Month of Conception and Odds of Premature Birth
Mom's exposure to flu during third trimester appears to be a key risk factor, researchers say
By Alan Mozes
MONDAY, July 8 (HealthDay News) -- What time of year a baby is conceived may raise the chances of a premature birth, new research suggests.
Investigators found that children conceived during the month of May faced a 10 percent increase in risk relative compared to babies conceived at other times of the year.
The study authors believe that may be a function of the expectant mother's increased exposure to the seasonal flu during January and February, exactly when a baby conceived in May would be nearing term.
"We were surprised that the relationship between potential flu exposure and premature birth appears to be so evident in the data," said study author Janet Currie, director of the Center for Health and Wellbeing at the Woodrow Wilson School at Princeton University. "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."
Currie, who conducted the study with researcher Hannes Schwandt, added that if pregnant women got flu shots, they might not be at risk of premature labor due to flu infection.
The findings were reported online in the current issue of the journal Proceedings of the National Academy of Sciences. The study uncovered an association between month of conception and premature birth, but it didn't prove cause-and-effect.
To explore the potential impact of conception timing on infant health, the researchers analyzed data on roughly 647,000 mothers in the northeastern region of the United States, all of whom had given birth to more than one child.
In addition to dates of birth and lengths of pregnancies, the data included information on maternal weight changes, race, education and smoking history.
The research team noted that by looking solely at the conception-to-birth experience of more than 1.4 million siblings (as opposed to non-related babies), they were able to compare apples to apples, and sidestep other complicating factors that might influence prematurity risk, such as a family's wealth or educational background.
The result: The authors identified a "sharp trough" in the length of pregnancies that began in May.
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.
What's more, babies conceived during the summer months tended to weigh slightly more at birth than babies conceived at other times of the year.
"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.