Pelvimetry

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What is pelvimetry? 

  • Measurement of the female pelvis that has theoretically been used to try and identify cephalo-pelvic disproportion.

  • Originally described by Dr. Caldwell and Dr. Moloy in “Anatomical Variations in the Female Pelvis: Their classification and Obstetrical Significance” in 1938.

  • Clinical evidence shows, however, that all pregnant women should be allowed a trial of labor regardless of pelvimetry results!

    • Cochrane review in 2017 that looked at deciding mode of delivery for cephalic fetuses at term:

      • X-ray pelvimetry vs. no pelvimetry or clinical pelvimetry was the only comparison used due to lack of trials identified that used other types of pelvimetry.

      • There was not enough evidence to support use of X-ray pelvimetry for deciding mode of delivery.

      • Women who underwent X-ray pelvimetry were more likely to have C-section, but there was no clear difference in perinatal outcomes in these groups.

    • Even the WHO in Feb 2018 stated that routine clinical pelvimetry may increase cesarean section without clear benefit for birth outcomes

  • Ok… so why are we even talking about pelvimetry? 

    • Historical purposes so that you know what people are talking about.

      • CREOGs sometimes test on the different measurements of the pelvis used! 

What are the traditional types of pelvises? 

  • Gynecoid - round to slightly oval inlet 

    • Traditionally the pelvis that is most likely in women; most “favorable” for SVD.

  • Android - triangular inlet, and prominent ischial spines, with more angulated pubic arch

    • Thought to lead to longer labor or cephalo-pelvic disproportion.

  • Anthropoid - the widest transverse diameter is less than the anteroposterior (obstetrical) diameter.

    • Traditionally thought to lead to more OP babies 

  • Platypelloid - Flat inlet with shortened obstetrical diameter. Wide or transverse oval appearance “kidney shaped.” 

    • Traditionally thought to be difficult for vaginal birth 

But remember! Clinical evidence shows that a trial of vaginal birth should be done for all women regardless of pelvimetry

What are the various measurements for pelvimetry?

Pelvic Inlet 

  • Transverse diameter of the pelvic inlet 

    • Measure the distance between the iliopectineal lines at the widest transverse distance (usually 13-14.5cm).

  • Obstetric conjugate 

    • Line between the closes bony points of the sacral promontory and the pubic bone next to the symphysis (normally 10-12 cm).

  • Interspinous distance 

    • The line between the closest bone points of the ischial spines (9.5-11.5cm) 

Pelvic outlet 

  • Sagittal pelvic outlet diameter aka obstetric AP diameter of the pelvic outlet 

    • Closest bony points of the sacrococcygeal joint and the pubic bone next to the symphysis (normally 9.5-11.5cm) 

  • Intertuberous diameter 

    • Closest bony points of the ischial tuberosities (normally 10-12 cm)