To document radiographically the changes in pelvic dimensions created by McRoberts' maneuver.
Women at least 37 weeks' pregnant who presented to labor and delivery were eligible for study entry. Anterior-posterior and lateral x-rays were taken with women in the dorsal lithotomy position and after application of McRoberts' maneuver, in which the maternal legs were hyperflexed 45 degrees onto the maternal abdomen. A two-tailed paired t test was used to assess the changes in the pelvic diameters, with P < .05 considered statistically significant.
Thirty-six subjects were enrolled in the study and 34 x-rays were suitable for analysis. McRoberts' maneuver was associated with an increase in the mean angle of inclination between the symphysis pubis and the sacral promontory (51.53 ± 2.03 versus 38.07 ± 1.96 degrees, P < .001). There was a 24% decrease in the angle created by drawing a line bisecting the symphysis pubis relative to the horizontal (P < .001). With McRoberts' maneuver the angle created by a line bisecting the longitudinal axis of the fifth lumbar vertebra and the longitudinal axis of the upper sacrum also increased (133.75 ± 2.25 to 140.14 ± 2.12 degrees, P = .04).
Ours are the first systematic observations of pelvic changes associated with McRoberts' maneuver, confirming the traditional thinking that the maneuver causes a significant cephalad rotation of the symphysis pubis and subsequent flattening of the sacrum.