Isn't the interspinous diameter, not the posterior sagittal diameter, the most significant measurement in obstetric assessments?

Interspinous Diameter is the Most Significant Obstetric Measurement, Not Posterior Sagittal Diameter

Yes, the interspinous diameter is more significant than the posterior sagittal diameter in obstetric assessments for predicting successful vaginal delivery. The interspinous diameter has been demonstrated to have higher correlation with successful vaginal delivery outcomes and is one of the key pelvic measurements used to assess pelvimetry in obstetric evaluations.

Key Pelvic Measurements in Obstetric Assessment

• The interspinous diameter (ISD) is a critical measurement of the mid-pelvis, representing the distance between the ischial spines, and is one of the most reliable predictors of successful vaginal delivery 1, 2
• The obstetric conjugate (OC), which measures the anteroposterior diameter of the pelvic inlet, is another important measurement but has less predictive value than the interspinous diameter for vaginal delivery outcomes 2
• The posterior sagittal diameter, while useful in some contexts, is not as significant as the interspinous diameter in predicting successful vaginal delivery 2

Evidence Supporting Interspinous Diameter's Significance

• In a comprehensive MR pelvimetry study of 781 women, the interspinous distance was significantly larger (112.3 mm ± 7.9) in women who had successful spontaneous vaginal deliveries compared to those requiring cesarean section or vacuum extraction (p<0.001) 2
• A CT biometric study found significant positive correlation between the pubic arch angle and the interspinous diameter (Pearson's correlation = 0.373, p<0.001), highlighting the interspinous diameter's importance in the birth canal configuration 1
• The interspinous diameter has demonstrated high reliability in measurement studies with intraobserver and interobserver reliability values of 0.92-0.95, making it one of the most consistently measurable pelvic parameters 2

Impact of Maternal Position on Pelvic Dimensions

• The interspinous diameter significantly increases in upright birthing positions (hand-to-knee and squatting) compared to the supine position (11.6 ± 1.1 cm and 11.7 ± 1.0 cm vs 11.0 ± 0.7 cm; p<0.0001), demonstrating its dynamic nature during labor 3
• This positional expansion of the interspinous diameter suggests its critical role in facilitating labor and delivery 3
• Dynamic external pelvimetry testing confirms that maternal shifting positions can create more room in the pelvis by modifying key obstetric diameters, including the interspinous diameter 4

Clinical Implications

• When evaluating pelvic adequacy for vaginal delivery, the interspinous diameter should be given particular attention as a key predictor of successful delivery 2
• In cases of suspected skeletal dysplasia or pelvic abnormalities, comprehensive evaluation including measurement of the interspinous diameter is recommended 5
• Finding of femur length below the 5th percentile during 18-22 week ultrasound warrants further evaluation, which should include assessment of pelvic dimensions including the interspinous diameter 5

Measurement Techniques

• MR pelvimetry provides reliable measurements of the interspinous diameter with high intra- and interobserver reliability 2
• CT pelvimetry can also accurately measure the interspinous diameter, though it involves radiation exposure 1, 6
• Ultrasonography remains the main imaging modality for initial assessment of suspected pelvic or skeletal abnormalities that might affect delivery 5

The evidence clearly supports that the interspinous diameter, not the posterior sagittal diameter, is the most significant measurement in obstetric assessments for predicting successful vaginal delivery outcomes.