Magnetic resonance pelvimetry for trial of labour after a previous caesarean section.

OBJECTIVES: To stratify the magnetic resonance (MR) pelvimetric diameters according to mode of delivery and establish possible reference values for pelvic diameters and outlet index for trial of labor after a previous caesarean section.
METHODS: This is a retrospective study of 125 patients at Sultan Qaboos University Hospital who underwent MR pelvimetry prior to a trial of vaginal delivery after a previous caesarean section between May 2001 and October 2003. Sagittal inlet, Transverse inlet, interspinous diameter, sagittal outlet and transverse outlet diameters were measured in all patients. The mean diameters were stratified according to delivery modality (vaginal delivery or caesarean section). The outlet index (sum of interspinous, sagittal outlet and intertuberous diameters), biparietal diameter of the foetus and head circumference were compared in women who delivered by caesarean section or vaginally.
RESULTS: All the diameters except sagittal inlet, were significantly larger (P < 0.05) in women who delivered normally as compared with those who had a caesarean section for any indication. The mean outlet index in the spontaneous delivery group with vertex presentation 31.89 ± 2.05, was significantly larger than that of the elective and emergency caesarean section, which were 29.69 ± 1.85 & 30.62 ± 1.80 respectively. The mean head circumference was also found to be significantly larger in the caesarean section deliveries.
CONCLUSION: An outlet index of 31.89 ± 2.05 and the pelvic diameters, transverse inlet 12.56 ± 0.80cm, sagittal outlet 10.54 ± 1.00 cm, interspinous diameter10.46 ± 0.89cm, and intertuberous diameter (transverse outlet) 10.89 ± 1.02cm are useful cut-off points for vaginal delivery in our population.