Not This Month, Honey: Why You Shouldn’t Conceive a Baby in May

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It turns out that it does matter what time of year you’re born — at least when it comes to your health.

We’re not talking about in the Athena Starwoman horoscope way, but in actual measurable health data. And what’s important may not be the month you’re born in but more the month in which you were conceived (which, after all, does dictate when you’re born).

According to the study from Princeton University, there’s a 10% higher rate of prematurity among babies conceived in May. The authors, Janet Currie and Hannes Schwandt, conclude that this may be because the moms hit their third trimester around flu season, and flu is a known factor in early delivery. Prematurity can contribute to a higher risk of asthma, learning disabilities and other developmental problems later in life. The two researchers also found that babies conceived in summer months were almost a third of an ounce (8 g) heavier, which is a tiny amount, but when you only weigh a few pounds, every fraction of an ounce counts.

(MORE: What Does Your Birthday Have to Do With Immune Disorders?)

Currie and Schwandt, researchers at Center for Health and Wellbeing at Princeton University, who reported their findings in the Proceedings of the National Academy of Sciences, compared more than 1.4 million siblings born to 647,050 mothers in New York City, New Jersey and Pennsylvania. The seasonal difference in birth outcomes has been found in previous studies, but those analyses pointed to factors like the socioeconomic status of the mother since wealthy, educated and nonteenage mothers usually have access to better prenatal care. But because the scientists compared brothers and sisters with similar genetic and socioeconomic environments, these factors were somewhat neutralized. “A high[-socioeconomic-status] mother getting pregnant in an unfavorable month will, on average, experience similarly poor birth outcomes as the typical (lower socioeconomic status) mother conceiving in [an] unfavorable month,” the study authors write.

Why does the flu make mothers deliver early? It’s possible that inflammation, which increases in response to the flu and has been linked in other studies with early delivery (and in one recent study, with autism), may be behind the premature birth. But while the flu may explain the premature births for babies conceived in May, it’s less clear what causes the summer-conception weight gain. “Women gain almost 1 lb. more when they conceive in June, July, or August than when they conceive in January,” says the study, “suggesting that gains in birth weight are driven, in part, by higher maternal weight gain during pregnancy.” The authors surmise this has to do with seasonally available nutrition (more ice cream during the hot summer months, perhaps?). They also found that smoking or marital status didn’t seem to be contributing to their results.

(MORE: Why Pregnant Women Should Get Flu Shots)

More studies are documenting the potentially important role that birth month may play in health; one recent analysis found a link between birth month and immune disorders, but this study provides some added weight to the idea, since it studied kids from the same moms.

So is timing really everything? Perhaps. At the very least, it’s one of the many things worth considering if you’re thinking of starting a family. And no matter what month you conceive, it’s important to get that flu shot.