Intensity modulated radiotherapy (IMRT) is often used in gynecologic cancer in the post-operative setting with defined volume guidelines. It delivers a highly conformal treatment, reducing normal tissue irradiation, but results in a sharp dose gradient at the treatment volume edge. Significant vaginal cuff motion can occur due to intra- and inter-fraction motion of the rectum and bladder, which may cause inadequate target coverage. There is no consensus on how to account for the effects of rectal filling on this motion. We sought to quantify inter-fraction rectal motion during the full course of IMRT treatment and to describe if rectal motion varied by week of treatment delivery or by level within the pelvis. We retrospectively reviewed thirty consecutive patients with endometrial or cervical cancer status post hysterectomy who received IMRT to the vaginal cuff and regional lymphatics. Two reviewers measured differences in rectal position between the initial planning CT and cone-beam CT (CBCT) scans obtained daily. Department policy recommended patients be treated with full bladder daily and those with significant rectal filling asked to empty their bowels. Differences were obtained for scans obtained the first day of each week of treatment and on the final treatment date for the five weeks. At each time point, reviewers quantified anterior rectal wall movement in the anterior-posterior axis at six pre-determined levels from S4-S5 superiorly to 4 cm below the inferior border of S5 inferiorly. The resulting measurements were reviewed, and any discrepancies > 2 mm were resolved by a tertiary review. The average displacement for all patients at all timepoints and pelvic landmarks was 0.36 cm posteriorly. The maximum anterior and posterior displacements of the sample population were 3.78 cm and 4.73 cm, respectively. The average displacement did not meaningfully vary by week, measuring 0.31 cm, 0.47 cm, 0.39 cm, 0.32 cm, and 0.33 cm, all posteriorly. However, greater displacement was noticed higher in the pelvis. From superior to inferior, average displacements were 0.47 cm, 0.43 cm, 0.48 cm, 0.35 cm, 0.32 cm, and 0.18 cm, all posteriorly. 95% of all displacements fell between 2.55 cm posteriorly and 1.82 cm anteriorly. No previous studies have examined rectal displacement in the anterior-posterior axis over the course of treatment and at different levels within the pelvis for cervical and endometrial cancers. Our findings indicate significant displacement exists and that pelvic IMRT planning may benefit from anterior and posterior margins above those recommended by current guidelines. At a minimum, the results support the need for daily CBCT imaging to assess rectal filling and to develop institution guidelines for managing large displacements. In addition, margin recommendations may differ based on the level within the pelvis.