Nine pregnant women who traveled from the U.S. to countries with Zika outbreaks contracted the virus and that one of them had a baby born with severe microcephaly, an abnormally small head that often is related to a host of developmental delays, the Centers for Disease Control and Prevention said Friday.
Of the other eight women, two had miscarriages, two aborted their fetuses after MRIs and ultrasounds showed evidence of brain malformation, two had healthy babies, and two other women are still pregnant with apparently healthy babies.
"We did not expect to see these brain abnormalities in this small case series of U.S. pregnant travelers," said Dr. Denise Jamieson, who's helping lead the CDC's Zika response. Her comment came during a telebriefing on Zika held Friday by the CDC.
The CDC is following 10 other pregnant women.
Dr. Tom Frieden, head of the CDC, said that while a direct connection between Zika and microcephaly is not yet proven, "the evidence is getting stronger and stronger" that there is one. If so, this would be the first time a mosquito-borne virus would be responsible for causing birth defects.
Frieden emphasized that proving a Zika-microcephaly connection is complicated, as there may be other factors involved, as well.
FILE - A blood samples from pregnant women are analyzed for the presence of the Zika virus, at Guatemalan Social Security maternity hospital in Guatemala City, Feb. 2, 2016.
Doctors believe microcephaly occurs in the first trimester of pregnancy when babies' brains develop. It is not known if Zika causes miscarriages, but Frieden noted that up to 20 percent of pregnancies end in miscarriage under normal circumstances.
While mosquitoes are the primary vector for Zika infection, the CDC has confirmed six cases of sexual transmission of the virus from men to their female partners. It also is investigating 14 reports of the Zika virus that may have been transmitted through sex, including to pregnant women.
Frieden said the agency did not anticipate so many cases of sexual transmission. Doctors don't know how long Zika remains in the semen, and Frieden said the report underscores the importance of proper condom use.
Frieden added that the CDC has developed a test for Zika and is sending it to public health laboratories in the U.S. and its territories, including Puerto Rico, where the virus is spreading.
One of the unknowns is whether an apparently healthy baby, born to a woman who had the virus during pregnancy, will remain healthy or if the virus will cause problems in years to come.
Doctors also addressed the issue of Guillain-Barré syndrome, a form of paralysis linked to the virus that can be deadly. Guillain-Barré is also associated with other viruses such as West Nile. Frieden said it is "almost certainly related" to Zika.
The CDC is working with the Health Ministries of Colombia and Brazil to learn more about the complications of Zika. Data on Guillain-Barré has been collected in Brazil, and research on the link with microcephaly is ongoing in both countries. The studies are expected to be completed by April.