After a study of more than 1.4 million babies born in the 1990s and 2000s, researchers have found that babies conceived in May had a greater chance of being born prematurely, perhaps due to cases where the mother had the flu late in her pregnancy.
The study from Princeton University collected data from over 600,000 mothers who each had more than one child. Noting that most of the babies in their data were born at the height of flu season, in early- to mid-winter, postdoctoral researcher Hannes Schwandt said the team saw a 10-percent increase in premature births for babies conceived in May.
The team singled out influenza as a potential risk factor due to its effects on a pregnant mother.
While an increase in premature births during flu seasons doesn't draw a causal link, only a correlative one, Schwandt said the likelihood of their findings being coincidental is low, operating with the knowledge that "flu infections late in pregnancy may lead to inflammation," and that this is "a known risk for premature birth."
The team's study examined women who had more than one child so that fixed characteristics, such as poverty, could be removed from the equation, letting researchers focus on other factors. Schwandt cited poverty as a thorny variable because past studies have shown that poorer women are more likely to give birth in the first half of the year.
In their analysis, the team found the average gestation length — the time from conception to birth — fell just under 39 weeks. Gestation length dipped for babies conceived early in the year, however, and was shortest for babies conceived in May, at 0.08 weeks (about half a day) shorter than average, Reuters reports. Gestation length returned to normal for the babies conceived in June.
"By following the same mother over time, we eliminate differences in fixed maternal characteristics as an explanation for seasonal differences in health at birth," the team noted. "We find a sharp trough in gestation length among babies conceived in May, which corresponds to an increase in prematurity of more than 10%."
Other infections not including influenza may be the cause, notes Lyndsey Darrow, researcher of birth seasonality at Emory University in Atlanta; the link between flu season and preterm births are purely associative.
"The patterns observed could be driven by other factors which vary by season, including infections other than influenza," Darrow told Reuters. "Nonetheless, this study provides some evidence for influenza as a trigger of preterm labor."
Schwandt followed up by saying that getting a flu vaccine is probably an easier alternative for many mothers than worrying about which month to conceive their child.
Noting that the study looked only at statistical averages, "For the individual mother who conceived (in May)," he said, "I don't think there's a big concern."
Source: Currie J, Schwandt H. Within-mother analysis of seasonal patterns in health at birth. Proceedings of the National Academy of Sciences. 2013.