Variation of the uterine flexion angle before and after voiding in nulliparous women with anteverted uterus: FLEXIMIX study

Abstract Background Resistance during intrauterine device insertion (IUD) may be influenced by the uterine flexion angle. While bladder filling has been used to facilitate catheter insertion for hysteroscopy and embryo transfer, the only study assessing this strategy for IUD insertion found no benefit. However, the physiological effect of bladder filling on uterine alignment has not been rigorously evaluated. This effect may be particularly relevant in nulliparous women with anteverted uteri. The study aimed to evaluate the variation in uterine flexion angle between full and empty bladder conditions in nulliparous women, and to determine the prevalence of uterine anteversion in a European population. Methods Prospective single-center study included 250 nulliparous women aged 18 years or older. Participant underwent two transabdominal ultrasounds: with a full bladder / after voiding. Uterine version and flexion were assessed by measuring the angles formed by the uterine body, cervix, and vaginal axis. Results Among the 215 participants with uterine anteversion (87.0%), paired flexion angle measurements were available for 212 (84.8% of the total cohort). Among these 212 women, the mean change in flexion angle was 48.0° (SD 36.7°, p < 0.001; Cohen’s d = 1.31). In 60.8% of cases, bladder filling changed an acute flexion angle to an obtuse one. The point of uterine flexion was located approximately 20–21 mm from the external cervical os, just beyond the internal os, suggesting a mechanical effect of bladder distension on uterine alignment. Conclusions Bladder filling significantly modifies uterine flexion in nulliparous women with anteversion. This physiological effect provides a plausible anatomical mechanism that may facilitate IUD insertion. Further interventional studies are needed to determine its clinical applicability. Trial registration ClinicalTrials.gov, trial registration number: NCT06669845, registration date: 2024-10-31.