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, NBC News reported.
The Centers for Disease Control and Prevention confirmed it had tested tissue from the fetus and found the Zika virus.
"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," the CDC said in a statement.
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.
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.