Analysis of McRoberts' maneuver by x-ray pelvimetry

Objective: To document radiographically the changes in pelvic dimensions created by McRoberts' maneuver. Methods: 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. Results: 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). Conclusion: 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. (Obstet Gynecol 2000; 95:43-7. (C) 2000 by The American College of Obstetricians and Gynecologists.).