Three babies with birth defects caused by Zika have been born in the US, the government reported on Thursday in its first accounting of pregnancy outcomes involving the virus.

Birth defects from Zika were also seen in three other pregnancies that ended.

The Centers for Disease Control and Prevention (CDC) has been tracking the pregnancies of women with Zika infections since the beginning of the year. So far, 234 pregnant women – residents and visitors – have been found to have Zika.

Some babies have been born with no immediate signs of problems, according to the CDC’s Dr Denise Jamieson, though she would not say how many. Most of the pregnancies are ongoing.

All the cases are connected to travel to areas with outbreaks of the mosquito-borne virus, primarily Latin America and the Caribbean. There has been no local spread of Zika in the US.

The CDC provided few details about the women, their pregnancies, the birth defects or their severity. Three cases ended in “pregnancy loss” but the CDC did not say whether it was from miscarriage, stillbirth or abortion.

Jamieson said the six cases of birth defects involved some women who had not experienced Zika symptoms. Most people infected never develop symptoms. Others get a fever, rash, joint pain or red eyes, and recover within a week.

During the Zika epidemic in Brazil, the virus was identified as a cause of fetal deaths and potentially devastating birth defects.

In its birth defects numbers, the CDC is counting a range of conditions. Chief among them is microcephaly, a severe birth defect in which a baby’s skull is much smaller than expected because the brain has not developed properly.

Also in the count are calcium deposits in the brain; excess fluid in and around the brain; abnormal eye development; and other problems resulting from damage to the brain that can include clubfoot or inflexible joints.

Dr Jamieson said the numbers were concerning but consistent with what has been seen in other countries affected by Zika outbreaks. Researchers estimate that for every 100 pregnancies involving women infected early in their pregnancy, 1% to 15% will develop severe birth defects.

The CDC report appears to include the two known cases of babies born in the US with Zika-caused birth defects. One was a baby girl born to a Honduran woman at a New Jersey hospital. The other was born in Hawaii to a woman who had lived in Brazil. Jamieson would not confirm the two were included, but said the cases meet the criteria.

The virus is spread mainly through the bite of a tropical mosquito called Aedes aegypti. It can be found in the southern US, but there is no evidence that they have been spreading the virus in the US.

Overall, 756 cases of Zika have been reported in the 50 states and Washington DC. All were in people who had traveled to outbreak areas, or had sex with someone who did.

In Puerto Rico and other US territories, 189 pregnant women are among those infected with Zika. The CDC did not report on birth defects in such areas.