OBJECTIVE. To determine whether fetal weight estimated by ultrasound on the day the external cephalic version (ECV) is related to the procedure's success rate. DESIGN. Retrospective cohort study. Setting. University hospital. POPULATION OR SAMPLE. Two hundred women with a singleton pregnancy undergoing an ECV attempt between March 2002 and March 2006. METHOD. Projected birthweight at delivery was calculated. Logistic regression analysis, by grams and by weight intervals, and multiple correspondence analysis were performed to determine whether or not the weight was related to the procedure's success. Specific statistical analysis was undertaken to minimize measurement bias in fetal weight estimation. MAIN OUTCOME MEASURE. Success of ECV in relation to estimated fetal weight. RESULTS. Measurement bias was ruled out because no statistically significant differences were identified with the Student's t-test (p>0.05), Bland-Altman's agreement analysis, and the interclass correlation coefficient analysis (ICC = 0.829; CI95% 0.772-0.871; p<0.01). By logistic regression analysis, a variable relation between fetal weight estimated by ultrasound and ECV success was detected, while multiple correspondence analysis suggested that variations in fetal weight are weakly related to the procedure's success or failure. No relation was identified between ECV and birthweight (p = 0.38) when the delivery took place in the same week as the version. CONCLUSIONS. Estimated fetal weight before ECV seems not to be related to the outcome of ECV.