We’ve known for decades that babies conceived at certain times of the year tend, on average, to be healthier than babies conceived at other times. But what the hell, right? Why should that be? By looking at the birth records for over 1.4 million children born in the 1990s and 2000s, two economists may have figured out how it happens.
Science deals with the big, messy soup of our world. Its eternal challenge lies in teasing out what’s truly connected from that which simply happens at the same time. Previous studies have shown the correlation between infants who are born in winter and a host of problems later in life, but no one knew why it was happening. Wintertime diseases? Higher winter pollution? It could’ve been almost anything. The questions were complicated by data showing that certain mothers, ones from a lower socioeconomic tier, are statistically more likely to have children with developmental and health problems. But they’re also more likely to give birth in the first half of the year. So what’s been causing what? To study the problem, scientists needed a way to control for things like a mother’s race, marital status and education level so they didn’t end up comparing apples to oranges.
Janet Currie and Hannes Schwandt, economists out of Princeton University, came up with a solution. They looked only at mothers who had given birth to more than one child—over 600,000 of them. That way, when the differences in outcomes were measured, it had more to do with when the baby was born than the particular social conditions of the mother.
What they noticed was kind of startling. For babies conceived in May, the study found more than a 10-freaking-percent increase in premature births. The average gestation length is only about half a day shorter, but it still matters. Being born prematurely is linked to all kinds of problems, including a weaker immune system, neurodevelopmental complications and impaired vision or hearing.
Clearly, this sucks and we need to find the culprit. The study’s authors think we can most likely blame the seasonal flu, which really gets roaring in January and February, when May-conceived babies are born. The 2009-2010 flu season was particularly nasty, infecting more people than usual, and corresponded to a more dramatic dip in gestation times.
Plenty more work needs to be done to see if the common winter flu is really the reason for the premature births and therefore the reason for the generally worse outcomes of babies conceived in May. Right now, it’s just an association—the outcomes could actually be caused by some other seasonal disease or by climate or temperature, which this study wasn’t able to control for. But by looking at large samples of already-existing data, Currie and Schwandt have given other researchers a strong lead for their inquiries. And knowledge inches forward once more.