Janet Currie and Hannes Schwandt of the Center for Health and Wellbeing, Princeton University suggest in their paper that this strange finding could be because of the expectant mothers' increased exposure to the seasonal flu during January and February, exactly when a baby conceived in May would be nearing term. The study was not directly looking for cause-effect.
"We were surprised that the relationship between potential flu exposure and premature birth appears to be so evident in the data," said Currie. "There has been some recent work suggesting that flu can induce premature labor in women late in pregnancy, and our results appear to corroborate this."
The findings were reported online in the current issue of the journal Proceedings of the National Academy of Sciences.
The researchers analyzed data on siblings born to roughly 647,000 mothers in the northeastern region of the United States, in order to eliminate differences in mothers' habits and economic status.
Although the study is of American mothers and their babies, it has implications for babies in other countries too. The role of seasonal ailments on birth outcomes has been established by this study.
By further lining up birth and month-by-month conception records alongside post-1997 influenza data that had been collected by the U.S. Centers for Disease Control and Prevention, the team found a correlation between May conceptions and a notable increase in flu exposure during the third trimester of such pregnancies.
The other significant finding - that babies conceived in summer weigh more - appears to be linked better diet in the summer months, the authors suggest.
"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 said. "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 said. "But we think the message of our paper is that parents should take steps to guard against known problems," suggesting that the most practical thing pregnant women can do is simply eat well and get a seasonal flu shot. "That would probably be a more sensible approach then trying to time conception to avoid May."