The role of previous birthweight on risk for macrosomia in a subsequent birth.

Macrosomia (birthweight > or = 4,500 gm) is associated with increased perinatal morbidity and mortality. Although past studies have evaluated risk factors for macrosomia, little is known about the effect of a prior macrosomic birth on the risk for a macrosomic infant in a subsequent birth. To assess the risk for delivery of a macrosomic infant subsequent to a previous macrosomic infant, we performed a population-based cohort study utilizing the Washington State linked infant birth file for 1984-1990. We identified 1,793 infants with birthweight > or = 4,500 gm who were linked to a subsequent birth and 3,596 randomly selected infants with birthweight of < 4,500 gm also linked to a subsequent livebirth. We then compared the risks for subsequent macrosomic births between the two groups. Infants with birthweights > or = 4,500 gm were 7.0 times more likely (95% confidence interval = 5.4-9.1) to have a subsequent macrosomic sibling than were infants with smaller birthweights, after controlling for pregnancy smoking status, parity, and gestational age. Race, maternal age or marital status, and diabetes mellitus did not materially affect this relation. The overall prevalence of macrosomic infants subsequent to a previous macrosomic birth was 22%, a proportion that did not vary notably with parity, or when paternity changed between successive births. Mothers with one macrosomic infant are at markedly increased risk for repeat macrosomic births.