Three women have given birth to children with Zika-related birth defects in the U.S. and three others have lost or terminated their pregnancies because of links to the virus, according to statistics released last week by the Centers for Disease Control and Prevention.
Authorities did not specify in what states the babies were born, citing the involved families' privacy.
The numbers are the first concrete examples of the Zika virus' effect on pregnant women in the United States amid fears that the mosquito-borne illness is likely to spread across the country.
"Although these outcomes occurred in pregnancies with laboratory evidence of Zika virus infection, we do not know whether they were caused by Zika virus infection or other factors," health officials said Thursday in a news release.
The question is one of many that scientists hope to answer through the CDC's U.S. Zika Pregnancy Registry, including how likely it is for a mother to pass on Zika to her unborn child and whether the way a mother is infected – from a mosquito's bite or by sexual transmission – has any effect on a child's likelihood of getting the virus.
The registry is a data-gathering collaboration between the CDC and state, local, tribal and territorial health departments.
"We urgently need to understand the magnitude of the potential risk, and what factors might affect that risk such as when the infection occurs during pregnancy," said Dr. Margaret "Peggy" Honein, co-team lead of the CDC's 2016 Zika Virus Response Team.
Tests have found evidence of Zika virus in 235 pregnant women in the U.S. as of June 9, according to health officials. An additional 190 pregnant women appear to have the infection in U.S. territories.
The numbers were collected as part of the CDC's first report on the U.S. Zika Pregnancy Registry. CDC officials announced that they would release additional updates from the registry once a week for the foreseeable future.
"(The) CDC's top priority for the Zika response is to protect pregnant women and women of childbearing age because of the potential risks associated with Zika virus infection during pregnancy," the CDC said in a news release.
Zika can be transmitted from mother to child and cause microcephaly in newborns, a defect which causes a child to be born with an abnormally small head and which often leads to intellectual disabilities and speech problems. The virus has also been linked to eye defects, hearing loss and impaired growth.
The virus is predominantly spread by mosquitoes of the Aedes species, an insect which has been found throughout the southern half of the United States. It can also be spread through sexual contact.
The virus can cause fever, rash, joint pain and red eyes in adults, although the symptoms are rarely severe enough to require hospitalization.