New Study Finds Influenza during Pregnancy is Associated with Increased Risk of Pregnancy Loss and Reduced Birthweight

October 29, 2020 — A CDC study published today suggests that flu infection during pregnancy may result in an increased risk of pregnancy loss and a reduction in birthweight.  CDC recommends flu vaccination among pregnant women, which has been shown to prevent flu illness in the mother and baby after birth. This new study provides data underscoring the potential importance of flu vaccination in pregnant women. In the United States last season, about 60% of pregnant women were vaccinated, but that proportion is much lower in some other parts of the world.

While it’s well established that pregnant women are at higher risk of severe illness associated with flu illness, there has been less information about the impact of flu illness in pregnant women on pregnancy outcomes. The new CDC study, published in Lancet Infectious Diseases, found that flu illness during pregnancy was associated with a reduction in the average birthweight of full-term newborns and an increased risk of late pregnancy loss (defined as pregnancy loss after 13 weeks gestation). In addition, women with respiratory illness symptoms who had fever were found to have an increased risk of preterm birth.

Another finding of the study was that pregnant women had the highest incidence of flu infection during their first trimester, and the risk of having influenza during pregnancy increased with each month that women were pregnant during the influenza season. These findings suggest it is best for pregnant women to be vaccinated either before flu viruses begin circulating in places with distinct influenza seasons or as early in pregnancy as possible in places with year-round influenza circulation to maximize protection of mothers and their babies throughout pregnancy.

This study included 11,277 pregnant women from India, Peru and Thailand during the 2017 and 2018 flu seasons. Only 13% of study participants had been vaccinated against flu. The study provides data for policymakers, healthcare providers and pregnant women in middle-income countries about the importance of flu vaccination among pregnant women and the benefits that flu vaccination may provide by preventing flu-associated poor pregnancy outcomes. Getting a flu vaccine is the first and most important step in protecting against flu. CDC recommends pregnant women receive a flu shot (not the nasal spray flu vaccine) during any trimester of pregnancy. For more information on flu vaccine and pregnancy visit the CDC website.

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