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Health & Pregnancy

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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 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.

"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."

Two doctors with Scott & White Healthcare in Texas described the study as "interesting," while stressing the need for pregnant women to get a flu shot.

"Avoiding conception during certain times of the year is an option," said Dr. Nancy Bertsch, an obstetrician and gynecologist at Scott & White's College Station Clinic. "But if the authors' theory is correct and the increased rate of prematurity is related to seasonal spikes in flu cases, a better option is for us to do a better job preventing the flu."

Bertsch said the American College of Ob/Gyns and the CDC both recommend that all pregnant women get a flu shot during pregnancy.

"It's a perfectly safe vaccine for both mother and baby," she said. "We already know that a pregnant woman with the flu can be much sicker than a non-pregnant woman, and can end up with much more serious consequences for both her and her baby. If getting a flu vaccine during pregnancy also lowers the risk of prematurity, that's just one more reason to get it."

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