Zika Appears to Pose Less of a Threat to Fetus if Contracted Late in a Pregnancy In one sample

ABNORMALITIES WERE NOT SEEN IN CHILDREN OF WOMEN WHO BECAME INFECTED IN THIRD TRIMESTER

BETSY MCKAY in the Wall Street Journal

Updated June 15, 2016 5:31 p.m. ET

Pregnant women who are infected with Zika in their third trimesters may not face a major risk that their babies will develop the type of abnormalities linked to the virus, according to a study published Wednesday.

None of the babies born to 616 women in Colombia who were diagnosed with Zika during their third trimesters was born with microcephaly or brain abnormalities, according to the study, published by public-health scientists in Colombia and at the U.S. Centers for Disease Control and Prevention in the New England Journal of Medicine. The majority—82%—of the babies were born at term at normal birth weight.

Microcephaly is a condition in which babies have heads that are abnormally small, a potential sign of brain damage. The CDC has said Zika can cause microcephaly and other brain abnormalities.

The findings were part of a study looking at all 65,726 cases of Zika in men and women reported in Colombia between August 2015 and early April, with the peak number of new cases reported in the first week of February. That total included 11,944 pregnant women. The findings were derived from a subgroup of 1,850 pregnant women whose time of Zika symptoms was known. Not all had Zika infection that was confirmed by a laboratory, however.

The findings offer some tentative proof that Zika may not affect women and their unborn babies at all stages of their pregnancies, but the study’s authors advised caution.

“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,” said Margaret Honein, a study author and a co-leader of a task force on pregnancy and birth defects task force that is part of the CDC’s Zika response. The babies will need to be monitored for potential eye or hearing problems or developmental delays that signal unseen prenatal damage, she said.

The study shed light on other risks in pregnancy. Four infants with microcephaly were born to women who had been infected with Zika but had no symptoms? Those findings support evidence that Zika can produce birth defects whether or not the mother herself becomes visibly sick from the virus.

The study’s findings add to a growing body of research on the risk that Zika poses in pregnancy, though researchers aren’t ready to draw firm conclusions to guide women of childbearing age. A preliminary report from Rio de Janeiro, published in March in the New England Journal of Medicine, found a range of abnormalities in 29% of Zika-infected women, including women who had been infected in their third trimesters. A retrospective examination of French Polynesia’s 2013-2014 outbreak, by contrast, found that women infected during their first trimester face a 1% risk that their babies will develop microcephaly, though it didn’t examine the risk of brain damage.

”Perhaps the answer is between those two somewhere,” Dr. Honein said of the risk to pregnant women.

Karin Nielsen, an author of the earlier paper from Rio, said that she believes the risk is somewhere between 20% and 30%. “Based on what we’re observing, I don’t think it’s negligible at all,” said Dr. Nielsen, who is a professor of clinical pediatrics in the division of infectious diseases at the David Geffen School of Medicine at the University of California, Los Angeles.

The cohort of women in the Rio study had all tested positive for Zika, while the women in the Colombia study had Zika symptoms but not all were confirmed by lab tests. But Dr. Nielsen said the findings on the possible effects of Zika in the third trimester were similar in both studies. Of the 616 women whose Zika infection was diagnosed in the third trimester, 2% had infants who were born with a low birth weight, 8% were preterm and 1% died, according to the study.

Third-trimester problems in the Rio study included stillbirths and fetal distress, Dr. Nielsen said.