Birth defects occur in 1 in 10 pregnancies with first trimester Zika infection

Analysis of pregnant women helps firm up numbers, timing of virus’s effect on fetuses

DANGER ZONE  Zika infection in the first trimester is especially risky, according to new results from an analysis of Zika-affected pregnancies in the United States.


For pregnant women infected with Zika virus in the first trimester, the future is foreboding.  

Nearly 11 percent of U.S. women likely exposed to Zika in the early weeks of or just before pregnancy had babies or fetuses with birth defects, researchers report online December 13 in JAMA. The new study offers the first results from the U.S. Zika Pregnancy Registry, which the Centers for Disease Control and Prevention and state and local health departments use to track Zika-affected pregnancies.

As of November 30, the registry had reported more than 3,800 pregnant women in the United States and its territories with evidence of Zika infection. Out of 442 pregnancies tracked so far, 26 — or 6 percent — resulted in infants or fetuses with birth defects, including microcephaly and other brain abnormalities. Of the 85 pregnancies where women were exposed only in the first trimester, 9 — 11 percent — resulted in birth defects. Researchers reported no birth defects in infants or fetuses of women exposed to Zika exclusively in the second or third trimester.

But the data might overlook babies with less obvious defects, pediatrician William Muller and obstetriciangynecologist Emily Miller, both of Northwestern University Feinberg School of Medicine in Chicago, point out in an accompanying editorial. And, they add, “longer-term neurologic outcomes are not yet available.”

Still, until now, the magnitude of Zika’s risk to pregnant women had been less certain. Earlier studies had estimated the risk of a baby developing microcephaly at 1 percent, or between 1 and 13 percent, for mothers infected in the first trimester.

The new study offers concrete numbers, and also answers an open question: Women with or without symptoms of Zika seem to be equally prone to bearing babies with birth defects.

Meghan Rosen is a staff writer who reports on the life sciences for Science News. She earned a Ph.D. in biochemistry and molecular biology with an emphasis in biotechnology from the University of California, Davis, and later graduated from the science communication program at UC Santa Cruz.

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