Are smoking and other lifestyle factors associated with female urinary incontinence? The Norwegian EPINCONT Study

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1 BJOG: an International Journal of Obstetrics and Gynaecology March 2003, Vol. 110, pp Are smoking and other lifestyle factors associated with female urinary incontinence? The Norwegian EPINCONT Study Yngvild S. Hannestad*, Guri Rortveit, Anne Kjersti Daltveit, Steinar Hunskaar Objective To examine whether modifiable lifestyle factors such as smoking, obesity, physical activity and intake of alcohol or caffeinated drinks were associated with urinary incontinence in women. Design Cross sectional population-based study. Setting The Norwegian Epidemiology of Incontinence in the County of Nord-Trøndelag (EPINCONT) Study is part of a large survey performed in a county in Norway during Population Women 20 years (n ¼ 34,755, 75% of the invited) attended the first part of the survey and received the questionnaire. There were 27,936 (80% of source population) women who completed the incontinence part of the questionnaire. Methods Questionnaire covering several health topics including urinary incontinence was received at a screening station. Logistic regression analysis was used to adjust for confounding and to establish associations with the different outcomes under investigation: any incontinence, severe incontinence and stress, urge and mixed subtypes. Main outcome measures Effect measure were odds ratios with corresponding 95% confidence intervals. Results Former and current smoking was associated with incontinence, but only for those who smoked more than 20 cigarettes per day. Severe incontinence was weakly associated with smoking regardless of number of cigarettes. The association between increasing body mass index and incontinence was strong and present for all subtypes. Increasing levels of low intensity physical activity had a weak and negative association with incontinence. Tea drinkers were at slightly higher risk for all types of incontinence. We found no important effects of high intensity physical activity, intake of alcohol or coffee. Conclusions Several potentially modifiable lifestyle factors are associated with urinary incontinence. Highest odds ratios were found for body mass index, heavy smoking and tea drinking. INTRODUCTION Epidemiological research has revealed several factors associated with urinary incontinence in women, the most commonly reported being age, pregnancy and childbirth 1 3. Modifiable risk factors have not been investigated to the same extent, and several of the studies that do address such factors do not control adequately for confounders. Examples of lifestyle factors possibly associated with incontinence are obesity, smoking, physical activity and diet. Two case control studies 4,5 have shown an association between smoking and incontinence. Bump and McClish 4 found a twofold increase in the risk for incontinence among both former and current smokers compared with never smokers. Only one population-based study states to have included smoking in multivariate analyses and reports an Section for General Practice and Section for Preventive Medicine, Department of Public Health and Primary Health Care, University of Bergen, Norway * Correspondence: Dr Y. S. Hannestad, Section for General Practice, University of Bergen, Ulriksdal 8c, N-5009 Bergen, Norway. D RCOG 2003 BJOG: an International Journal of Obstetrics and Gynaecology doi: /s (02) odds ratio for any incontinence of 1.9 (95% CI ) for smokers compared with never smokers 6. No populationbased studies have investigated the potential effect of smoking on the different types of incontinence or evaluated a potential dose effect of cigarettes in adjusted analyses. Obesity and high body mass index are now well established risk factors for incontinence 1. Stress incontinence has turned out to be the type most closely associated with body mass index 7 9, but two studies have shown an effect of body mass index also on urge incontinence in women under 60 years of age 7,9. A corresponding effect could not be shown in a study of postmenopausal women 8. While no population-based studies have evaluated the effect of tea intake on incontinence, some have included the intake of coffee in their analyses and no association has been found However, in a case control study, the mean intake of coffee was significantly higher in women with detrusor instability than in controls 13 and an increase in detrusor pressure after administration of caffeine has been shown experimentally 14. An association between a decrease in amount of dietary caffeine consumed and fewer daytime episodes of incontinence was found in an intervention study, although it was not statistically significant 15.

2 248 Y.S. HANNESTAD ET AL. A possible effect of alcohol intake on incontinence has been investigated in some studies, but an association has not been shown 8, Some studies have evaluated the impact of levels of physical activity, but the results are inconclusive The aim of the present study was to investigate whether modifiable lifestyle factors such as smoking, obesity, physical activity and intake of alcohol or caffeinated drinks were independently associated with urinary incontinence in women. The study was performed in a large and unselected population of women to allow for analyses of risk factors also for the different types of incontinence and for different severity of symptoms. The demonstration of independent associations from a cross sectional survey can lead to hypotheses regarding preventive strategies for incontinence that can be tested in intervention studies. METHODS The Norwegian Epidemiology of Incontinence in the County of Nord-Trøndelag (EPINCONT) Study is a substudy of the Nord-Trøndelag Health Survey 2 (HUNT 2), a large population-based survey performed in one county in Norway during the years Urinary incontinence was one of several topics covered by the complete HUNT 2 survey. All women aged 20 years or more (N ¼ 47,313) residing in the county were invited to participate. Invitations were sent by mail along with Questionnaire 1, which was to be returned at attendance. A total of 34,755 communitydwelling women participated in HUNT 2 (74%) and completed the full screening, including the measurement of several clinical parameters. These women are defined as the source population of the EPINCONT Study. At the screening station the participants received Questionnaire 2, which included a section about urinary incontinence. Questionnaire 2 was to be completed at home and returned by mail. The questions about urinary incontinence were answered by 27,936 women, giving an overall response rate for the EPINCONT Study of 80%. A more complete description of the survey and questionnaires has been published elsewhere 20. Some relevant results and study data will be referred here for background information. The entry question was whether or not the participant experienced involuntary loss of urine. If the answer was affirmative (6876 women, 24.6% of the study population), the woman was categorised as having any incontinence. A severity index developed by Sandvik et al. 21,22 was used to characterise the degree of incontinence. The index was calculated based on the answers regarding frequency and amount of leakage, whereby the incontinence was categorised into slight, moderate or severe (43%, 31% and 26% of the incontinent women, respectively). Typically, slight incontinence denotes leakage of drops a few times a month, moderate incontinence daily leakage of drops and severe incontinence larger amounts at least once a week. The severity index has been validated against a 48 hour pad weighing test 21,22. According to this test, slight incontinence means a leakage of 6 g/24 hours (95% CI 2 9), moderate incontinence means a leakage of 17 g/24 hours (95% CI 13 22) and severe incontinence means a leakage of 56 g/24 hours (95% CI 44 67). Women who confirmed involuntary loss of urine in connection with coughing, sneezing, laughing or lifting heavy items were classified as having symptoms of stress incontinence (86% of the incontinent women, 21% of all women). Those whose involuntary loss of urine was connected with a sudden and strong urge to void were classified as having urge symptoms (47% of the incontinent women, 11% of all women). Of the incontinent women, 36% (9% of all women) had both stress and urge symptoms and were thus classified as having mixed type incontinence, while 50% had pure stress type (12% of all women) and 11% pure urge type incontinence (3% of all women). Due to incomplete answers, 3% (1% of all women) could not be classified. The validity of type classification based on these questions has been tested by Sandvik et al. 23. The classification was compared with a final diagnosis by a gynaecologist after urodynamic evaluation as gold standard. The clinical diagnosis shifted many women with mixed incontinence to the group of genuine stress incontinence while the percentage of urge was slightly affected. The results from our epidemiological survey are in agreement with other comparable studies 1. Smoking habits were recorded by several questions. These variables were aggregated into a main variable called smoking status (never/former/current smoker) that was further subdivided on the basis of number of cigarettes smoked daily. The number of pack years was calculated by multiplying the cigarettes consumed per day (in packs) by years of smoking. Physical activity in leisure time was covered by two questions. Participants were asked to give the average number of hours of low intensity (not sweating/out of breath) and high intensity (sweating/out of breath) physical activity per week for the past year. Coffee and tea intake was given in cups per day, while alcoholic beverages were reported as glasses of beer, wine or spirits per two weeks. Body mass index was derived from the measurements of height and weight (kg/m 2 ) at the screening station. Proportions were used to describe the univariate relationship between urinary incontinence and the ordered variables. Where relevant, stratification was performed and possible interactions were tested by Breslow Day s test for homogeneity between odds ratios on the different levels. Confounding was evaluated by stratification and logistic regression analysis. Factors that confounded the effect of one or several of the exposure variables under investigation were included in the final models. Logistic regression analysis was used to adjust for confounding and establish independent risk factors for

3 LIFESTYLE FACTORS AND URINARY INCONTINENCE 249 incontinence. Odds ratios with corresponding 95% confidence intervals were the effect measures. Effects were denoted as strong when odds ratios were 1.8 or more, and weak when odds ratios were Analyses were performed separately for each of the different outcomes under investigation: any incontinence (all incontinent women), severe incontinence and the different subtypes. For all logistic regression analyses, continent women served as the reference group. Missing answers were categorised as Not given for each variable so as to allow the inclusion of all the women in the multivariate analyses. Statistical analysis was performed using the programme SPSS The level of statistical significance was set at Age is a well known risk factor for urinary incontinence and was also a confounder for all the lifestyle factors investigated. In order to assess the extent to which smoking affects incontinence other than by producing or aggravating respiratory symptoms, three variables (former or current asthma, periodic daily coughing, episodes of wheezing or dyspnoea during the last 12 months) were evaluated as possible confounders. Periodic coughing and wheezing/ Table 1. Percentage of the participating women (n ¼ 27,936) reporting any incontinence (n ¼ 6876), severe incontinence (n ¼ 1592), stress type incontinence (n ¼ 3414), mixed type incontinence (n ¼ 2417) and urge type incontinence (n ¼ 756) according to various potentially associated factors*. All women (n) Any incontinence (%) Severe incontinence (%) Stress incontinence (%) Mixed incontinence (%) Urge incontinence (%) Age , , þ Body mass index <25 12, , þ Smoking, no. of cigarettes/day Never smoked 13, Former þ Current þ Cups of tea/day þ Cups of coffee/day þ 19, No. of glasses of alcoholic beverages/two weeks , þ Low intensity physical activity/week <1 hour hours hours High intensity physical activity/week <1 hour 13, hours hours * The prevalence for the three types does not add up to the prevalence for any incontinence as the data for the women who could not be classified (3%) are not reported.

4 250 Y.S. HANNESTAD ET AL. dyspnoea positively confounded the effect of both smoking and body mass index on incontinence. Body mass index was included as a confounder in the models estimating the effects of smoking and beverages. Coffee was a weak positive confounder of the effect of smoking and vice versa. Smokers have been shown to enter menopause at an earlier age than non-smokers 24, but neither the use of hormone replacement therapy nor menopausal status were confounders of the effect of smoking. Number of children confounded the effect of body mass index and physical activity. We found no significant correlation between the two variables for physical activity, which were therefore included in the same model. Because neither age nor number of children satisfied the assumptions of linearity, these were included as categorical variables in the adjusted analyses. We found no important effect modification in the models we present. RESULTS Table 1 displays the crude prevalence of any urinary incontinence, severe incontinence and stress, urge and mixed type incontinence according to age and various lifestyle factors. The prevalence of any urinary incontinence increased with age and higher body mass index. Former smokers reported a higher rate of incontinence than current smokers, or those who had never smoked. High intakes of tea or coffee were positively associated with incontinence, whereas the relationship was inverse for the intake of alcoholic beverages. The crude prevalences also suggested a negative association between physical activity and incontinence. The prevalence of severe incontinence and the different subtypes of incontinence varied according to the different factors in a pattern similar to that for any incontinence (Table 1). An exception was stress incontinence, which was more prevalent among middle-aged than among older women. Multiple regression analysis showed that neither former nor current smoking increased the risk for any incontinence compared with never smoking, but a weak and significant association was present for severe incontinence (Table 2). When smoking status was explored further, based on the number of cigarettes smoked, a dose response relationship appeared. There was no association between any incontinence and former or current smoking of less than 20 cigarettes a day, although there was a slightly increased and significant odds ratio for both former and current smoking of 20 cigarettes or more daily. A weak dose response relationship was also present between any urinary incontinence and number of pack years for both former and current smokers. The association between the smoking variables and severe incontinence followed a pattern similar to that for any incontinence, but the effects were stronger (Table 2). For the different subtypes of incontinence, smoking status had an independent and significant effect only for mixed incontinence (Table 2). For mixed type, both former and current smokers showed a strong dose response effect depending on number of cigarettes per day, with the highest odds ratio shown by former smokers of 20 or more cigarettes per day. Table 2. Multiple regression analysis of the association between smoking and urinary incontinence*. Values are expressed as OR (95% CI). Incontinence Stress type incontinence Mixed type incontinence Urge type incontinence All Severe All Severe All Severe All Severe Smoking Never Former 1.1 ( ) 1.4 ( ) 1.1 ( ) 1.1 ( ) 1.2 ( ) 1.5 ( ) 1.2 ( ) 1.1 ( ) Current 1.0 ( ) 1.4 ( ) 0.8 ( ) 1.1 ( ) 1.2 ( ) 1.5 ( ) 1.2 ( ) 1.2 ( ) No. of cigarettes/day Never smoked Former ( ) 1.2 ( ) 1.1 ( ) 1.1 ( ) 1.2 ( ) 1.3 ( ) 1.1 ( ) 1.0 ( ) 20þ 1.7 ( ) 2.5 ( ) 1.3 ( ) 1.4 ( ) 2.2 ( ) 3.3 ( ) 1.9 ( ) 2.1 ( ) Current ( ) 1.3 ( ) 0.8 ( ) 1.1 ( ) 1.1 ( ) 1.4 ( ) 1.2 ( ) 1.3 ( ) 20þ 1.3 ( ) 2.1 ( ) 1.2 ( ) 1.8 ( ) 1.6 ( ) 2.4 ( ) 1.3 ( ) 1.3 ( ) Pack years Never smoked Former < ( ) 1.2 ( ) 1.1 ( ) 1.0 ( ) 1.1 ( ) 1.3 ( ) 1.1 ( ) 0.8 ( ) 15þ 1.5 ( ) 2.0 ( ) 1.3 ( ) 1.7 ( ) 1.6 ( ) 2.0 ( ) 1.9 ( ) 2.8 ( ) Current < ( ) 1.1 ( ) 0.7 ( ) 1.0 ( ) 1.0 ( ) 1.1 ( ) 1.3 ( ) 1.5 ( ) 15þ 1.2 ( ) 1.7 ( ) 1.0 ( ) 1.4 ( ) 1.4 ( ) 1.9 ( ) 1.2 ( ) 1.1 ( ) * Adjusted for age, body mass index, coughing, wheezing/dyspnoea and coffee.

5 Table 3. Multiple regression analysis of the association between BMI, physical activity and urinary incontinence*. Values are expressed as OR (95% CI). Incontinence Stress type incontinence Mixed type incontinence Urge type incontinence All Severe All Severe All Severe All Severe Body mass index < ( ) 2.0 ( ) 1.4 ( ) 1.9 ( ) 1.7 ( ) 2.3 ( ) 1.1 ( ) 1.6 ( ) ( ) 3.1 ( ) 1.7 ( ) 2.8 ( ) 2.3 ( ) 3.5 ( ) 1.5 ( ) 3.0 ( ) ( ) 4.2 ( ) 2.0 ( ) 3.2 ( ) 3.5 ( ) 5.5 ( ) 1.4 ( ) 2.4 ( ) 40þ 2.7 ( ) 5.0 ( ) 2.4 ( ) 4.2 ( ) 3.7 ( ) 6.0 ( ) 1.8 ( ) 3.8 ( ) Low impact physical activity/week <1 hour hours 0.9 ( ) 0.6 ( ) 1.0 ( ) 0.8 ( ) 0.8 ( ) 0.6 ( ) 1.0 ( ) 0.8 ( ) 3 hours 0.8 ( ) 0.5 ( ) 0.8 ( ) 0.6 ( ) 0.7 ( ) 0.5 ( ) 1.0 ( ) 0.4 ( ) High impact physical activity/week <1 hour hours 1.1 ( ) 1.2 ( ) 1.1 ( ) 1.4 ( ) 1.0 ( ) 1.0 ( ) 1.0 ( ) 1.3 ( ) 3 hours 1.0 ( ) 1.4 ( ) 1.0 ( ) 1.5 ( ) 0.9 ( ) 1.2 ( ) 1.0 ( ) 1.2 ( ) * Adjusted for the other factors in the table in addition to age, no. of children, coughing and wheezing/dyspnoea. LIFESTYLE FACTORS AND URINARY INCONTINENCE 251 The analyses displayed in Table 2 were also performed without adjusting for coughing and wheezing/dyspnoea (data not shown). Typically, the odds ratios for former smoking remained unchanged or were slightly higher, while the odds ratios for current smoking rose to the levels for former smoking or higher. For the outcome of severe incontinence with regard to number of cigarettes smoked, the odds ratios were therefore as follows: never smoked (reference), former smoker <20 cigarettes (OR 1.2, 95% CI ), former smoker 20þ cigarettes (OR 2.7, 95% CI ), current smoker <20 cigarettes (OR 1.5, 95% CI ), current smoker 20þ cigarettes (OR 3.0, 95% CI ). Increasing body mass index was strongly associated with all outcomes, and in most cases, it showed a clear dose response effect. The odds ratios were highest for severe mixed type incontinence (Table 3). Higher levels of low intensity physical activity slightly lowered the odds for all outcomes except for urge incontinence. High intensity physical activity showed only weak and non-significant associations with the various outcomes (Table 3). Daily tea drinkers had raised odds for all outcomes (Table 4) although not significantly for urge. For coffee, the analyses showed conflicting results. Coffee had no effect on any incontinence, a weak but significant negative effect on severe and severe mixed type incontinence and a weak but significant positive effect on stress type incontinence. The intake of alcohol showed no association with any of the outcomes (Table 4). Table 4. Multiple regression analysis of the association between urinary incontinence and the intake of tea, coffee and alcohol*. Values are expressed as OR (95% CI). Incontinence Stress type incontinence Mixed type incontinence Urge type incontinence All Severe All Severe All Severe All Severe Cups of tea/day ( ) 1.3 ( ) 1.1 ( ) 1.3 ( ) 1.3 ( ) 1.4 ( ) 1.1 ( ) 1.1 ( ) 3þ 1.3 ( ) 1.5 ( ) 1.3 ( ) 1.4 ( ) 1.5 ( ) 1.6 ( ) 1.3 ( ) 1.4 ( ) Cups of coffee/day ( ) 0.8 ( ) 1.2 ( ) 0.9 ( ) 0.9 ( ) 0.6 ( ) 1.2 ( ) 1.4 ( ) 3þ 1.0 ( ) 0.8 ( ) 1.2 ( ) 1.0 ( ) 1.0 ( ) 0.7 ( ) 1.0 ( ) 1.0 ( ) No. of glasses of alcoholic beverages/two weeks þ 1.1 ( ) 0.9 ( ) 1.1 ( ) 1.0 ( ) 0.8 ( ) 0.8 ( ) 1.0 ( ) 1.1 ( ) * Adjusted for the other factors in the table in addition to age, body mass index and smoking.

6 252 Y.S. HANNESTAD ET AL. DISCUSSION In this cross sectional study, we demonstrate that heavy smoking and intake of tea is related to incontinence while increasing hours of low intensity physical activity is associated with a decreased risk of incontinence. We also confirm the strong association between incontinence and body mass index. Because exposure and disease status are measured simultaneously, we can formulate hypotheses but not conclude whether the exposure preceded or resulted from the disease. A longitudinal cohort study design may handle this dilemma. Intervention studies are needed to test the hypothesis that the removal or reduction of the associated factors will prevent incontinence. Lifestyle factors may change over time and subjective interpretation of questions in a survey is a source of uncertainty. However, our questions concerning incontinence have been used in several studies, and the severity index and type classification have been validated 22,23,25. The size of our study, covering almost 7000 incontinent women and 21,000 controls, is a strength. Even so, some aspects of the analysis still did not reach statistical significance as a result of lack of power in small subgroups. In contrast to any incontinence, prevalence estimates of severe incontinence are shown to be relatively consistent in several studies 26 and we therefore elaborated on this group specifically. Because pathophysiology differs for each type, the associated risk factors are likely to differ accordingly. We therefore also analysed type of incontinence as outcome. We found that former and current smoking was associated with incontinence, but the effect was limited to those who smoked more than 20 cigarettes per day or had a life consumption of 15 pack years or more. Severe incontinence was weakly associated with smoking regardless of number of cigarettes. Several cross sectional studies in different age groups have found no association between smoking status and incontinence 10 12,27,28, without considering a possible dose effect or controlling for relevant confounders. Two case control studies have investigated the effect of the intensity of cigarette consumption, or of lifetime exposure, on incontinence, and these also reported a dose response relation 4,5. These studies included women who underwent urogynaecological assessment in secondary care, and their study populations may therefore be comparable to our group of severely incontinent women. The American study showed strong dose effects of current smoking on stress and urge symptoms and mixed type. An increased prevalence of incontinence among smokers has been explained in terms of strong and frequent coughing among smokers, the anti-oestrogenic effect of smoking and interference with collagen synthesis 4. As we found that the effects of smoking on incontinence were reduced when adjusted for coughing and wheezing/dyspnoea, our results partly support the first theory. However, the adjusted results show a remaining effect of smoking not mediated through airway symptoms. This has not previously been investigated in epidemiological studies. It is also noteworthy that for former smokers, the risk of incontinence was equal to or even higher than it was for current smokers. We may thus infer that a possible effect of smoking on incontinence does not seem to decrease after smoking cessation. An alternative explanation could be that women with incontinence are more likely to quit smoking than continent women are. Several cross sectional studies have found a significant association between body mass index and incontinence 3,9 12,29. Weight loss in obese women has been shown to improve incontinence symptoms 30,31. The current study elaborates on the contribution of body mass index to incontinence. In addition to confirming the strong association between body mass index and any incontinence, we show a close link with severe symptoms. The risk of severe incontinence among obese women was three times higher than it was for normal weight women. We have also demonstrated that body mass index is a risk factor for all the different types, but with different strengths of the associations; weakest for urge and strongest for mixed type. Women with a body mass index of 40 or more had an odds ratio for severe mixed symptoms that was six times higher than for women with normal weight. It is a general belief that women who are physically fit also have strong pelvic floor muscles. On the other hand, some activities may unmask symptoms of stress incontinence during the exercise. Minimal stress incontinence is common among young exercising women 16,18,32, and women taking part in highly strenuous activities more often experience leakage in connection with exercise than those involved in only moderately strenuous activities 19. We found that women who participated in low intensity activities for one hour or more per week had less incontinence than those who did not. There was, however, a tendency towards more severe incontinence and severe stress symptoms among women who reported high intensity activities, but these results were not statistically significant. What could be interpreted as a protective effect of low intensity activity on incontinence might just as well be an effect of selection: there may be a tendency among incontinent women to quit exercising because of the leakage they experience. The way in which the questions about physical activity are understood may also vary for women of different age and for different levels of activity. Some women may include only regular exercise, others heavy housework. There still remains uncertainty as to whether there is an important association between physical activity and incontinence, or whether the nature of this association is causal or adaptive. In spite of reports indicating that high caffeine intake is related to detrusor instability 13, which in turn may cause urinary incontinence, and a trial suggesting that a reduction in caffeine intake may lead to a reduction in the number of leakage episodes per day 33, no association between

7 LIFESTYLE FACTORS AND URINARY INCONTINENCE 253 coffee and incontinence has been found in cross sectional studies We investigated the effect of two caffeinated drinks: coffee and tea. Per unit of volume, the caffeine content of tea is approximately one-third of that of coffee 13. However, it was intake of tea that turned out to be positively associated with incontinence overall, while the effects of coffee were conflicting. Tea might contain components other than caffeine that might aggravate incontinence and thus account for this difference. No effect of alcohol intake on incontinence was found, and this is in agreement with previous cross sectional studies 11,12. CONCLUSIONS This large study is the first to show a dose response effect of smoking on incontinence in an unselected population and investigate the effect on the different types. We found that former and current heavy smoking (more than 20 cigarettes per day) was associated with any incontinence while severe incontinence was weakly associated with smoking regardless of the number of cigarettes. A significant dose response effect was found for mixed type. The study also suggests an effect of smoking additional to that of coughing. The association between body mass index and any incontinence was confirmed. In addition, a relation between body mass index and severe symptoms was shown as well as an effect on all subtypes. Increasing levels of low intensity physical activity had a weak and negative association with incontinence. Tea drinkers were at higher risk for all types of incontinence. We found no important effects of high intensity activity, intake of alcohol or coffee. Intervention studies are needed to investigate whether a reduction of number of cigarettes smoked or intake of tea will reduce incontinence symptoms. The relation between physical activity and incontinence should also be further explored. Acknowledgements The Nord-Trøndelag Health Study (The HUNT Study) is a collaboration between the HUNT Research Centre, Faculty of Medicine, Norwegian University of Science and Technology (NTNU), Verdal, the Norwegian Institute of Public Health, and Nord-Trøndelag County Council. The EPINCONT Study was also supported by the Research Council of Norway. References 1. Hunskaar S, Arnold EP, Burgio K, Diokno AC, Herzog AR, Mallett VT. Epidemiology and natural history of urinary incontinence. Int Urogynecol J 2000;11: Rortveit G, Hannestad YS, Daltveit AK, Hunskaar S. Age- and typedependent effects of parity on urinary incontinence: the Norwegian EPINCONT study. Obstet Gynecol 2001;98: Thom DH, van den Eeden SK, Brown JS. Evaluation of parturition and other reproductive variables as risk factors for urinary incontinence in later life. Obstet Gynecol 1997;90: Bump RC, McClish DK. 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