Birthweight and perinatal mortality of second births conditional on weight of the first

A woman's successive offspring tend to have similar birthweights. We use data from the Medical Birth Registry of Norway to describe weight and perinatal mortality of second births given the weight of the mother's first birth. Mean weights among second births differ by as much as 1000 grams, depending on the weight of the first. Furthermore, the survival of the second baby at any given weight is strongly affected by its weight relative to the first baby's weight. A baby may be average size compared to the whole population, but small compared to its sibling; such a baby has the increased mortality that goes with being relatively small. For example, an infant of 3250 grams is relatively large if the mother's previous baby was 2250 grams, but relatively small if the previous birth weighed 4250 grams. In the first case, the mortality risk of the 3250-gram baby is 2.2 per thousand, while in the second case, risk for the same weight infant is 9.0, or four times higher. Implications of these observations for the more general analysis of birthweight and perinatal mortality are discussed.