US babies mysteriously shrinking

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. 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. Unmeasured factors, such as diet or exercise, could explain why babies are being born lighter. NewScientist