The aim of the present study was to assess the extent to which delivery mode and antepartal and postpartal risk factors increase the risk of postpartum stress and urge urinary incontinence.

We conducted a prospective cohort study to investigate risk factors for pelvic floor injuries and pelvic floor dysfunction after pregnancy and delivery. All eligible nulliparous women registering for maternity health care in the region during early pregnancy between 1 October 2014 and 1 October 2017 were invited to participate by the midwife in charge. Participants were asked to answer the web-based questionnaires on four occasions: at entry into the study in the early pregnancy, at 36 weeks of gestation, at 8 weeks postpartum, and at 1 year postpartum. Primary outcome measures were stress and urge urinary incontinence, reported at 1 year postpartum, in women who were continent before pregnancy. Associations between potential risk factors and the various types of urinary incontinence were evaluated using unadjusted and adjusted generalized linear models

Of the 1049 women included in the study, 670 responded to the first question about urinary leakage both in early pregnancy and at 1 year postpartum, thus qualifying to be included in the present substudy. Stress urinary incontinence was the predominant subtype of de-novo urinary incontinence at 1 year postpartum, reported by 21% of participants, whereas urge urinary incontinence was reported by 8%. Both stress and urge urinary incontinence during pregnancy increased the risk of the respective subtype postpartum (RR=2.48 [95% CI: 1.86, 3.3] and RR=4.07 [95% CI: 2.1, 7.89], respectively), whereas vaginal delivery increased the risk of postpartum stress incontinence only (aRR=2.63 [95% CI: 1.39, 5.01]). When stratifying by incontinence status during pregnancy, vaginal delivery was a statistically significant risk factor for stress urinary incontinence both in women who were continent during pregnancy and in those who were not (RR=2.49 [95% CI: 1.05, 5.93] and RR=3 [95% CI: 1.04, 8.66], respectively). Familial pelvic floor dysfunction and/or connective tissue deficiency increased the risk of both subtypes (RR=1.47 [95% CI: 1.03, 2.08] and RR=1.94 [95% CI: 1.06, 3.52], respectively). Postpartal factors were analysed as potential risk factors for urinary incontinence 1 year postpartum, but none of them were statistically significant. The population attributable fraction of stress incontinence associated with vaginal delivery was 0.58 (95% CI: 0.23, 0.77). Vaginal delivery thus constituting the major risk factor at population level in this cohort.

We found stress urinary incontinence to be more prevalent compared to urge urinary incontinence at 1 year postpartu. The majority of previous studies on postpartum urinary incontinence have found similar relations between stress and urge incontinence, but due to the heterogeneity of study designs and outcome definitions, comparing prevalence between studies is difficult in most cases. 
Vaginal delivery was associated with increased risk of stress urinary incontinence (aRR=2.63 [95% CI: 1.39, 5.01]) at 1 year postpartum compared to caesarean section, whereas no significant association with urge urinary incontinence was found. Chan et al. (1) and van Brummen et al. (2) both found equivalent results to ours, with a significant association between vaginal delivery and stress but not urge urinary incontinence compared to caesarean section, though no measures of association were presented in either study.
We found that vaginal delivery accounted for 58% (95% CI: 23–77%) of all stress urinary incontinence as compared to if all women delivering vaginally had instead delivered by caesarean section, showing vaginal delivery to be by far the largest risk factor for de novo stress urinary incontinence on the population level. Solans-Domènech et al. found a corresponding population attributable fraction of 67.8% (95% CI: 50.2–85.4%), though this was regarding unspecified type of urinary incontinence at 7 weeks postpartum (3). In the search for preventive measures against stress urinary incontinence, caesarean section may thus appear a tempting solution. However, our stratification for continence status during pregnancy showed largely overlapping risk ratios for stress urinary incontinence after vaginal delivery compared with caesarean section, thus not supporting the idea that any group would have a greater benefit of caesarean section than another.

This prospective study of primiparous women showed essentially different risk factors for stress and urge urinary incontinence, supporting stress incontinence as being the subtype mostly associated with pregnancy and childbirth. At population level, vaginal delivery was the major risk factor for stress urinary incontinence, followed by reporting stress incontinence during pregnancy. However, we found no evidence that caesarean section offers a greater protective effect against stress urinary incontinence postpartum in women reporting stress incontinence during pregnancy.

Chan SS, Cheung RY, Yiu KW, Lee LL, Chung TK. Prevalence of urinary and fecal incontinence in Chinese women during and after their first pregnancy. Int Urogynecol J. 2013 Sep;24(9):1473-9. doi: 10.1007/s00192-012-2004-8. Epub 2012 Dec 11. PMID: 23229419.
van Brummen HJ, Bruinse HW, van de Pol G, Heintz AP, van der Vaart CH. The effect of vaginal and cesarean delivery on lower urinary tract symptoms: what makes the difference? Int Urogynecol J Pelvic Floor Dysfunct. 2007 Feb;18(2):133-9. doi: 10.1007/s00192-006-0119-5. Epub 2006 Apr 21. PMID: 16628375.
Solans-Domènech M, Sánchez E, Espuña-Pons M; Pelvic Floor Research Group (Grup de Recerca del Sòl Pelvià; GRESP). Urinary and anal incontinence during pregnancy and postpartum: incidence, severity, and risk factors. Obstet Gynecol. 2010 Mar;115(3):618-628. doi: 10.1097/AOG.0b013e3181d04dff. PMID: 20177295.