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 (eight grams) heavier, which is a tiny amount, but when you only weigh a few pounds, every fraction of an ounce counts.

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 such as the socioeconomic status of the mother, since wealthy, educated and non-teenage 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 [socio-economic 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.