In patients with breech presentation secondary cesarean section is often caused by a failure to progress in labor due to a disproportion between fetal breech and maternal pelvis. The aim of the present study was to select such patients for primary cesarean section by prenatal use of magnetic resonance imaging. In 39 patients with breech presentation at term, maternal obstetric conjugate, transversal inlet diameter, sagittal mid-pelvis diameter, and interspinal distance, as well as fetal transversal and sagittal breech diameters, were measured by magnetic resonance imaging 1-7 days before delivery. All obstetricians that were involved in this study were blind to the measured values. For statistical evaluation the Mann-Whitney U-test was used. In 13 of these patients a secondary cesarean section was performed due to failure to progress in labor and in 9 due to other indications (intrauterine asphyxia, etc.). Seventeen women were delivered vaginally. The proportion between maternal pelvic inlet and fetal breech diameters was significantly less favourable in patients with failure to progress in labor (n = 13) than that in patients who were delivered vaginally (n = 17) (0.97 +/- 0.06 vs. 0.86 +/- 0.05, P < 0.001). Furthermore, by comparing the maternal pelvic inlet diameters with the corresponding fetal breech diameters, three groups of patients could be selected: one group (A; n = 8), in which the patients could be delivered only by cesarean section, a second group (B; n = 10), in which delivery by both cesarean section and vaginally was possible, and a third group of patients (C; n = 12), in which vaginal delivery was exclusively successful.(ABSTRACT TRUNCATED AT 250 WORDS)