On Friday evening, at the start of a long holiday weekend, the U.S. Centers for Disease Control and Prevention issued a travel warning for pregnant women and those trying to get pregnant: stay clear of places where Zika virus is present.

The warning about the mosquito-borne illness encompasses 14 countries and areas, which include: Brazil, Colombia, El Salvador, French Guiana, Guatemala, Haiti, Honduras, Martinique, Mexico, Panama, Paraguay, Suriname, Venezuela and the Commonwealth of Puerto Rico. Moreover, the number of areas with active transmission of Zika will increase, says Lyle Petersen, director of CDC’s Division of Vector Borne Infectious Diseases.

“This is a fairly serious problem,” says Petersen. CDC issued the statement today instead of waiting until after the weekend, he says, because “the virus is spreading fairly rapidly throughout the Americas” and there is mounting evidence connecting Zika and microcephaly.  The agency was originally slated to issue its announcement Friday afternoon but delayed it until Friday evening.

The agency is issuing this recommendation after CDC scientists tested samples provided by Brazilian health authorities from two infants with diagnosed microcephaly who died shortly after birth and from two pregnancies that ended in miscarriage. All four mothers reported having symptoms consistent with Zika virus disease during their pregnancy. CDC’s analysis found that Zika virus was present in the brains of both full-term infants. Outside of this Zika link, microcephaly is typically caused by genetic factors, exposure to environmental toxins or diseases during pregnancy including rubella or herpes.

CDC is now recommending that women in any trimester of pregnancy “should consider postponing travel to the areas where Zika virus transmission in ongoing.” So far, most evidence suggests the risk to pregnant women is greatest during the first trimester. For women who are trying to get pregnant CDC recommends consulting with a healthcare provider before traveling to these areas. People traveling to any of the 14 areas covered under the warning should make sure windows in their lodgings have screens and strictly follow steps to prevent mosquito bites during the trip, including wearing long sleeves, long pants and applying insect repellents containing substances like DEET. “Insect repellents containing DEET, picaridin, and IR3535 are safe for pregnant and nursing women and children older than 2 months when used according to the product label,” the agency said, in a statement.

The mosquito-borne disease caused by the Zika virus was considered minor and rare prior to a large outbreak in Micronesia in 2007. The disease itself is relatively mild – leading to a week of symptoms including rash, fever and joint pain – but its recent links with microcephaly prompted CDC to take this step. “We cannot quantify the risk,” Petersen said.

Last year, Brazil reported about 20 times its average level of microcephaly. Between October 2015 and January 2016 there have been more than 3,500 cases of the incurable condition, CDC says, citing Brazilian health authorities. Babies with the condition are born with abnormally tiny heads, and often, debilitating brain damage. “It is critically important to confirm or dispel a causal link between Zika infection of pregnant women and the occurrence of microcephaly by doing intensive investigative research, including careful case–control and other epidemiologic studies as well as attempts to duplicate this phenomenon in animal models,” Anthony Fauci, director of the National Institute for Allergy and Infectious Disease, and his colleague David Morens wrote in the New England Journal of Medicine earlier this week.

Petersen says it is too early to speculate if the Zika-microcephaly travel warning will still be in effect by the time of Olympics in Rio de Janeiro this summer.

So far, Zika has not yet been locally transmitted in the United States, although 26 U.S. travelers have been infected with the virus elsewhere, Petersen says. The species of mosquitoes biologically capable of transmitting the virus, however, are present in the United States.

Earlier this week Harris County, Texas, confirmed that a middle-aged female who traveled to El Salvador in November had subsequently developed symptoms of Zika and that the disease had been confirmed by laboratory testing. “She is now fully recovered,” says Umair Shah, the executive director of the Harris County Public Health and Environmental Studies. When it comes to the possibility of Zika showing up in their local mosquitoes, however, “for us it’s not a matter of if - it’s a matter of when,” he says.

In the greater Houston area, doctors have been informed to be on the lookout for Zika symptoms and take careful travel histories of people who have traveled to Zika-affected areas, according to Shah. The county’s mosquito control office is also testing local mosquitoes for any signs of dengue – a test that it is using as a proxy for Zika screening because there is no readily available field test for Zika, says Mustapha Debboun, Director of the Mosquito Control Division for the county.