May conception increases risk of premature birth

Love might be in the air in May, but so is the flu nine months later. Scientists believe the latter might explain why babies conceived in May are more susceptible to premature birth and low birth weight.

According to a new study out of Princeton University published in the journal Proceedings of the National Academy of Sciences, flu season is a significant contributing factor to the increased premature births in mid-winter.

"We can explain year-to-year variation in prematurity using the year-to-year variation in how bad the flu season was," says lead author Janet Currie, an economics and public affairs professor at Princeton. "To go into labour early you have to be at the right stage of pregnancy. It’s those babies who are at risk of being premature right at the height of flu season."

The researchers studied and compared the conception-to-birth data from 647,050 American families with more than one child — gestation time, birth weights and prematurity rates — and found that the average prematurity rate is abut 10 per cent greater for babies conceived in May (and scheduled to be born in February).

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"We were surprised that the relationship between potential flu exposure and premature birth appears to be so evident in the data," says Currie. "There has been some recent work suggesting that flu can induce premature labour in women late in pregnancy, and our results appear to corroborate this."

"Our motivation in this study was to examine how season alone affects birth outcomes," Currie tells Healthline. "The most striking thing we found was that babies conceived in May were at a 10 percent higher risk of being born prematurely."

Birth weight followed a similar trend: babies conceived in May were, on average, 10 grams lighter than those conceived in January, and 20 grams lighter than those conceived in June.

"The birth weight results suggest that infants conceived during the summer have higher birth weight in part because mothers tend to gain more weight during pregnancy when they conceive in summer," Currie says. "It seems likely that this is because they have a better diet, though we cannot directly observe that in our data."

"We cannot rule out other factors that might also be important for pregnancy outcomes," she says. "But we think the message of our paper is that parents should take steps to guard against known problems."

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"The most important implication, because it's the most actionable, is that pregnant women should get flu shots," Currie says of the study's practical applications.

The Public Health Agency of Canada agrees with the recommendation:

"Expectant mothers, at any stage of pregnancy, should get a flu shot to protect themselves and their babies from the influenza virus. Pregnant women are at increased risk for complications from influenza. Research also demonstrates that vaccination of pregnant women protects their newborns from influenza and influenza-related hospitalization. Also, infants born during influenza season to vaccinated women are less likely to be premature, small for gestational age, and low birth weight."

Earlier this year, a large study out of Norway confirmed that the flu shot is safe during pregnancy and may even reduce the risk of infant death.

"That would probably be a more sensible approach than trying to time conception to avoid May," Currie adds.