One big question about the Zika virus has been how big a risk the virus might pose in the United States.
Studies earlier this year suggested that birth defects and other problems were mainly limited to babies born in some parts of Brazil.
A study out Tuesday provides a sense of the effects on women who were exposed while pregnant in other countries and then came to the United States. About 6 percent of those pregnancies resulted in defects in the fetus or baby.
"It's not specific to one geographic location," says Margaret Honein of the Centers for Disease Control and Prevention, which published the study in the Journal of the American Medical Association. "This really helps make the point that Zika virus infection poses a major risk to pregnant women and their fetuses."
The study involved 442 women who completed pregnancies in the U.S., either by pregnancy loss or live birth. Twenty-six of the fetuses or babies, or 6 percent, had at least one birth defect that might be linked to Zika, the researchers report.
The women had Zika symptoms or were exposed while in 12 countries — Barbados, Belize, Brazil, Colombia, Dominican Republic, El Salvador, Guatemala, Haiti, Honduras, Mexico, the Republic of the Marshall Islands and Venezuela.
Among women who had symptoms or were exposed in the first trimester, 11 percent had fetuses or infants with birth defects. There were no reports of birth defects in fetuses of infants with exposure only in the second or third trimester.
The study used laboratory tests to tell if the mother, fetus or baby had at some point been infected with Zika or similar viruses. But the tests couldn't necessarily tell which virus, or when. Four percent of the babies or fetuses had microcephaly, a devastating abnormality that causes very small heads and brain damage.
That's far greater than the usual risk for microcephaly in the United States, which is about 0.007 percent. "This is a major increase," Honein says.
Other birth defects seen in the study included spinal cord damage, other brain and nervous system abnormalities, deafness, eye damage and clubfeet.
"Most of these babies had very serious brain abnormalities," Honein says. And she speculates that Zika could cause other problems later on. "We also think this may expand over time with further follow-up," she says. "There also may be some babies who were apparently healthy at birth but as we do more follow-up and more careful imaging of the brain may find additional infants who have adverse effects."
In fact, another paper released Tuesday by the journal Emerging Infectious Diseases found that the Zika virus can continue reproducing in the brain after a baby is born. That raises the possibility that the virus could cause brain damage later in life, or pose a risk to very young children.
A third paper released Tuesday in the New England Journal of Medicine found that 46 percent of 125 women infected in Brazil while pregnant had some kind of serious complication, including miscarriages and birth defects. Rates of birth defects have been much higher in some parts of Brazil than in other countries, and it's not clear why.
"Zika virus infection poses a serious risk to pregnant women and their fetuses," says Honein, who wrote an editorial accompanying the NEJM report. "It is critically important pregnant women not travel to areas with active transmission of Zika virus."
Other researchers cautioned about reaching any conclusions based on these studies. "We still really don't know for sure," says William Muller, a professor of pathology at the Northwestern Feinberg School of Medicine, who wrote an editorial accompanying the CDC study in JAMA. Limitations in both the JAMA and NEJM reports mean they could over- or underestimate the risk, he says.
About 80 percent of Zika infections don't cause symptoms, so it's difficult to know how many women have been exposed while pregnant and have normal babies. Women who have a child with birth defects are more likely to get tested for Zika, and more likely to be in this sort of study. And it's not certain that Zika caused the problems reported.