The subsequent need for surgery for stress urinary incontinence (SUI) and pelvic organ prolapse (POP) may be a more valid outcome to assess childbirth as a risk factor for pelvic floor dysfunction than reports based purely on pelvic floor symptoms. It is also interesting to examine whether changes during pregnancy and after delivery were transient or if they persisted over time.
The aim of this study, therefore, was to examine the relationship of parity, mode of delivery, and pregnancy to the absolute and relative risk of surgery for POP and SUI, using nulliparous women not affected by childbirth as reference.

We collected information from the Swedish National Quality Register of Gynecological Surgery (GynOp). All women who had SUI or POP surgery from 2010 to 2017 and were ≥45 years of age were eligible for the study (n=59,415). The total cohort was stratified according to parity and mode of delivery into three groups: nulliparous women, women with all deliveries by acute or elective caesarean sections (CS), and women with ≥1 vaginal delivery (VD). Women were also stratified by the number of births (0, 1, 2, 3, ≥4). Data were registered prospectively and consecutively, including a preoperative evaluation (with postal- or web-based questionnaires) and based on hospital records from admission, surgery, discharge, and a questionnaire completed 1-year postoperatively. The reference group consisted of all women ≥45 years of age (n=2,309,765) from the Total Population Register (TPR) of Statistics Sweden. Information in the Swedish Medical Birth Register (MBR) was used to determine the rate of women with caesarean and vaginal deliveries and their respective parity based on women born in 1960. 
Surgical codes for POP and SUI surgery were based on the International Classification of Diseases, 10th revision. Continuous variables were presented as mean and standard deviation. Categorical data were presented as number, percentage, and 95% confidence interval (CI). Fisher´s exact test and the Mann-Whitney U test were used for categorical and continuous variables for pairwise comparisons. Results are presented as the mean difference for continuous variables and as the difference in percentages for categorical variables, 95% CI and P-value. The absolute risk (AR, per thousand = ‰) of having surgery for POP or SUI was calculated by dividing the number of UI or POP surgeries in GynOp by the number at risk in the reference population presented with a 95% CI. An observed/expected ratio was calculated, and the method of Ulm1 was used to assess the CI of the relative risk (RR) when comparing proportions between groups in GynOp with the respective proportions in the general female population ≥45 years. Statistical significance was set at P<0.05. Statistical analyses were performed using SAS 9.4 (SAS Inc, Cary, NC, USA).

There was 20,488 SUI and 39,617 POP surgeries from 2010 to 2017. Among women with SUI surgery, 93.1% had ≥1 vaginal delivery, 2.6% had ≥1 CS, and 4.3% were 0-para. In women with POP surgery 97.8% had ≥1 vaginal delivery, 0.4% ≥1 CS, and 1.9% were 0-para. Compared with the proportion in the general female population aged ≥45 years, the VD-group was overrepresented in GynOp by the RR 1.22 (95% CI, 1.21-1.24) in women with SUI surgery, and by RR 1.28 (95% CI, 1.27-1.29) with POP surgery, both P<0.001. The reverse applied to the 0-para and CS groups that were equally underrepresented by RR 0.31 (95% CI, 0.29-0.33) and RR 0.26 (95% CI, 0.24-0.28), both P<0.001, respectively for SUI surgery, and by RR 0.14 (95% CI, 0.13-0.15) and by RR 0.004 (95% CI, 0.031-0.043), both P<0.001 respectively for POP surgery. 
The AR of POP surgery was lowest in CS delivered women [0.09‰ (95% CI, 0.08-0.11)]. In women with ≥1 vaginal births, POP surgery was >23 times higher [AR 2.11‰ (95% CI 2.09-2.13)]. There was a consistent cumulative increase of AR for POP and SUI surgery with parity in women with vaginal births. This trend was not observed in women with all their deliveries by CS and was in addition on par with the AR of surgery in 0-para women (Figure 1). The first VD carried the highest increase of AR for POP surgery (x6) and SUI surgery (x3). The second vaginal birth added the lowest AR for POP surgery ( about 1/4 of the risk at first birth) and for SUI surgery (about 1/10 of the risk compared with the first VD) (Figure 2 A+B).

In women with one or more pregnancies delivered exclusively by C-sections, the risk of surgery for POP and SUI was negligible and on par with that in 0-para women. In contrast, the risk of surgery after VD increased consistently with the number of births. The first VD brought the largest risk increase for prolapse and incontinence surgery.

The result of the present study did not support the assumption that one or more pregnancies in themselves may cause long-term effects on the pelvic floor leading to POP and SUI surgery.

Ulm K. A simple method to calculate the confidence interval of a standardized mortality ratio (SMR). Am J Epidemiol 1990;131:373-5.

Continence 2S2 (2022) 100239
DOI: 10.1016/j.cont.2022.100239