Babies conceived in May are 10% more likely to be premature, a new Princeton University study finds. And since premature babies have a higher risk of suffering from asthma, learning disabilities, and developmental problems, you may want to avoid getting pregnant during that particular month. Why May? The authors think it's because the third trimester arrives right around flu season; the flu is known to contribute to early delivery, Time reports. In general, the study found babies conceived in the first half of the year spent less time in the womb, but May was the worst month, the Los Angeles Times reports.
The study also found that infants conceived in the summer were heavier than other infants—just about a third of an ounce heavier, but that's significant for a baby. Previous studies have also found differences between babies born in different seasons, but those studies highlighted factors like the mother's socioeconomic status. This study compared siblings in order to neutralize factors like that, and found that no matter what your socioeconomic status, certain months are just more "unfavorable" in which to get pregnant. (Read more conception stories.)