Getting lighter, but why?

Birthweights in the US are falling but no one knows why, according to a study of 36.8 million infants born between 1990 and 2005.

A 52-gram drop in the weight of full-term singletons – from an average of 3.441 to 3.389 kilograms – has left Emily Oken’s team at Harvard Medical School scratching their heads. It can’t be accounted for by an increase in caesarean sections or induced labours, which shorten gestation. What’s more, women in the US now smoke less and gain more weight during pregnancy, which should make babies heavier. Oken suggests that unmeasured factors, such as diet or exercise, could explain why babies are being born lighter.

“For your average baby, 50 grams probably makes no difference at all,” she stresses. But those born substantially lighter could be at increased risk of heart disease and diabetes later in life.

Journal reference: Obstetrics & Gynecology, DOI: 10.1097/aog.0b013e3181cbd5f5