Although obesity is considered to be one of the risk factors for pelvic organ prolapse (POP), relationship between BMI and the condition has not yet been established. Cases of POP in underweight women are also not clinically uncommon. What happens in underweight women with  POP? We paid our attention to a problem of the posture.We consider that  the spine becomes out of alignment and the vector of abdominal pressure changes, increasing chances of POP to occur .  As a result, abdominal pressure on pelvic floor becomes bigger.Benting outward of the spinal  column caused posterior inclination of a pelvis.Then, we hypothesised that the inclimination of pelvis was bigger  in POP patients than stress urinary incontinence(SUI) patients and examined retrospectively.

Subjects of the study were the patients who had preoperative chain urethrocystography for treatment of stage2 POP or above and SUI in our hospital between December 2016 and March 2019. Sacral slope (SS) defined as the angle between the sacral plate and the horizontal plane was measured using standing lateral images taken in chain urethrocystography.The  smaller the SS, the more inclined the pelvis backwards.  We compared age, the number of vaginal delivery, BMI, presence or absence of spinal disease, and SS with the patients of POP and SUI. Statistical analysis was conducted using Mann–Whitney U test and Pearson's correlation coefficient.

The average age of 29 POP patients and 16 SUI patients were 71.3±9.8 years and 67.8 ±13.1 years, mean BMI were 24.4±4.7 and 24.4±5.0. No difference was found in BMI, age, the number of delivery,presence or absence of spinal disease  in 2 groups. With regards to physiological curvature, a normal value of SS is considered to be between 25° and 45°. In this study, the average of measured SS of POP and SUI were 29.8±8.0° and 31.0±9.3°. There was no significant difference in 2 groups. (p=0.31). There were no correlation with BMI and SS in 2 groups and all patients.

Although obesity is one of the risk factors of POP, there are clinical cases of severe POP in underweight women. Bad posture abnormally increases abdominal pressure, even with light weight. Abnormal abdominal pressure puts a strain on the pelvic floor and causes pelvic organ prolapse. Benting outward of the spinal  column caused posterior inclination of a pelvis.Therefore, it was considered that posterior pelvic tilt could be a risk factor of POP. Cine MRI and transperineal ultrasound are used in image diagnosis of POP, although all examinations are carried out in the recumbent position and not in the standing position. So the study attempted to evaluate pelvic inclination in the standing position with sacral slope measured using standing lateral images taken in chain urethrocystography. 
   We expected the result that the pelvis was tilted backward largely in the POP group than in the SUI group, but there was no difference between the two groups. So,  it could not be said that the backward tilt of the pelvis was the cause of the POP.In this study  we considered SUI as a control. But even in SUI patients,  the pelvic floor is loaded, though less than POP.  In the future study, it was considered that comparing the cases with normal women is necessary, since there may be  difference between  the SUI patients and normal women. 
 In addition, BMI and SS did not show any correlation in both groups. It was not possible to prove that pelvic retrogradation was the cause of SUI and  POP in light weighted patients.

The study investigated pelvic inclination of patients with POP and SUI. We could not prove that the posterior tilt of pelvis was the cause of POP and SUI in this study.