Relative birthweights of twins

In a series of 182 twin gestations second-born twins were more often heavier than first-born twins (55%), but overall they weighed less by an average of 21 g. This paradox was shown to be due to the skewed distribution of weight differences favoring the first-born twin, most apparent among pairs in which one or both weighed at least 3000 g. The larger the difference between birthweights, the greater the likelihood that the heavier twin would be delivered first.