Getting pregnant in May raises the risk that your baby will be born prematurely. Conceiving in the summertime, on the other hand, boosts the odds of having a heftier infant.

The new findings add to growing evidence that the timing of pregnancy can influence the health of the baby. Other studies have linked birth month to eventual intelligence, mental health, longevity and even wealth.

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A closer look at previous studies, however, has uncovered a potential flaw: Socioeconomic status seems to have a strong influence on the months that mom deliver their babies, and minority women who are poor, young, less educated and single tend to give birth in the months that have been linked with the worst consequences for babies.

For the new study, researchers from Princeton University got around that problem by looking at siblings born to the same mothers. They collected data on 1.4 million births to nearly 650,000 moms in New York City, New Jersey and Pennsylvania. Then, they compared the outcomes of children conceived to identical moms but at different times of year.

When babies are conceived in May, the researchers report today in Proceedings of the National Academy of Sciences, they are 10 percent more likely to be born prematurely compared to babies conceived in other months. Premature birth is usually linked to lower birth weights.

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The researchers couldn’t definitely say why May was a riskier month to get pregnant, but they also found a direct link between infections with the influenza virus during pregnancy and early births. And because the flu is most common in January and February, when May conceptions become full term, the influenza virus could explain why pregnancies that start in mid-Spring may be more likely to end early.

Waiting until June, July or August to get pregnant, on the other hand, seems to provide a boost to birth weight by about a third of an ounce. Women who conceive in those months gain nearly a full pound more than women who conceive in January do, the researchers report. That larger pregnancy weight-gain is likely to contribute to bigger babies.

“By 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 co-authors write.

“Our results may have some implications for public policy, because they suggest that seasonal variations in nutrition matter for birth outcomes, even in rich countries, and that flu shots might be effective in fighting the seasonal deterioration in length of gestation.”