in affected areas can avoid having children with severe brain damage.

The W.H.O. says men and women of reproductive age “should be correctly informed and oriented to consider delaying pregnancy.” The guidance was originally issued last week but did not garner wide notice among experts until Thursday, when the W.H.O. issued a clarification, distinguishing between people who visit Zika-affected countries and those who live in them.

People living in those countries are not advised to delay for any specific amount of time, but that guidance “means delaying until we have more answers, more evidence, more science,” said Nyka Alexander, a W.H.O. spokeswoman.

“But it’s important to understand that this is not the W.H.O. saying, ‘Hey everybody, don’t get pregnant.’ It’s that they should be advised about this, so they themselves can make the final decision.”
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Dr. Peter J. Hotez, dean of the National School of Tropical Medicine at the Baylor College of Medicine, said of the W.H.O. recommendation, “It’s about time.”

His medical school is in Houston, and he has urged that women in areas where the virus is circulating or may circulate this summer avoid pregnancy if they can. “What happens when Zika hits Texas and the Gulf Coast this summer?” he asked.

Dr. William Schaffner, head of preventive medicine at Vanderbilt University School of Medicine, called the W.H.O.’s advice “excellent.”

“Now we just have to provide both the education and the means so that couples can implement,” he added.

Brazil has noted the W.H.O.’s new guidance in a statement for its citizens, a spokesman for the Health Ministry said. But the ministry itself has now taken a softer stance, saying, “Pregnancy is a personal decision that should be evaluated and considered by a woman together with her family.”

Some Brazilian virologists have pushed for emphatic advice to delay. The W.H.O. guidelines “understand the gravity of the situation,” said Dr. Artur Timerman, president of the Brazilian Society of Dengue and Arbovirus.

His society recommends that women living in areas of active transmission postpone getting pregnant and that men who return from such areas use condoms for six months. Brazilian health officials did not provide enough leadership on the issue, he said.

Dr. Celso Granato, a virologist at the Federal University of São Paulo, called the new W.H.O. guidelines “an important recommendation.”

“At this moment, what we know for sure is that the infection of the fetus may be catastrophic,” he said, “so I think that all the possible ways to avoid these situations have to be taken.”

In Atlanta, a C.D.C. spokeswoman said officials thought the W.H.O. guidelines were largely in line with the C.D.C.’s. On the issue of how long a man should wear a condom after visiting a Zika-affected country, they are now identical, which they were not before.

For people living in areas with Zika transmission, C.D.C. guidelines say doctors or nurses “should discuss the risks of Zika, emphasize ways to prevent Zika virus infection, and provide information about safe and effective contraceptive methods.”

When the epidemic began, health officials in Brazil, Ecuador, Colombia, Jamaica and El Salvador asked women in their countries to delay pregnancy if they could. In some cases, they suggested waiting a few months; officials in Jamaica suggested a year, and those in El Salvador suggested waiting until 2018.

The advice was sharply criticized by reproductive rights’ groups, which said it was hypocritical coming from governments that often outlawed abortion and made it difficult for women to get birth control. Some Roman Catholic archbishops also objected.

Some infectious disease experts, however, said delaying pregnancy is the only sure way to prevent birth defects. Mosquito control had not previously stopped mosquito-borne viruses such as dengue or chikungunya, they pointed out, and a Zika vaccine is years away.

If women were able to delay pregnancy for just one season in which Zika was widely transmitted, it is likely that so many people would gain immunity from having been bitten that the virus would either completely disappear — as happened in previous Pacific island outbreaks — or would circulate at only very low levels the following year.

From the outset, the W.H.O. did endorse making birth control more available. Until recently, however, it had stopped short of advising women to consider delaying pregnancy.

“Theoretically, many have thought it may work,” Dr. Bruce Aylward, the organization’s chief of emergency responses, said in February. But, he said, at that time experts thought the best approach was to fight mosquitoes while scientists worked on a vaccine.

The revised guidelines are a result of meetings of global experts in mid-March “and further input from experts in the editing process,” said Ms. Alexander of the W.H.O.

They were issued May 30, and the recommendation that some couples be “oriented” to consider delaying pregnancy was included in Section 4, Paragraph 1.c.

But the agency drew no attention to it, and the new section was confusingly worded. On Thursday, the W.H.O. issued a clarification saying: “Men and women of reproductive age living in affected areas should be informed and orientated to consider delaying pregnancy. This was the original intention of the guidance. The correction makes this more clear.”

Later Ms. Alexander said, “We should have done a better job of highlighting it.”