It's true: Babies conceived at specific times of the year appear healthier than those conceived at other times. And scientists may now know exactly why the correlation happens.
In the 1930s, researchers took note that children born in winter were at a higher risk for a slew of health problems later in life including slower growth, mental illness and early death. While many sought explanations - diseases, harsh temperatures and higher pollution levels that come with the winter - the phenomenon was more complicated.
When looking at demographics, economists discovered mothers who are nonwhite, unmarried or don't have a college education are more prone to having children with health and developmental problems and are also more likely to conceive during the first half of the year.
Janet Currie and Hannes Schwandt, economists of Princeton University, used data from the vital statistics offices in New Jersey, New York and Pennsylvania about births between 1994 and 2006. Controlling for socioeconomic status, they only looked at siblings born to the same mother.
Their findings appear in the new report in the Proceedings of the National Academy of Sciences. May is the most unfavorable time to get pregnant, according to the study. Babies conceived in May were 13 percent more likely to be born premature, and their gestation time was about a week below the average.
Low birth weight and prematurity can lead to health problems like weaker immune systems, poorer vision and hearing and slower cognitive development. For conceptions between January and May, gestation length declined by about a week before reaching the average length again in June.
When looking at the Centers for Disease Control and Prevention data, the researchers found that the drop in gestation length aligns with the time when most patients visited the doctor for flulike symptoms. The researchers think that the flu may cause mothers to deliver too early.
"I think it really gives support to the idea that pregnant women should be vaccinated for the flu," Currie said.
