Summer is the best season to conceive the healthiest babies
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 surprising impression may actually be true — and they think they may know why it happens.
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.
Supporting these findings, economists Janet Currie and HannesSchwandt of Princeton University took a new approach to resolving this long-standing question, using data from 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, representing 1,435,213 children. 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 shorter than average.
As for birth weight, summer was the best time to conceive.
The researchers then looked at data from the Centers for Disease Control and Prevention and found that the annual dip in gestation length closely aligns with the time when the most patients visited a doctor for flulike symptoms.
In 2009, when the H1N1 pandemic struck about two months earlier than a typical flu season, the dip in gestation time came earlier, too, and it was more dramatic. Currie and Schwandt suggest that flu might cause mothers to deliver early. The researchers support the idea that pregnant women should be vaccinated for flu.