Obstetrics and gynaecology

Prevalence of urinary incontinence among nulliparous women and its association with underweight body mass index: a secondary analysis of a nationwide cross-sectional study in China

    To estimate the prevalence of urinary incontinence (UI) and its subtypes among nulliparous Chinese women with associated risk factors. The prevalence of UI among those living in urban or rural communities was also analysed with potential risk factors.

    This is a secondary analysis of epidemiological survey data on UI in Chinese women. The original study was designed as a nationwide cross-sectional study involving 56 460 adult women conducted from October 2019 to December 2021.

    Seven geographic regions of China.

    Nulliparous women who were aged ≥20 years old and were permanent residents were included in this secondary analysis. Participants who had severe mental or physical disorders or were pregnant were excluded. Data on demographic characteristics, health status and medical history were collected.

    The primary outcome was the prevalence of UI, whereas secondary outcome measures were adjusted odds ratios (aOR) for risk factor analysis.

    A total of 6244 nulliparous women were included in the analysis. The prevalence of UI was 1.9% for nulliparous Chinese women, with stress, urgency and mixed UI being prevalent at 0.9%, 0.3% and 0.7%, respectively. The prevalence of UI was 2.1% and 1.6% for urban and rural subgroups. Abnormal body mass index was significantly associated with UI in the nulliparous group (underweight, aOR: 1.88, 95% CI: 1.03 to 3.45, p=0.041; overweight, aOR: 2.26, 95% CI: 1.37 to 3.73, p=0.001; and obesity, aOR: 3.64, 95% CI: 1.86 to 7.15, p<0.001) and the subgroup in urban areas (underweight, aOR: 2.43, 95% CI: 1.18 to 5.00, p=0.016; overweight, aOR: 3.31, 95% CI: 1.76 to 6.25, p<0.001; and obesity, aOR: 4.53, 95% CI: 1.88 to 10.92, p<0.001); however, no significant association was found for those in rural areas.

    UI among nulliparous women deserves greater public attention. Abnormal body mass index, including underweight status, was found to be a risk factor for UI among Chinese nulliparous women. Further research is required to investigate the mechanism underlying the association between underweight status and UI.

    The study was approved by Peking Union Medical College Ethics Committee (No. S-K970) and conducted according to the Declaration of Helsinki. All participants signed consent forms before data collection. A completed STROBE checklist detailing compliance with all 22 items is provided.

    Data are available upon reasonable request. The data from the present research that were used and analysed are accessible from the corresponding author upon reasonable request.

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    Urinary incontinence (UI) is defined as any complaint of involuntary leakage of urine according to the International Continence Society.1 UI demonstrates a strongly negative impact on female quality of life through not only mental dysfunction, such as embarrassment, depression, and low self-esteem, but also deteriorated physical ability, resulting in decreased recreational activity time and social isolation.2 3 UI prevalence in China was estimated to be 30.9% in 2006 and decreased to 16% in 2021 with significant efforts being made to implement pelvic floor rehabilitation in the past fifteen years.4 In spite of this, it is estimated that over 90 million women in China are still suffering from UI.5 There are several well-known risk factors for UI, including childbirth, ageing, obesity and chronic medical conditions.6 Considerable attention was paid to how childbirth affects female UI symptoms; however, it cannot be neglected that nulliparous women can also experience UI episodes. Minimal research has reported the prevalence of UI among nulliparous women. A systematic review published in 2017 found that UI prevalence varied between 1% and 42.2% in nulliparous women worldwide,7 which suggested that UI among the nulliparous group is with great uncertainty and highlighted the urgent need for public attention and better preventative strategies for this underserved population.

    Due to significant delays in childbirth and the transition of childbirth preference of Chinese couples in the past two decades10, the number of nulliparous women with greater ages also significantly increased in rural China. Previous research noted that women from rural areas may suffer from more UI episodes due to lower educational level 7 with less personal health awareness.8 To our knowledge, there was also very limited published study reporting the prevalence of UI as well as risk factors in the rural nulliparous population. Therefore, the objective of this study is to estimate the prevalence of UI and its subtypes among nulliparous Chinese women with associated risk factors. UI prevalence among those living in urban or rural communities was also analysed with potential risk factors.

    This current study is a secondary data analysis of the nulliparous subgroup from a national cross-sectional research project which was conducted between October 2019 and December 2021 using a six-stage sampling strategy.5 Detailed description of the study design, sample size calculation and participant recruitment for the original study has been published.5 Inclusion criteria were women aged ≥20 years old and permanent residents defined as living in their current residence for 1 year or longer. For this secondary analysis, we only include nulliparous women from the original research. Participants who had severe mental or physical disorders or were pregnant were excluded from the study. The original study has been approved by Peking Union Medical College Ethics Committee (No. S-K970) and conducted according to the Declaration of Helsinki. All participants signed consent forms before data collection. A completed STROBE checklist detailing compliance with all 22 items is provided in online supplemental table 1.

    This study included participants’ basic information, such as demographic characteristics, medical history and health behaviours. Demographic characteristics included age, urbanisation level, ethnicity, education level, marital status, occupational status, monthly family income and menopause status. Body mass index (BMI) was categorised as underweight (BMI<18.5 kg/m2), normal (18.5 kg/m2≤BMI≤24.9 kg/m2), overweight (25 kg/m2 ≤BMI≤29.9 kg/m2) and obese (BMI≥30 kg/m2). Medical history includes chronic constipation, diabetes, high blood pressure and the history of other gynaecological diseases (ie, pelvic inflammatory disease, pelvic pain, endometriosis, fibroids, gynaecological malignant tumours and pelvic organ prolapse). By using a six-stage sampling strategy, participants were then divided into rural and urban subgroups based on their residence during data collection. The primary outcome was UI, which was defined as the complaint of any involuntary leakage of urine,9 and UI was categorised into stress UI (SUI), urgency UI (UUI), mixed UI (MUI) and other UI according to the Chinese version of the International Conference on Incontinence Questionnaire UI Short Form in the original study.10 The type of UI was determined by the following two questions: (1) ‘During the past 4 weeks, did you leak urine when you were performing some physical activity, such as coughing, sneezing, lifting or exercise?’ (yes or no); and (2) ‘During the past 4 weeks, did you leak urine when you had the urge or the feeling that you needed to empty your bladder, but you could not get to the toilet fast enough?’ (yes or no). Women who answered yes for question (1) were categorised as having SUI. Women who answered yes for question (2) were categorised as having UUI. Women who answered yes to both questions were categorised as having MUI. Other types of UI were defined as those with a negative response to the two preceding questions about leakage (SUI or UUI) among those who reported having UI during the past 4 weeks. No nulliparous participant recruited was categorised into the other UI subgroup; thus, only SUI, MUI and MUI were analysed in this current study.

    Number and percentage were used to describe categorical variables. Participants’ baseline characteristics were compared using χ2 test or Fisher’s exact test of independence for categorical variables. The Cochran-Armitage trend test was used to demonstrate linear trends for ordinal categorical variables. We used a binary logistic regression model to obtain OR and 95% CI to detect risk factors of UI for the nulliparous population and subgroups living in urban or rural areas. In addition, multinomial logistic regression was applied to estimate risk factors of UI subtypes, including SUI, UUI and MUI, among the nulliparous population. The reference category was the group of participants without UI. Statistical significance was set at p<0.05 (two-tailed). All analyses were performed using SPSS, V. 24.0 (IBM).

    Patients or the public were not involved in the design, conduct, reporting or dissemination plans of this study.

    A total of 6244 nulliparous women were selected from the cohort of 54 346 participants in the original study (figure 1). Overall, 2718 (43.5%) participants were from rural areas, whereas 3526 (56.5%) were from urban areas (table 1). No significant differences were noted in BMI, menopausal status and the medical history of coughing, diabetes and high blood pressure (p>0.05) between the two groups. Rural population tends to be younger (p<0.001), unmarried (p<0.001), unemployed (p<0.001), belongs to an ethnic minority group (p<0.001), has lower education background (p<0.001), lower monthly family income (p<0.001) and less probability to have medical history of chronic constipation (p=0.001) and other gynaecological disease (p<0.001) (table 2).

    Figure 1

    Table 1

    Demographic characteristics of the sample population

    Table 2

    Variables and UI occurrence among urban/rural nulliparous Chinese women

    The prevalence of UI was 1.9% among Chinese nulliparous women, while that of SUI, UUI and MUI was 0.9%, 0.3% and 0.7%, respectively (table 3). For urban and rural subgroups, the prevalence of UI was 2.1% and 1.6%. The trend of prevalence was also analysed among BMI, age groups and menopause status. The nulliparous population and urban subgroup showed that the normal BMI group had the lowest UI prevalence compared with underweight, overweight and obese subgroups, whereas a positive trend of UI prevalence was noted among BMI groups for the rural subgroup (figure 2). As nulliparous women became ageing and postmenopausal, higher UI prevalence was shown (online supplemental figure S1). The urban subgroup demonstrated higher UI prevalence during the premenopausal period but lower prevalence during the postmenopausal period than the rural subgroup (online supplemental figure S2).

    Table 3

    Association between variables and UI and its subtypes among nulliparous Chinese women

    Figure 2

    Figure 2

    The prevalence of urinary incontinence (UI) among different body mass index (BMI) groups. Bars indicate the prevalence of UI among BMI subgroups in the nulliparous population and urban and rural subgroups, respectively. The graph demonstrates the trends of UI prevalence in each population group as BMI becomes greater.

    Table 4 shows the analysis of risk factors of UI and its subtypes among nulliparous Chinese women. In binary logistic regression, the results showed that unmarried (aOR: 0.31, 95% CI: 0.18 to 0.54) and employed status (mentally employed, aOR: 0.53, 95% CI: 0.29 to 0.98; physically employed, aOR: 0.55, 95% CI: 0.31 to 0.96) were both protective factors for nulliparous women. Nulliparous women with chronic constipation had the strongest association with UI occurrence (aOR: 5.75, 95% CI: 2.59 to 12.74), followed by ageing 30–39 years (aOR: 4.16, 95% CI: 2.05 to 8.45), having postmenopausal status (aOR: 2.22, 95% CI: 1.22 to 4.05) and belonging to an ethnic minority (aOR: 2.03, 95% CI: 1.14, 3.64). This current study also noted that abnormal BMI is significantly associated with UI occurrence in nulliparous female groups, regardless of under- or overweight status (underweight, aOR: 1.88, 95% CI: 1.03 to 3.45; overweight, aOR: 2.26, 95% CI: 1.37 to 3.73; and obesity, aOR: 3.64, 95% CI: 1.86 to 7.15).

    Table 4

    Potential risk factors of UI and its subtypes among nulliparous Chinese women

    In multinomial logistic regression, nulliparous women who have chronic coughing (aOR: 7.11, 95% CI: 2.80 to 18.01), are of older age (≥40 years, aOR: 5.31, 95% CI: 2.22 to 12.70), are urban residents (aOR: 2.27, 95% CI: 1.16 to 4.46) and belong to an ethnic minority (aOR: 3.05, 95% CI: 1.52 to 6.15) are more likely to report SUI. Nulliparous women who are overweight (aOR: 2.54, 95% CI: 1.29 to 4.98) or obese (aOR: 2.97, 95% CI: 1.12 to 7.89) demonstrated significant odds of developing SUI, while obesity is a statistically significant risk factor for UUI (aOR: 4.42, 95% CI: 1.01 to 19.37) and MUI (aOR: 4.19, 95% CI: 1.49 to 11.78). Nulliparous women with a medical history of other gynaecological diseases were found to have an increased risk of developing UUI (aOR: 6.23, 95% CI: 1.86 to 20.88). Postmenopausal nulliparous women have the strongest association with MUI occurrence (aOR: 8.79, 95% CI: 1.98 to 39.01).

    Table 5 shows the analysis of risk factors of UI among urban and rural nulliparous Chinese women. Subgroup analyses revealed that abnormal BMI is significantly associated with UI occurrence for nulliparous women residing in urban areas (underweight, aOR: 2.43, 95% CI: 1.18 to 5.00; overweight, aOR: 3.31, 95% CI: 1.76 to 6.25; and obesity, aOR: 4.53, 95% CI: 1.88 to 10.92); however, BMI is not a significant risk factor for those living in rural areas. For participants in both residential areas, unmarried status continues to be a protective factor for nulliparous women (rural, aOR: 0.34, 95% CI: 0.11 to 1.00; urban, aOR: 0.32, 95% CI: 0.17 to 0.61) while chronic constipation demonstrates the strongest association with UI occurrence (rural, aOR: 5.66, 95% CI: 1.14 to 28.15; urban, aOR: 6.29, 95% CI: 2.51 to 15.80). In the rural subgroup, participants aged over 30 years showed increased odds of developing UI (age 30–39 years, aOR: 4.29, 95% CI: 1.17 to 15.79; age ≥40 years, aOR: 7.28, 95% CI: 1.87 to 28.24); however, only age greater than 40 years old (age ≥40 years, aOR: 2.94, 95% CI: 1.25 to 6.92) is a risk factor for those residing in urban communities.

    Table 5

    Potential risk factors of UI among rural and urban nulliparous Chinese women

    To the best of our knowledge, this research is the first to report the prevalence of UI and its subtypes in nulliparous women with the largest sample size through a nationwide study. This current study revealed that the prevalence of UI among nulliparous Chinese women was 1.9%, which is slightly higher compared with a previous study (1%) conducted in 2003 within a smaller Chinese nulliparous population.11 The prevalence of UI found in this current study among nulliparous women living in urban areas (2.1%) was reported to be higher than that in rural areas (1.6%). It is agreeable in this current study that chronic constipation and overweight status were risk factors for UI development; however, underweight status was for the first time also identified as an influencing factor for UI development among the nulliparous population.

    A systematic review including multiple cohort or cross-sectional studies investigated UI prevalence among nulliparous women in different countries, such as 10.8% in Australia,12 14% in Denmark,13 19.9% in Portugal14 and 38.6% in Sweden,15 which were all higher than results in this current study. One possible explanation for the ratio differences could lie in different study designs, including data collection method and use of UI definition. The study design included a six-stage sampling strategy to accurately represent the national population. Additionally, we used an internationally accepted definition of UI, as well as a reliable international standardised questionnaire to measure outcomes. The results showed that SUI is the most prevalent subtype among Chinese nulliparous women, which was consistent with several previous high-quality research studies conducted in other countries.16 Despite low prevalence compared with parous women, UI among nulliparous women remains an important health issue requiring increased public attention. A shift in the marriage conception and fertility preference from Chinese women has been noted with the national postponement of parenthood, such as late marriage, long marriage and conception interval and advanced pregnancy.17 Nulliparous women with advanced age may present with UI symptoms before pregnancy, which can result in more severe adverse outcomes and postpartum pelvic floor dysfunction.16 18–20 Thus, advocating for pelvic floor checks before pregnancy for the nulliparous population could assist in preventing the detrimental effects of UI after childbirth with increased age.

    Our study is also the first one to report UI prevalence of nulliparous Chinese women living in urban and rural areas with a rate of 2.1% and 1.6%, respectively. Several risk factors contribute to UI development in underdeveloped areas. A recent study found that urban women with better knowledge of UI and its negative impact were more likely to seek medical care, while rural women with similar lower education levels were less aware of UI as a treatable condition.5 This is further supported by the finding that 83.7% of UI women from rural areas had only senior school or below educational backgrounds. Occupation demands also explain the UI prevalence difference between urban and rural communities. Rural women were more likely to do labour work and retire late, with 28.7% rural women in this current study performing mainly physical work compared with only 14.9% of urban women.21 The rural National Retirement Pension programme, phased in since 2009, provided less coverage than urban areas but did not require quitting agricultural activities, exposing rural women to activities inducing long-lasting increased abdominal pressure longer. Due to the unequal distribution of medical resources, UI prevalence has the potential to be underreported in rural areas. As inadequate access to healthcare and limited access to information exist, only a small proportion of Chinese rural citizens had consulted a healthcare provider for UI symptoms.22 23 A lack of health literacy not only affects patients and their care providers, but also rural health providers who had insufficient knowledge of the impact of UI; therefore, inappropriate prevention and management strategies were undertaken.24 Rural women may perceive occasional ‘urine drops’ as a natural process of ageing and prioritised other life-threatening conditions, such as cardiac vascular disease, which accounts for 46.74% mortality in rural areas.25 UI, affecting quality of life but not life threatening, received less priority in rural communities. However, higher limitations in social and physical aspects due to incontinence in the rural population can lead to a loss of personal identity and more severe mental health deterioration, as social life with fewer boundaries is more important in rural daily life than in urban areas.3 26

    In 2020, the rate of childlessness of women aged 49 and above reached 5.16% in China.27 Considering those who chose to be lifelong nulliparous, this study revealed that multiple risk factors contribute to UI occurrence even if no pregnancy or childbirth happened. Our result is consistent with previous research showing that chronic constipation is detrimental to the development of UI. Poor living habits, such as the widespread use of the ‘bearing down’ toilet strategy, may contribute to increased abdominal pressure and chronically weak pelvic floor muscles.28 29 It is globally agreeable that obesity with long-lasting increased abdominal pressure is a significant risk factor to UI, as multiple studies proved that weight loss is an effective strategy to improve UI symptoms.30 31 Our study agreed that overweight or obese BMI is a risk factor; however, the results surprisingly showed that underweight BMI also increased the possibility of UI occurrence for Chinese nulliparous women. We speculated that underweight BMI is associated with loss or weakening of whole-body muscle mass and strength, which includes that of pelvic floor muscles. One previous research study showed that underweight BMI was significantly associated with an increased likelihood of sarcopenia, which is often diagnosed with lower hand grip strength to overall represent whole-body muscle mass and strength.32 Zhang et al discovered that lower levels of hand grip strength are significantly associated with an increased prevalence and severity of SUI, suggesting that decreased muscle mass may correlate with UI development.33 No significant association among the rural population can possibly be explained due to the smaller sample size in the rural subgroup.

    The robust study design of the original nationwide cross-sectional study was the key strength of this research. It used a six-stage sampling strategy in six regions of China with a total of 56 460 women for a more accurate nationally representative population,5 allowing this secondary analysis to be the first one showing UI prevalence in nulliparous women with the largest sample size. However, this study also has several limitations. First, it is a cross-sectional design, which cannot infer causality. Despite this, this study design is effective for providing a snapshot of UI prevalence in this population and offering a solid base for generating hypotheses that could be tested in future longitudinal studies. Second, women with severe mental dysfunction who could not complete the survey were not included in this study, which might lead to an underestimation of UI prevalence in this population. However, the exclusion was necessary to ensure the accuracy of responses, as the excluded population may have difficulty providing reliable information during the survey. Third, all information was derived from self-reported data, which may lead to recall and information bias. Even though we did not use an objective test to collect results, a standardised reliable questionnaire was chosen for its feasibility in a large-scale study.

    In conclusion, even though low UI prevalence was reported for nulliparous Chinese women, action should be taken for this underserved population to enhance their awareness of UI symptoms and its negative life impact. Abnormal BMI, especially underweight status, was also found to be a risk factor for UI in nulliparous women. Although obesity-related UI has dominated clinical practice, the association between low BMI and UI deserves urgent clinical attention with progress in both clinical assessment and targeted intervention strategies. Further research could focus on investigating the mechanism underlying the association between underweight BMI and UI development.

    Data are available upon reasonable request. The data from the present research that were used and analysed are accessible from the corresponding author upon reasonable request.

    Not applicable.

    This study involves human participants and was approved by the institutional review board of Peking Union Medical College Hospital (Number: S-K970). Participants gave informed consent to participate in the study before taking part.

    We thank all participants in this study.

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