Medical literature is rife with research linking birth months to the risk of various chronic conditions. While intriguing, many of these studies have been flawed methodologically. But a recent study in Proceedings of National Academy of Sciences may be the largest to demonstrate a persuasive connection between the season of conception and at least one important outcome :preterm birth. The researchers, at Princeton University, studied more than 1.4 million births in New York city, New Jersey and Pennsylvania. They found that babies conceived in May were 10 per cent more likely to be premature. But they suspected that this was because mothers were nearing full term in January and February, when seasonal flu causes spike. They also found that babies conceived during the summer weighed up to 9g more than other infants, which they chalked up to "seasonal patterns in pregnancy weight gain." As reported in IE on July 21, 2013 through NYT.