The studies selected for this review revealed a wide range of prevalence rates for overall UI and the specific types of UI, which may be due in part to the country’s size. China is a large country with 9.6 million square kilometers and it includes numerous cultures in urban and rural locations. Genetic factors, diet, lifestyle, local environment, climate, economic development level, occupation types, and toileting behaviors may differ across these regions. These factors could act as determinants of UI, and thus affect variation in UI prevalence rates. Other important reasons for the variations in UI prevalence rates and UI types are the different research definitions and statistical and sampling methods used in the selected studies. For example, researchers used different methods when creating samples (i.e., based on general outpatient  or gynecological clinics  or physical examination  or nursing institutions ). The study participants had different occupations (i.e., medical personnel , nurses  and railway workers ), and different living conditions (i.e., rural  and urban ). Also, in some studies, clinicians conducted physical examinations and documented medical histories, which may have yielded different results from women’s self-reported questionnaires.
Researchers identified several factors associated with overall UI in Chinese women. Some of these factors are modifiable, such as weight, BMI, education, smoking and drinking alcohol. Some factors can be remedied or controlled (i.e., constipation, hypertension, chronic cough, diabetes, respiratory diseases, and vaginitis). Lastly, although some of the identified factors are unmodifiable (i.e., age, vaginal delivery, cesarean section delivery, and menopause), interventions can nonetheless be designed and tested to promote bladder health and help delay the onset or slow worsening of UI.
Age is often associated with UI risk factors such as education level, number of pregnancies and deliveries, menopause, and chronic conditions such as hypertension, diabetes, and respiratory diseases. Thus, studying clusters of factors that increase UI risk across the lifecourse is important. Moreover, using a lifecourse perspective is advocated in bladder health research .
Chinese scholars have largely confined their research on prevalence of UI in adult women (≥17 years old), but in their findings of subgroup analyses for discrete age groups were not reported. Despite this limitation, when using the prevalence rate range of 2.6%  to 30%  for young women aged 17 to 40 years old, the number of Chinese women in this age group who are affected by UI is estimated to be between 6.6 million and 75.8 million. This finding alone indicates that screening young women for risk factors, especially modifiable ones, and taking actions to minimize or eliminate the effects of these factors could potentially prevent or delay incident cases of UI throughout the lifecourse and especially later in life.
In a prospective cluster-randomized controlled trial of UI among young women (18-40 years), SUI prevalence was 14.3% ; 49.5% of these women had SUI during pregnancy, 43.6% had postpartum SUI, and 6.9% had SUI before pregnancy. Because muscle, connective, and nervous system pelvic structures are subjected to anatomical, morphological, functional, and hormonal changes during pregnancy, clinicians should initiate primary prevention interventions. The pelvic floor also undergoes an enormous amount of stretching to allow the passage of a newborn during vaginal delivery . Evident or hidden injuries to the pelvic floor may manifest as urinary and fecal incontinence, prolapse symptoms, or sexual dysfunction, all of which have a considerable impact on quality of life. Because pregnancy and childbirth can put young women at risk of developing UI , research and clinical attention should be focused on understanding the underlying mechanisms of UI as well as developing effective strategies (perform pelvic floor muscle training, maintain normal weight, avoid constipation ) to preserve bladder health for young Chinese nulligravid women.
In our research, there was only one study focused on the young women (18-26 years old) and it found that age (21-26 years old comparing with 18-20 years old) (OR = 0.87, 95% CI = 0.77 - 0.98), constipation (OR = 2.40, 95% CI = 1.49 - 3.84), alcohol consumption (OR = 1.76, 95% CI = 1.11 - 2.79), often/always delayed urination (OR = 1.74, 95% CI = 1.31 - 2.31), and/or often/always strained to urinate (OR = 1.43, 95% CI = 1.11 - 1.85) were associated with UI. The prevalence of UI in this study was 23.6% . Evidence from studies of young women who live outside of China also provides compelling evidence that UI in young women should be investigated further and intentionally. For example, the prevalence rate of UI for Swedish women (N = 653) between the ages of 18 and 30 years was 12% . In nulligravid Australian women aged 16 to 30 years (average age 22.5 ± 3.2 years), the prevalence of overall UI was 12.6%  , and women who were sexually active and those who were not using oral contraceptives had the highest rates of UI . Mishra et al.’s study found that the UI prevalence rate for Australian women aged 22 to 27 years was 6.8% at baseline and increased to 16.5% nine years later . These researchers also reported that women with depressive symptoms or a history of depression were more likely than those without depressive symptoms to report subsequent UI symptoms . For nulligravid women living in Italy between 15 and 25 years old, age, BMI, depression/ anxiety/panic attacks, eating disorders, and constipation were risk factors for UI . Participating in organized sports that involves high-volume exercise for competition also increased the risk of developing UI (OR = 2.53, 95%CI = 1.3 – 2.7) . Other studies conducted outside of China showed that UI is an issue for many nulliparous female athletes .
Cultural differences may be evident with regard to UI risk and associated factors in China and abroad. A relatively new factor under investigation is toileting behaviors, i.e., actions women take immediately prior to and during urination [77-80]. Toileting behaviors play a role in developing or worsening urinary symptoms, but more research is required, especially studies that focus on young women in China. It is often during youth and young adulthood when women develop habitual behaviors and form beliefs and attitudes about bladder health for themselves and their children. This period in women’s lifecourse may be pivotal in influencing prevalence rates because evidence is mounting that research to prevent or reduce UI in this age stage is important .
This review also found that UI prevalence rates for middle-aged Chinese women ranged from 8.7%  to 47.7% , which represents 15.5 to 85.0 million middle-aged women with UI. The UI prevalence rate for women between 45 and 60 years old living in Brazil was 23.6% . The associated factors relate mainly to obstetrics-related ones, such as parity, perineal laceration, and postpartum UI, and gynecological factors, such as menstrual disorder, menopause, pelvic organ prolapse, pelvic operation history, and hormone replacement therapy (see Table 1). UI prevalence rate was found to be significantly higher in a postmenopausal group than a premenopausal age group , which may be related to the change of hormone levels in postmenopausal women . Research conducted in China found that the protective effect of cesarean section delivery compared with vaginal delivery was more obvious at five years postpartum than at one year postpartum . Vaginal delivery appears to aggravate pelvic floor structure injuries. Compared to this study, studies conducted in other countries had similar findings. In Norway, a survey of middle-aged women (average age 47 years) who had delivered either vaginally or by Caesarian section 15 to 23 years previously had 46.9% UI prevalence. In addition, caesarian section delivery was a protective factor as UI prevalence was lower than in this group of women as compared to women who delivered vaginally. . Further research is needed to determine the mechanism(s) of injury during childbirth and identify associated factors and interventions that prevent or lessen adverse effects of childbirth on bladder health.
Although UI is common across the lifecourse, its prevalence peaks in the older age group of women . China's older female UI prevalence rate ranged from 16.9%  to 61.6% , which translates to more than 12.8 to 46.7 million older Chinese women living with UI. Scholars outside of China have found similar UI prevalence rates for older women. A survey of 622 older women living in Brazil (average age 64 years) revealed that the UI prevalence rate was 52.3%  and for women over 65 years old living in Turkey the UI prevalence was 51.6% . In China, risk factors associated with UI include being 80 years old and over, BMI, malnutrition, low educational level, sleep disorders , unspecified number and types of medications, history of pregnancy, urine leakage during pregnancy, chronic respiratory disease, cardiovascular disease, gynecological diseases, diabetes, urinary tract diseases , chronic pain, constipation, and restricted activity  (see Table1). Compared to middle-aged women, older women have a higher prevalence of chronic diseases (i.e., respiratory disease, hypertension, diabetes, etc.), limited physical activity, malnutrition, and other factors that could be contributing risk factors for UI. A longitudinal study of older women (baseline ages of 51 to 74 years) conducted in the United States years who did not have UI found the incidence of UI was 37.2% ten years later . This study also found that UI prevalence in later in life had strong associations with obesity, functional ability, and medical comorbidities, but not with parity .
Although we used rigorous methods to conduct this epidemiological review, some limitations are noted. First, although we searched one English database (i.e., PubMed), and the PRISMA review process was followed, we may have inadvertently overlooked eligible articles. Second, research into UI prevalence rates has not been conducted in all 34 provinces in China (e.g., 22 of the 34 provinces are represented in the included studies), which could result in an incomplete picture of the distribution of UI across the Chinese female adult population. The quality of the included studies was not formally assessed in order to include a broad representation of the literature. It is recommended that future studies include this important element. This paper, however, provides important information and raises awareness about prevalence of UI in women living in China.