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hree infants in the United States have been born with birth defects linked to the Zika virus, the Centers for Disease Control and Prevention reported Thursday.

Three other pregnancies in the US have also been affected but they were not carried to term, the agency said. It listed them as “pregnancy losses” — meaning a miscarriage, stillbirth, or termination after the fetus was seen to have birth defects and tested positive for the virus.

The early signs of what the US may experience from Zika infections among pregnant women are worrying, admitted Dr. Denise Jamieson, co-lead for the CDC’s Zika pregnancy and birth defects task force.

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“I’m very concerned,” she told STAT. “What we’re seeing among US travel-associated cases and US travelers is the same pattern that we’re seeing in other places like Colombia and Brazil.”

These six cases involve women in the 50 US states and the District of Columbia. They do not include infected pregnant women in Puerto Rico — where one case of Zika-related microcephaly has been reported — or other US territories.

Some of these cases have already been reported publicly. In January, a woman in Hawaii who had been infected with Zika in Brazil early in her pregnancy gave birth to a baby with severe microcephaly.

And the CDC reported in February that two women had miscarried after having contracted Zika abroad. Testing of the fetuses confirmed Zika infection.

Also included in these figures is a woman from Latin America who gave birth to an infant with microcephaly in New Jersey. The woman, believed to be from Honduras, was visiting relatives when she went into labor.

Jamieson declined to comment on individual cases but said the figures include all babies born in the US with birth defects that are linked to Zika infection.

She said the CDC will update these numbers weekly and will eventually provide similar statistics for the US territories.

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None of the cases involved local infection in the United States from a mosquito.  It’s not clear if any of the women were infected through sexual transmission of the virus from a male partner who had become sick while abroad.

“They’re all travel-associated cases. So they’re all women who either traveled to a Zika-affected area or had sex with someone, a man, who traveled to a Zika-affected area,” said Jamieson, who declined to clarify further.

The US has reported 11 cases of sexual transmission of Zika.

The figures were posted on the CDC’s website on Thursday, the first time the agency has provided a tally of Zika-linked pregnancies and birth defects in the United States.

The CDC reported that, as of June 9, 234 pregnant women in the US have been diagnosed with Zika infection. Jamieson said most of those pregnancies are still ongoing. There have also been at least 189 women in US territories who have been diagnosed with the Zika virus.

Zika infection during pregnancy can cause significant birth defects in the developing fetus including microcephaly, a condition in which infants are born with abnormally small heads and sometimes underdeveloped brains.

Scientists are still trying to determine how many women infected with Zika during pregnancy are likely to give birth to babies with birth defects.

Jamieson said the CDC’s best estimate at the moment is that there is a 1 percent to 15 percent risk, if the infection occurs in the first trimester of pregnancy.

Some people have characterized that risk as relatively low, but that’s not the way Jamieson sees it.

“I think a 1 percent chance of having a baby with a major birth defect such as microcephaly is very concerning. I’m not reassured by that.”

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