How exposure to influenza affects birth and infant health outcomes

A University of Minnesota study is the first of its kind to use U.S.-wide data to examine the impact of influenza on a range of birth outcomes, finding that if a baby is exposed to the flu while in utero it can have negative effects on health outcomes. In the study, Assistant Professor Audrey Dorélien found pregnant women who are exposed to the flu virus are more susceptible to preterm births, lower birth weight infants and neonatal mortality.

Published in the journal Population and Development Review, the study examined the birth outcomes of women who conceived at the same time of year in the same county, but in years with stronger and weaker flu seasons. Specifically, data — which included birth weight, gestational age, and neonatal and infant mortality — was examined from counties in the contiguous U.S. with more than 250,000 residents from 1989-1991 and 1995-2004. 

The study found:

• exposure to a moderate (75th percentile) flu season compared to a weak (25th percentile) flu season during the first trimester led to a 45% increased risk of neonatal and infant mortality, with a majority of these deaths due to congenital abnormalities;
• exposure to the flu during the first trimester increased the risk of preterm births by about 10% and by 16% in the last trimester, which is in line with evidence showing that exposure during the third trimester can trigger premature labor;
• exposure to a strong flu season in the third trimester increased the risk of low birth weight by 15%, with the majority of this effect due to earlier births.

“For adults, the flu vaccine is important and necessary to both protect oneself and provide herd immunity to others,” said Dorélien, who is with the Humphrey School of Public Affairs and faculty affiliate of the Minnesota Population Center. “For pregnant women and women at risk of pregnancy, flu vaccination is also necessary to protect newborn children and unborn fetuses.”

Dorélien — who is an expert in demography, public health and international development — urges women at risk of pregnancy to speak with their physician about obtaining their flu vaccination prior to becoming pregnant, as even the earliest exposure to the virus while in utero can be harmful to the fetus.

This research was supported by the Robert Wood Johnson Foundation Health & Society Scholar Program, and by a grant to the Minnesota Population Center at the University of Minnesota from the Eunice Kennedy Shriver National Institute of Child Health and Human Development Population Research Infrastructure Program (P2C HD041023).