A US study of Zika-infected pregnancies has found that six per cent of them ended in birth defects.
The rate was nearly twice as high for women infected early in pregnancy.
The study is the first published research on outcomes in the US with the authors finding the results echo what's been reported in Brazil and other countries with Zika outbreaks.
It did not include the US territory of Puerto Rico, which has been hit hard by Zika, a virus which is primarily spread through mosquito bites
The rate of birth defects was the same for women who didn't show any signs of infection during their pregnancy as those that did.
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Margaret Honein, one of the study's authors, said that finding was important as most people who get infected don't have any symptoms.
All of the women in the study included were infected while in a Zika outbreak country or had sex with someone who got Zika overseas.
Out of the 442 pregnancies, 26 had birth defects linked to Zika and among women who were infected in the first three months, the figure rose to 11 per cent.
Twenty-one babies were born with birth defects and five of the cases were stillbirths, miscarriages or abortions.
Defects included microcephaly, in which a baby's skull is small because the brain hasn't developed properly.
Scientists are still trying to determine the full range of birth defects that Zika may cause, including whether there are some that may not be diagnosed for months or years.
