This study describes and compares the urethral motion profile (UMP) of primiparae and multiparae in the immediate postpartum period. The aim is to determine the impact of multiparity on urethral mobility.

This descriptive prospective study recruited 65 women (29 primiparae, 36 multiparae) at the maternity ward of a tertiary hospital between March 2021 and December 2021. A sample size of 114 was needed to compare the means of the two sample groups with a power level of 80% (b=0.20) and a significance level of 95% (a=0.05). Women were included if they had a vaginal delivery one to seven days prior to recruitment. Patients underwent a two dimensional translabial ultrasound (TLUS) and a standardized interview based on possible confounders such as age, BMI, gestational age, days postpartum at the moment of TLUS performance, birthweight, (history of) grade of perineal rupture and (history of) delivery mode.
46 of the archived ultrasound volumes were eligible for offline analysis. To evaluate UMP, the urethra was manually traced and divided in five segments with six equidistant points on both rest and valsalva volumes (figure). The mobility vector (MV) for each of the six points was calculated using the formula √{(Vy-Ry)² +(Vx-Rx)². The intra-rater reliability was calculated using intraclass correlation coefficient (ICC) on a test-retest series of five datasets, suggesting a moderate to good intra-rater reliability (ICC 0.86, 95% CI [0.60-0.98], p<.001).  
An independent T-test was performed for all normally distributed data. When the assumption of normality was not fulfilled, a non-parametric Mann-Whitney test was performed. Afterwards, all UMP vectors and the possible confounding factors were correlated to each other and a univariate regression analysis was executed. Data was processed using IBM SPSS version 28.0.

Both groups (primiparae and multiparae) were comparable in terms of age, BMI, gestational age and birthweight. The majority of participants (86.2%, n = 56) was Caucasian. 80.0% (n = 52) had given birth by non-instrumental vaginal delivery, 20.0% (n = 13) by vacuum extraction, none by forceps. 
A significant difference between the MV1 to 4 of primiparous and multiparous women was demonstrated, with T-values and p-values of respectively t(44)=3.88 (p <.001) for MV1, t(44)=3.82 (p <.001) for MV2, t(44)=2.65 (p =.012) for MV3 and t(44)=2.54 (p =.015) for MV4. MV 1 and 2 had the greatest effect sizes with Cohen’s d=0.54 (95% CI [-1.77, -.52]) and d=0.48 (95% CI [-1.73, -.48]) respectively, followed by MV3 (d=0.39, 95% CI [-1.36, -.16]) and MV4 (d=0.28, 95% CI [-1.33, -.14]). The assumption of normality was not fulfilled for MV5 and MV6. A significant difference between primiparae and multiparae was found for MV6 (U=150.00, exact sig. two-tailed =.012), but not for MV5 (U=199.50, exact sig. two-tailed =.237). MV1 and MV2 were almost perfectly correlated to each other and strongly correlated with MV3 and MV4. Of all tested confounders, only the overall highest grade of perineal rupture was found to have a significant negative effect on proximal urethral mobility. Univariate generalized linear regression analysis was performed for each of the mobility vectors to investigate the predictive value of parity and the statistically significant correlated factors. Multiparity predicted urethral mobility for MV1 (R2=.26, F(1,44)=15.1, p<.001), MV2 (R2=.24, F(1,44)=14.1, p<.001), MV3 (R2=.13, F(1,44)=6.76, p=.013), MV4 (R2=.13, F(1,44)=6.27, p=.016) and MV6 (R2=.12, F(1,44)=6.20, p=.017). When overall highest grade of perineal rupture is added to the regression model, the difference in MV1 is predicted for adjusted R2=.28, F(2,43)=9.79, p<.001 and in MV2 for adjusted R2=.29, F(2,43)=10.15, p<.001.

The urethral motion profile was found to be significantly greater in multiparae compared to primiparae, with the greatest effect seen in the proximal urethra. Since MV1 and MV2 were strongly correlated to each other and to MV3 and MV4, the added value for clinical practice of measuring all mobility vectors compared to bladder neck mobility can be questioned. Only MV6 was not strongly correlated to the other five mobility vectors. However, the clinical significance of this most distal segment is not yet investigated. Multiparity was found to predict up to 26% of the difference in urethral mobility, being most predictive for MV1 and MV2.

This was the first study, as far as the authors know, to assess the UMP of primiparae and multiparae on TLUS in the first week postpartum. The association between multiparity and bladder neck mobility, proven by previous research, can now be extended to the entire urethra (1, 2). However, more research is needed on the superiority of UMP compared to bladder neck mobility, as for clinical significance of MV6. The impact of distinct parity grades on urethral mobility must be further investigated.

Meyer S, De Grandi P, Schreyer A, Caccia G. The assessment of bladder neck position and mobility in continent nullipara, mulitpara, forceps-delivered and incontinent women using perineal ultrasound: A future office procedure? International Urogynecology Journal. 1996;7(3):138-46
Pomian A, Majkusiak W, Horosz E, Tomasik P, Zwierzchowska A, Barcz E. Urethral mobility. Demographic features. Neurourology and Urodynamics. 2017;36:S401-S2

Continence 2S2 (2022) 100410
DOI: 10.1016/j.cont.2022.100410