Antecedents of high birthweight (macrosomia) were studied using the state birth certificates of White singleton infants born in three large metropolitan counties of Washington State from 1984 to 1986. Cases consisted of 2082 live‐born macrosomic infants, defined by a birthweight of over 4.5 kg. A random sample of 4440 live births with birthweights of 2.5‐4.0kg was selected as a comparison group. Estimates for the independent risks associated with gestational and established diabetes, male sex, parity, duration of gestation, maternal smoking during pregnancy, maternal age, and median income of maternal residential area were obtained and combined in a single logistic model. Maternal smoking was associated with a decreased risk of macrosomia (OR 0.4, 95% CI 0.3‐0.5). Established diabetes (OR 6.4, 95% CI 2.7–15.4), gestational diabetes (OR 3.2, 95% CI 2.1–5.1) and male sex of the infant (OR 2.4, 95% CI 2.2‐2.7) were associated with an increased risk. Increasing parity was related to an increasing risk from para one (OR 1.4, 95% 1.2‐1.6) to para six and greater (OR 3.3, 95% CI 1.5–7.4). Increasing duration of gestation was associated with an increasing risk from 33–36 weeks (OR 0.8, 95% CI 0.5‐1.2) to 43–45 weeks (OR 3.3, 95% CI 2.5‐4.2). Maternal age, median income of maternal area of residence, and maternal marital status were not significantly associated with macrosomia.