Seasonality of Premature Birth

Babies conceived in May have an elevated risk of being born early.

Jul 9, 2013
Kate Yandell

FLICKR, DANIEL LOBOConceiving a child in May raises the risk of giving birth prematurely by 10 percent, according to a study published yesterday (July 8) in Proceedings of the National Academy of Sciences. Surprisingly little is known about the causes of premature labor, but the authors argue that babies conceived in May are nearing the end of their terms at the peak of flu season, causing the observed spike in prematurity.

The researchers studied nearly 1.5 million pregnancies in some 647,000 US mothers. Past critics of studies on the effects of birth season on health have argued that women of low socioeconomic status are more likely to give birth at certain times of year, skewing the results. The new study avoided this potentially confounding factor by looking at women who had multiple pregnancies and comparing siblings’ conception seasons and whether or not they were premature.

Flu has previously been associated with preterm birth, and women who get flu vaccines during pregnancy have shown to be less likely to give birth prematurely. The researchers pointed out that a woman who conceived in May would be in her final months of pregnancy in January and February, when flu is at its peak. Flu could lead to inflammation, which could trigger preterm labor, the researchers said.

The study also indicated that babies conceived in the summer tend to have higher birth weights than babies conceived in other seasons. This pattern matches other data showing that women who conceive in the summer are likely to gain more weight during pregnancy than women who conceive during other seasons.

“[B]y focusing on births to the same mother, our work provides evidence that there are seasonal patterns in birth weight and gestation that are not entirely driven by the fact that women with different characteristics tend to give birth at different times,” the paper’s authors wrote.