When in Pregnancy Is Zika Most Dangerous?

There’s early evidence that the mosquito-borne virus can harm and even kill fetuses in the third trimester.

A woman works in a factory of citronella candles in Sao Joao da Boa Vista, Brazil. (Paulo Whitaker / Reuters)

Pregnancy is a waiting game. Mothers-to-be are used to counting their way to a baby’s arrival—in weeks, in ultrasounds, in kicks, and in contractions. For women who experience complications, particularly, nine months can seem like an excruciating slog. The waiting game can be especially intense for women now that the Zika virus is spreading, because scientists still don’t know if there is a point in pregnancy when contracting the illness isn’t potentially serious.

Early evidence is alarming. Zika appears to cause “grave outcomes” for many fetuses, regardless of when in pregnancy the virus is contracted, according to a study published by the New England Journal of Medicine in March.

Though the study sample size in the study was relatively small—88 pregnant women—the findings were troubling. Many of the fetuses of women who tested positive for Zika developed serious and sometimes fatal complications including growth problems, too-small heads, brain disorders, disruptions to the central nervous system, and abnormal blood flow.

There were also two fetal deaths after the 30-week mark, which is well into the third trimester. In one of these cases, the woman had been infected with Zika at 25 weeks pregnant; in the other, the woman was infected at 32 weeks. “Our findings are worrisome,” the study’s authors wrote. “These were all healthy women with no other risk factors for adverse pregnancy outcomes.”

The findings also run counter to earlier theories that Zika—like rubella, another infectious disease that causes a spate of birth defects—might be most harmful to the fetuses of women in their first and second trimesters of pregnancy, but less of a threat as pregnancy progresses. Peggy Honein, who is a leading researcher into birth defects on the Centers for Disease Control’s Zika Response Team, says the study is an “important piece of evidence,” but that more research is needed.

“We really want to understand which time period in pregnancy poses the highest risk, and if there are periods of pregnancy that pose no risks,” Honein told me. “That would be really helpful. It is a critical question.”

In the meantime, she and other health officials say pregnant women and their partners should follow guidelines issued by global health leaders—including using insect repellent, avoiding travel to places where Zika is locally transmitted, and practicing safe sex to prevent transmission of the virus.