What is the anatomy of the pelvis relevant to labor and delivery?

Updated: Jan 24, 2019
  • Author: Sarah Hagood Milton, MD; Chief Editor: Christine Isaacs, MD  more...
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The pelvis can also be assessed either by clinical examination (clinical pelvimetry) or radiographically (CT or MRI). The pelvic planes include the following:

  • Pelvic inlet: The obstetrical conjugate is the distance between the sacral promontory and the inner pubic arch; it should measure 11.5 cm or more. The diagonal conjugate is the distance from the undersurface of the pubic arch to sacral promontory; it is 2 cm longer than the obstetrical conjugate. The transverse diameter of the pelvic inlet measures 13.5 cm.

  • Midpelvis: The midpelvis is the distance between the bony points of ischial spines, and it typically exceeds 12 cm.

  • Pelvic outlet: The pelvic outlet is the distance between the ischial tuberosities and the pubic arch. It usually exceeds 10 cm.

The shape of the mother's pelvis can also be assessed and classified into 4 broad categories based on the descriptions of Caldwell and Moloy: gynecoid, anthropoid, android, and platypelloid. [30] Although the gynecoid and anthropoid pelvic shapes are thought to be most favorable for vaginal delivery, many women can be classified into 1 or more pelvic types, and such distinctions can be arbitrary. [2]

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