Pelvic Diameters

Conjugate Diameters

  • External conjugate: It’s the space between the upper margin of pubic symphysis to the tip of the spine of S1 vertebra.
  • True conjugate: It’s the space between the midpoint of sacral promontory to the upper margin of the pubic symphysis. It corresponds to the anteroposterior diameter of the pelvic inlet.
  • Angled conjugate: It’s the space from midpoint of the sacral promontory to the lower margin of the pubic symphysis. It’s the most useful measurement medically. Normally it measures about 5 inches (12.5 cm). It can be measured about by per vaginal (P/V) evaluation, of course without distress to the patient.
  • Obstetrical conjugate: It’s the shortest distance between the pelvic surface of the pubic symphysis and sacral promontory.

Diameters At The Pelvic Inlet

  • Anteroposterior: It goes from the midpoint of the sacral promontory to the midpoint of the upper margin of pubic symphysis.
  • Oblique: It goes from the sacroiliac joint of 1 side to the iliopectineal eminence of the other side.
  • Transverse: It’s the maximum transverse diameter of the pelvic inlet (i.e., best width of the pelvic inlet).

Diameters At The Mid-Pelvic Cavity

  • Anteroposterior: It goes from the middle of the pubic symphysis to the middle of 3rd sacral vertebra.
  • Oblique: It goes from the lower end of the sacroiliac joint of a single side to the middle of the obturator membrane of the different side.
  • Transverse: It’s the best width of the pelvic cavity.

Diameters At The Pelvic Outlet

  • Anteroposterior: It stretches from the tip of the sacrum to the lower margin of the pubic symphysis.
  • Oblique: It stretches from the middle of the sacrotuberous ligament of a single side to the junction of ischiopubic ramus of the opposite side.
  • Transverse: It stretches between the inner aspects of both ischial tuberosities.

The amounts of measurement at pelvic inlet, mid-pelviccavity, and pelvic outlet of true pelvis can be easilyremembered by the pupils in the create (in cm).

Obstetrical Pelvic Measurements (In Cm)

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During pelvic measurement the obstetricians dismiss the coccyx because of its freedom and they take the tip (apex) of the sacrum as the posterior boundary of the pelvic outlet.

Clinical Significance

  • Adaptation of the fetal head in the pelvis during parturition: During the process of parturition, the baby’s head accommodates itself to the measurements of the pelvic cavity in order to go through it easily. Consequently, during fixation of the head of fetus, the occiput of the head faces toward right/left, i.e., anteroposterior diameter of the head is located transversely at the inlet (13 cm in diameter). Afterward the head rotates about 90 ° so the occiput of the head generally faces anteriorly, i.e., anteroposterior diameter of the head is located anteroposteriorly at the pelvic outlet (13 cm in diameter).
  • Evaluation of adequacy of the pelvis during obstetrical evaluation: It’s done as follows:
  • Transverse diameter of the pelvic outlet is evaluated by measuring the space between the ischial tuberosities along a plane passing on the other side of the anus.
  • Anteroposterior diameter of the pelvic outlet is quantified from the pubis to the sacroiliac joint.
  • Angled conjugate (most essential) is evaluated by per vaginal evaluation.
  • Size of the subpubic arch. In normal gynecoid pelvis, examiner’s knuckles (with fist clenched) should be comfortably accommodated between the ischial tuberosities below the pubic symphysis.

 

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