A pregnant woman in Puerto Rico has become the first American whose fetus developed microcephaly because of a Zika infection acquired in the United States, the territory’s health department said on Friday.

Dr. Ana Ríus, the island’s health secretary, said the fetus, which was not carried to term, had developed a shrunken skull and calcified spots in the brain, suggesting inflammation and cell death.

The Centers for Disease Control and Prevention, which confirmed the presence of the virus in brain tissue from the fetus, released a statement saying the case “saddens and concerns us as it highlights the potential for additional cases and associated adverse pregnancy.”

The agency has estimated that 20 percent or more of the island’s 3.5 million residents will become infected with Zika this year.