Babies conceived in the spring may be at increased risk of being born prematurely, a new study suggests.
The results show that the rate of premature birth is about 10 percent higher among babies conceived in May, compared with those conceived in other months.
This finding may be related to the seasonal pattern of the flu virus, the researchers said. Babies conceived in May are due in February, which is typically the height of flu season.
"Because influenza is known to cause premature labor, these infants are at higher risk of short gestation," the researchers write in the July 8 issue of the journal Proceedings of the National Academy of Sciences.
In fact, there was a strong link in the study between the prevalence of flu and the risk of preterm birth. During the H1N1 flu pandemic in 2009, when the flu season started early, the highest risk for preterm birth shifted to babies conceived in February and March. [See 7 Ways Pregnant Women Affect Babies].
The findings suggest that flu shots might be effective at preventing seasonal increases in preterm birth, the researchers said. In the United States, doctors already recommend flu shots for pregnant women, who are at increased risk for complications from the flu.
The study also found a link between a baby's month of conception and his or her weight at birth. Babies conceived during the summer months tended to weight about 8 to 9 grams (0.28 to 0.32 ounces) more than babies conceived at other times of the year. This increase in birth weight may be tied to the mother's weight gain in pregnancy.
"Women gain almost one pound more [in pregnancy] when they conceive in June, July or August than when they conceive in January," the researchers said.
This finding suggests that seasonal patterns in food consumption may affect birth outcomes, even in developed countries, the researchers said.
Previous studies have linked a child's month of conception with a slew of outcomes, including IQ and risk for neurological disorders. However, in many of these studies, it was possible that differences between mothers, such as level of income, may have be the real reason for the link, rather than month of conception.
But in the new study, the researchers accounted for this possibility by comparing siblings born to the same mother. The study examined information from 1.4 million children born to 647,050 mothers in New York, New Jersey and Pennsylvania.
However, the researchers said they still could not investigate all potential reasons why month of birth may be related to preterm birth and birth weight; seasonal allergies and changes in temperature may also play a role, the researchers said.
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Rachael is a Live Science contributor, and was a former channel editor and senior writer for Live Science between 2010 and 2022. She has a master's degree in journalism from New York University's Science, Health and Environmental Reporting Program. She also holds a B.S. in molecular biology and an M.S. in biology from the University of California, San Diego. Her work has appeared in Scienceline, The Washington Post and Scientific American.