Low Risk of Microcephaly if Zika Infection is During Third Trimester

Ruth J Hickman, MD

Results of a preliminary study just published in the New England Journal of Medicine suggest that infection with Zika virus may pose less of a threat during the third trimester. The authors studied fetal outcomes for women who contracted the virus during pregnancy in Columbia. One of the study’s co-authors is Margaret Honein, MD, MPH, a co-leader of a task force on pregnancy and birth defects that is part of the CDC response to the outbreak.

Zika and microcephaly – previous studies

The recent outbreak of Zika virus infection has created a global health crisis. Infection with the virus, which is largely spread by infected mosquitoes, has been shown to cause a range of birth defects both in clinical and animal models. These include major brain abnormalities such as microcephaly, eye defects, and abnormally small overall size. The virus can also damage the placenta, causing fetal death.
An earlier study published in March looked at outcomes from women infected with the virus during pregnancy in Rio de Janeiro, Brazil. This study in the New England Journal of Medicine included 88 women, 23.6% of which were infected during the third trimester. The researchers found problems with pregnancy in 29% of fetuses, some of which were infected during the third trimester. Third trimester effects included stillbirths and fetal distress. These problems surprised researchers, since the risk posed by many different types of viral infections decreases with fetal age.

Zika and microcephaly latest study

Dr Honein and colleagues provided outcomes from a group of 616 women infected with the Zika virus during their third trimester of pregnancy. In this group, no infants were born with any apparent neurological abnormalities, including microcephaly. But 2% of infants were born with low birth weight, 8% were preterm, and 1% died. The study also reported a high incidence of infection in pregnant women: 73%.
Because the outbreak is relatively recent, the researchers are not yet able to tally results from women infected in the first or second trimesters.
Dr Honein commented on the findings to the Wall Street Journal. “It is somewhat reassuring that we don’t see microcephaly and structural brain abnormalities in those third-trimester infants, but it doesn’t mean everything is good. The babies will need to be monitored for potential eye or hearing problems or developmental delays that signal unseen prenatal damage.” It may be that more subtle problems will be detected in some of the infants over time.
The results are preliminary, and they will not change the public health response any time soon. The CDC recommends that pregnant women in any trimester avoid travel to parts of the world affected by Zika outbreak.
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