What is the least diameter of the pelvic canal?

The Least Diameter of the Pelvic Canal

The least diameter of the pelvic canal is the interspinous diameter, which measures approximately 10-11 cm in the normal female pelvis. This diameter represents the narrowest part of the pelvic canal and is a critical measurement in obstetric assessment.

Pelvic Canal Dimensions and Clinical Significance

The pelvic canal has several key diameters that are important in obstetrics:

• The interspinous diameter (distance between the ischial spines) represents the narrowest part of the pelvic canal, measuring approximately 10-11 cm in females 1
• The obstetric conjugate (anteroposterior diameter of the pelvic inlet) typically measures around 12.4 cm in the supine position 1
• The intertuberous diameter (transverse diameter of the pelvic outlet) measures approximately 12.4 cm in the supine position 1

Factors Affecting Pelvic Canal Dimensions

Several factors can influence the dimensions of the pelvic canal:

• Birthing position: Upright positions (hand-to-knee or squatting) can significantly expand certain pelvic dimensions compared to the supine position 1

  • The interspinous diameter increases from 11.0 cm in supine to 11.6-11.7 cm in upright positions 1
  • The intertuberous diameter widens from 12.4 cm in supine to 12.7 cm in squatting position 1
  • The sagittal outlet expands from 11.5 cm in supine to 11.7-11.8 cm in upright positions 1

• Body proportionality and geographical variations: Different populations show variations in pelvic canal geometry 2

  • High-latitude populations tend to have larger and broader pelvic canals 2
  • Low-latitude populations often display relatively expanded inlet anteroposterior dimensions 2

Clinical Applications and Assessment Methods

The measurement of pelvic dimensions has important clinical applications:

• Prediction of successful vaginal delivery: The interspinous diameter, along with the pubic arch angle and obstetric conjugate, are critical factors in assessing the likelihood of successful vaginal birth 3

• Imaging techniques for assessment:

  • MRI pelvimetry can accurately measure all pelvic dimensions and assess the impact of different birthing positions 1
  • Three-dimensional CT reconstructions can provide precise measurements of the pelvic canal 3
  • Transperineal ultrasound can be used to measure the pubic arch angle, which correlates with other pelvic dimensions 3

Correlation Between Pelvic Measurements

Research has demonstrated significant correlations between different pelvic measurements:

• The pubic arch angle shows a positive correlation with both the interspinous diameter and the obstetric conjugate diameter 3
• This correlation allows for estimation of the interspinous diameter (the narrowest part of the canal) based on measurements that are more accessible through ultrasound 3

Clinical Implications

Understanding the least diameter of the pelvic canal has important implications:

• In obstetrics, the interspinous diameter being the narrowest part of the birth canal means it can be a potential site of obstruction during labor 3, 1
• Changing maternal position during labor can significantly affect pelvic dimensions, potentially facilitating delivery when the fetus must pass through this narrowest part 1
• Assessment of this diameter can help predict potential complications during vaginal delivery 3