It almost seems like a mystical correlation. Babies conceived at certain times of the year appear healthier than those conceived during other times. Now, scientists have shown that the bizarre phenomenon is actually true—and they think they may know why it happens.

The work is "a really long-overdue analysis," says economist Douglas Almond of Columbia University, who was not involved in the study. "This is maybe not quite a smoking gun," he says, "but it's much stronger than the previous evidence."

As early as the 1930s, researchers noticed that children born in winter were more prone to health problems later in life: slower growth, mental illness, and even early death. Among the proposed explanations were diseases, harsh temperatures, and higher pollution levels associated with winter, when those expectant mothers and near-term fetuses might be most vulnerable. But recently, as economists looked at demographics, the picture got more complicated. Mothers who are nonwhite, unmarried, or lack a college education are more likely to have children with health and developmental problems. They are also more likely to conceive in the first half of the year. That made it hard to tease out the socioeconomic effects from the seasonal ones.

Economists Janet Currie and Hannes Schwandt of Princeton University took a new approach to resolving this long-standing question, using data from the vital statistics offices in New Jersey, New York, and Pennsylvania about births between 1994 and 2006. To control for socioeconomic status, their study looked only at siblings born to the same mother. And lo and behold, seasonal patterns persist, they report online today in the Proceedings of the National Academy of Sciences.

May is the most unfavorable time to get pregnant, the study finds. Babies conceived this month (and thus delivered in winter) were 13 percent more likely to be born premature, and their gestation time was almost a week below the average, Currie and Schwandt report. Because low birth weight and prematurity have been linked to diverse health problems—weaker immune systems, poorer vision and hearing, and slower cognitive development—this variation could help explain differences later in life. The study found that for conceptions between January and May, gestation length declined by about a week before shooting back up to average length in June.

In terms of birth weight, summer was the best time to conceive. The team found that mothers who conceived from June through August gained more weight during their pregnancies and gave birth to infants who were, on average, about 8 grams heavier than in other months.

The researchers then looked to Centers for Disease Control and Prevention data and found that the annual dip in gestation length closely aligns with the time when the most patients visited the doctor for flulike symptoms. In 2009, when the H1N1 pandemic struck about 2 months earlier than a typical flu season, the dip in gestation time came earlier, too, and was more dramatic. Currie and Schwandt suggest that flu could cause mothers to deliver early. "I think it really gives support to the idea that pregnant women should be vaccinated for flu," Currie says.

A few days' difference in gestation length is small but significant, notes Hyagriv Simhan, a maternal and fetal doctor at the University of Pittsburgh School of Medicine in Pennsylvania who was not involved in the work. Although babies born less than a month premature don't usually face significant health costs, he says that the difference is meaningful when averaged over a large population. Simhan, whose own research has suggested a link between influenza and early delivery, says that flu is a likely factor in newborn health, but not the only one. For example, he notes, vitamin D levels that dip at important points in fetal development could also be important.

*This story provided by ScienceNOW, the daily online news service of the journal *Science.