Background: Urethral hypermobility is one of the theories developed to explain stress urinary incontinence. Traumatic damage to urethral supports during vaginal childbirth may be an important contributor.Aims: To establish a methodology to study urethral mobility, to determine the urethral motion profile in a cohort of young nulliparous women, and to determine changes in urethral mobility after childbirth.Methods: Ultrasound volume datasets of 44 nulligravidae and the 4D antepartum and post‐partum datasets of 73 nulliparous women were assessed using post‐processing software. Volumes were acquired translabially, at rest and on Valsalva, after voiding while supine. We marked six equidistant points from bladder neck to external urethral meatus and determined the position of these points relative to the inferoposterior symphyseal margin. Mobility vector distances for these points were determined by the formula √{(Vy − Ry)²+ (Vx − Rx)²} (V = Valsalva; R = rest). Reproducibility was determined in a test–retest series of 21 patients. Changes in urethral mobility after delivery were determined by comparing antepartum and post‐partum mobility vectors.Results: The method was highly reproducible, with an intraclass correlation coefficient of 0.80 (confidence interval 0.73–0.86) for mobility vectors. The distal urethra was consistently found to be less mobile than the proximal part (P < 0.001). After childbirth, urethal mobility was significantly increased in five out of six segments. There was a trend towards more marked changes in mobility after vaginal operative delivery.Conclusions: Urethral mobility can be studied using 3D/4D translabial ultrasound. There is an increase in urethral mobility after childbirth, especially after instrumental delivery, suggesting an alteration in urethral support. This increase in mobility seems to affect the entire urethra.

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