In a study published today in Proceedings of the National Academy of Sciences, Janet Currie and Hannes Schwandt report that women who conceive at certain times of the year are predisposed to early labor, most likely because the end of their gestation period occurs during the height of the flu season.
Janet Currie is the Henry Putnam Professor of Economics and Public Affairs at Princeton University. “Our motivation in this study was to examine how season alone affects birth outcomes," Currie told Healthline. "The most striking thing we found was that babies conceived in May were at a 10 percent higher risk of being born prematurely.”
Using data from the Vital Statistics Natality program, they selected 647,050 women who had multiple births in New York, New Jersey, and Pennsylvania from 1997 to 2006—more than 1.4 million births in all.
By selecting mothers who had more than one child, the researchers were able to “deselect” socioeconomic status—a factor known to be associated with poor birth outcomes.
Comparing the season of conception to infant health at birth, the researchers found a sharp spike in early deliveries during the months of January and February—approximately nine months after conception in May.
“We also found a pattern of higher birth weight of 8 to 9 grams among babies conceived during the summer months, likely due to seasonal patterns in pregnancy weight gain,” Currie said.
To test the role of seasonal influenza on birth outcomes, they merged flu data collected by the Centers for Disease Control and Prevention from 1997 forward. “The incidence of prematurity was twice as great during the peak of the 2009 pandemic as during the peak of the seasonal flu in other years,” the researchers wrote.
So, should women avoid getting pregnant in May? “No, get a flu shot,” said Currie.
Why Is It Dangerous to Get the Flu While Pregnant?
According to the study authors, “Influenza infections are known to trigger adverse birth outcomes, perhaps by causing inflammation, which has itself been linked to the cascade of events that trigger labor.”
Women who get the flu while pregnant are at risk for serious complications, including bacterial pneumonia and dehydration. During pregnancy, changes in the immune system, lungs, and heart make flu infections more dangerous, both to the mother and to the unborn child. The worst-possible outcome of maternal influenza is fetal death.
According to the National Institutes of Health, one in eight babies are born prematurely in the U.S. each year. Full-term pregnancy is 40 weeks, and premature birth occurs before the 37th week.
Babies born early weigh less, and their organs may not be fully developed—causing severe complications. The lungs are the last organs to mature, and respiratory problems are the most frequent complication for premature babies.
Studies conducted since 2009 have shown that, during the flu season, pregnant women who get a flu shot are less likely to deliver prematurely than those who don’t.
A large Norwegian study reported earlier this year that babies had a greater risk of death from maternal flu infections, and that getting a flu shot reduced this risk. The researchers concluded: “We found no basis for withholding influenza vaccination from pregnant women in their second or third trimester—an important group, given that these women can be particularly vulnerable to the severe effects of influenza virus infection.”
According to Currie, most pregnant women didn’t get flu shots before the 2009 flu pandemic, possibly to avoid taking drugs that might affect their unborn child.
“Now, public health recommendations include flu shots during pregnancy, and our study supports that,” Currie said.
The World Health Organization has also issued new recommendations designating pregnant women as the “highest priority group” to receive flu vaccines.