ObjectiveTo compare pre-defecation straining without rectal gel and post-defecation straining with the defecation phase, and to investigate their roles in evaluating pelvic organ prolapse (POP). Material & methodsMagnetic Resonance Defecography (MRD) images of 65 patients with a clinical diagnosis of POP were retrospectively reviewed by two independent readers. Measurements were taken at rest, pre-defecation straining without rectal gel, defecation, and post-defecation straining. The presence, sizes, and/or grades of cystocele, uterovaginal prolapse, widened levator hiatus, perineal descent, cul-de-sac hernia, rectocele, and rectal intussusception were evaluated and compared across the four phases. ResultsCompared to pre-defecation straining, both defecation and post-defecation straining detected more cases, larger sizes, and higher grades of prolapse in all compartments. When comparing defecation and post-defecation straining, the latter diagnosed four more cases of cystocele (80 % vs 73.9 %, p = 0.2) with larger size (−34.1 vs −29.0, p < 0.01) and higher grade (p = 0.19). Post-defecation straining also identified eight more cases of uterovaginal prolapse (89.2 % vs 73.9 %, p < 0.01) with larger size (−32.9 vs −26.4, p < 0.01) and higher grade (p < 0.01) compared to defecation. Conversely, defecation detected eight more cases of rectocele (46.2 % vs 33.9 %, p < 0.01) with larger size (9.2 vs 6.2 cm, p < 0.01) and higher grade (p = 0.26). ConclusionPost-defecation straining effectively depicts the maximal extent of prolapse in the anterior and middle compartments, and should be performed whenever there is a clinical need for a comprehensive assessment of prolapse in these compartments.