Puerto Rican health officials said Friday they had confirmed the territory's first confirmed case of microcephaly caused by the Zika virus.
The mosquito-borne virus is spreading across the island, and health experts fear there will be more cases of severe birth defects caused by Zika.
The Centers for Disease Control and Prevention confirmed it had tested tissue from the fetus and found the Zika virus.
"CDC has confirmed the first case of Zika virus disease in a fetus in Puerto Rico. CDC conducted the laboratory test that confirmed the diagnosis and has shared the results with the Puerto Rico Department of Health," the agency said in a statement.
"This case of Zika virus disease in a pregnancy saddens and concerns us as it highlights the potential for additional cases and associated adverse pregnancy outcomes," it said.
Puerto Rican health officials did not say if the pregnancy ended in a miscarriage or abortion. Some women who find out they have fetuses with microcephaly have chosen to have abortions, but abortions are strictly regulated in Puerto Rico.
"I want to urge any pregnant women with even the slightest concern of infection to go see a doctor," Health Secretary Ana Rius said.
Zika's known to cause a range of birth defects, including devastating brain damage that results in a smaller-than-normal head, a condition known medically as microcephaly. It also causes paralyzing conditions such as Guillain-Barre syndrome and it killed one Puerto Rican man who died from bleeding.
Puerto Rico has confirmed Zika infection in 18 pregnant women, and says 27 patients infected with the virus have been sick enough to be hospitalized.
But in most patients, Zika causes very mild symptoms. The biggest danger is to the babies of pregnant women.
CDC says 48 pregnant American women have been confirmed to have Zika infections and said in March that five affected babies or fetuses had either miscarried or shown evidence of birth defects including microcephaly.
Rius said people were unnecessarily worried about the spread of Zika in Puerto Rico, which depends heavily on tourist dollars. Major League Baseball to scrap a series scheduled for the end of May because of Zika fears.
"This is creating … unnecessary chaos," she said.
More than 500 confirmed Zika cases have been brought to the U.S. by travelers from affected areas and U.S. health officials say they expect more - including localized outbreaks once mosquito season gets started.
Puerto Rico has 925 confirmed Zika cases.
President Barack Obama has asked Congress for $1.9 billion in emergency funding to fight the epidemic and get states prepared. Congress has so far refused, although the Senate votes on some compromise measures next week.