Correlation between internal and external bony pelvis dimensions in nulliparous women

Abstract Purpose: The study correlates internal and external bony pelvis dimensions of the birth canal to estimate the size and proportions of the birth canal. Material and Methods: MRI (Magnetic Resonance Imaging) is used to identify the bony pelvis landmarks in 67 nulliparous women. The internal and external dimensions in the anterior-posterior and the transverse directions are calculated based on the landmarks and the correlation between them is set by means of the Pearson correlation coefficient ρ. The pelvic type estimation using the external bony pelvis dimensions is calculated. Results: In the sagittal plane in the plane of inlet, the study shows a very strong correlation between APD (anterior-posterior diameter) and COE (conjugata obstetrica externa) (ρ = 0.8%) and between SCIPP (sacro-coccygeal inferior pubic point line) and DSB (distance between PSA and SJ), where PSA refers to the upper edge of the pubic symphysis and SJ refers to the sacro- coccygeal joint (ρ = 0.93%). In the transverse plane, there is a very strong correlation between TD (transverse diameter) and the average of DTI (distance between left and right tuber ischiadicum) and DBC (distantia bicristalis) (ρ = 0.8%). IPA (internal pubic angle) does not correlate strongly with any of the proposed dimensions. Finally, the correct estimation of the pelvic type is shown to be 63% using only the external dimension. Conclusion: The study proves very strong correlations between the chosen internal and external pelvis dimensions in all major directions. However, the pelvic type estimation based on external dimensions is unclear.