Our objective was to determine among postterm gestations (gestational age > or = 41 weeks) the accuracy of a parous patient's (n = 70) estimate of her newborn's birth weight and to compare the accuracy of clinical prediction with the assessment by the parturient (n = 40). During early labor 70 postterm parous subjects were asked to predict the birth weight of their neonate on the basis of their prior obstetric experiences. For comparison, a concurrent clinical estimate of the birth weight by the provider was also obtained in 40 of these patients. The incidence of macrosomia (birth weight > or = 4000 gm) among postterm parous mothers was 25.7%. The mean standardized error of the maternal estimate of birth weight was 85 +/- 76 gm/kg and 71.4% of their estimates were within +/- 10% of actual birth weight. Maternal estimate of birth weight > or = 4000 gm had a 94% specificity and 77% positive predictive value for a macrosomic fetus. Among 40 postterm parous women the clinical estimate had a mean standardized error (75 +/- 71 gm/kg) comparable to that of the maternal prediction (92 +/- 81 gm/kg) of birth weight (p = not significant). The mean standardized error among estimates for 11 macrosomic newborn infants who underwent both methods of estimation of birth weight was not significantly different. In conclusion, postterm parous women are at risk for delivery of a macrosomic fetus. Among these parturients, an estimate of birth weight by parous women is comparable to the clinical prediction of newborn weight by their providers.