NEW YORK (Reuters Health) - Research has long shown slight health differences among babies born at different times during the year, and a new study suggests seasonal flu may be a contributing factor.
In the study of over 600,000 women who each had more than one child, researchers found a dip in average pregnancy length - and thus an increase in premature births - for infants conceived in May.
Most of those babies were born at the height of flu season, in early- to mid-winter.
"When you plot the data over time, you see how there are really seasonal patterns," said Hannes Schwandt, who worked on the research at Princeton University in New Jersey.
Past studies also found trends in many aspects of babies' health by birth season. But it's been unclear, the researchers explained, if those differences could be attributed to their mothers.
Although no one is sure why, multiple studies have shown that poorer women are more likely to give birth during the first half of the year, Schwandt said.
By including only women who delivered more than one baby over the decades studied, his team was able to adjust for characteristics of the mother, like poverty, and look only at other factors, he said.
The researchers analyzed birth records for about 1.4 million babies born in New Jersey, New York and Pennsylvania in the 1990s and 2000s. During that time, average gestation length - the time from conception to birth - was just under 39 weeks.
Gestation length fell 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. Gestation length returned to normal for the babies conceived in June.
(Note: this study was originally published stating erroneously that babies conceived in May had a gestation period 0.8 weeks shorter than average, not 0.08 weeks. A correction will be published.)
The study findings include adjustments for a variety of maternal characteristics that might influence pregnancy, including the women's age, race, education level and whether they were married.
The difference in gestation length would translate to an extra one percent of all babies being born prematurely, the study team wrote in Proceedings of the National Academy of Sciences.
The trend in gestation length matched up closely with flu season, so that babies conceived in May were born around January and February, when the flu is typically at its peak.
That was especially true for data from the 2009-2010 flu season when a new strain of the H1N1 virus infected more people than usual - offering the "strongest evidence that it was not just a coincidence," Schwandt told Reuters Health.
He said flu infections late in pregnancy may lead to inflammation, a known risk for premature birth.
The study doesn't prove the flu causes dips in gestation length - it only shows an association. The researchers could not account for the effects of temperature and climate, for example.
"The patterns observed could be driven by other factors which vary by season, including infections other than influenza," said Lyndsey Darrow, who has studied birth seasonality at Emory University in Atlanta.
"Nonetheless, this study provides some evidence for influenza as a trigger of preterm labor," Darrow, who wasn't involved in the new research, told Reuters Health in an email.
Schwandt said the findings represent only "statistical averages" and that it may make more sense for pregnant women to get a flu vaccine rather than avoid conceiving in certain months.
"For the individual mother who conceived (in May), I don't think there's a big concern," he said.
SOURCE: bit.ly/1ay7kKW Proceedings of the National Academy of Sciences, online July 8, 2013.