Evolutionary theorist David Haig has proposed that human mothers withhold some energy resources during pregnancy for her own survival and future reproduction. As a result, babies are born slightly lighter than they otherwise might be. The withholding of resources for future reproductive effort is predicted to be more prominent during first pregnancies than later ones, because mothers have fewer potential future reproductive opportunities as they age and with each successive pregnancy. This sets up the expectation that first born children should be born smaller than each successive offspring. Maternal withholding of resources in pregnancy is also thought to increase reproductive conflict during pregnancy, in which maternal and paternally derived genes engage in a tug of war over nutrient delivery to the fetus.
Also: An association between birth order and the risk of diabetes