The objective in this study was to determine whether there was a difference in the accuracy of sonographically determined birth weight estimate in fetuses presenting by the breech as compared to the vertex. Prospectively, 104 consecutive singleton fetuses presented by the breech presentation were identified who also had a sonographic estimate of birth weight within 72 h of delivery. The control group (n = 104) consisted of fetuses presenting by the vertex of women who had the following hospital number and who were of similar demographic characteristics; the birth weight estimate in the control group had to be within 100 g of that of the women with the breech fetus. The estimate of the birth weight was derived for both groups using seven published models. Prediction limits were calculated for an estimated vs. actual birth weight for parturients with breech presentation. Among the study group, the mean standardized absolute error was similar for the seven models (range 120 +/- 104 to 138 +/- 118 g/kg; p = 0.08). For six of the models, the mean standardized absolute error was significantly higher for fetuses in the breech presentation than for fetuses in vertex presentation (range 80 +/- 63 to 112 +/- 95 g/kg; p = 0.02 to < 0.0001). Prediction limit calculations indicate that 99% of the time the actual birth weight was between 1500 and 3999 g if the estimate of birth weight was 2400-2900 g. We conclude that, for fetuses in a breech presentation, the error with most models is significantly higher than for a singleton in the vertex position.