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Risk factors for urinary incontinence among middle-aged women

      Objective

      The purpose of this study was to identify risk factors for urinary incontinence in middle-aged women.

      Study design

      We conducted a cross-sectional analysis of 83,355 Nurses' Health Study II participants. Since 1989, women have provided health information on mailed questionnaires; in 2001, at the ages 37 to 54 years, information on urinary incontinence was requested. We examined adjusted odds ratios of incontinence using logistic regression.

      Results

      Forty-three percent of the women reported incontinence. After adjustment, black (odds ratio, 0.49; 95% CI, 0.40-0.60) and Asian-American women (odds ratio, 0.57; 95% CI, 0.46-0.72) were at reduced odds of severe incontinence compared with white women. Increased age, body mass index, parity, current smoking, type 2 diabetes mellitus, and hysterectomy all were associated positively with incontinence. Women who were aged 50 to 54 years had 1.81 times the odds of severe incontinence compared with women who were <40 years old (95% CI, 1.66-1.97); women with a body mass index of ≥30 kg/m2 had 3.10 times the odds of severe incontinence compared with a body mass index of 22 to 24 kg/m2 (95% CI, 2.91-3.30).

      Conclusion

      Urinary incontinence is highly prevalent among these middle-aged women. Potential risk factors include age, race/ethnicity, body mass index, parity, smoking, diabetes mellitus, and hysterectomy.

      Key words

      Urinary incontinence affects women of all ages. In middle-aged women, the prevalence estimates range from 30% to 40% and rise to approximately 50% in older women.
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      Incontinence is associated with embarrassment and anxiety, which may negatively affect social participation, intimate relationships, and self-esteem.
      • Herzog A.R.
      • Diokno A.C.
      • Fultz N.H.
      Urinary incontinence: medical and psychosocial aspects.
      • Wyman J.F.
      • Harkins S.W.
      • Fantl J.A.
      Psychosocial impact of urinary incontinence in the community-dwelling population.
      Moreover, the economic impact of incontinence is substantial; in 2000, the estimated total cost of urinary incontinence in the United States was $19.5 billion.
      • Hu T.W.
      • Wagner T.H.
      • Bentkover J.D.
      • Leblanc K.
      • Zhou S.Z.
      • Hunt T.
      Costs of urinary incontinence and overactive bladder in the United States: a comparative study.
      Although several large-scale studies have focused on risk factors for urinary incontinence in older women,
      • Brown J.S.
      • Grady D.
      • Ouslander J.G.
      • Herzog A.R.
      • Varner R.E.
      • Posner S.F.
      Prevalence of urinary incontinence and associated risk factors in postmenopausal women: Heart & Estrogen/Progestin Replacement Study (HERS) research group.
      • Brown J.S.
      • Vittinghoff E.
      • Wyman J.F.
      • Stone K.L.
      • Nevitt M.C.
      • Ensrud K.E.
      • et al.
      Urinary incontinence: Does it increase risk for falls and fractures? Study of Osteoporotic Fractures Research group.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      limited data are available in middle-aged women. Primary risk factors for incontinence may differ over a woman's lifetime (eg, parity appears to be a more important risk factor in women aged <60 years compared with older women),
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      • Brown J.S.
      • Nyberg L.M.
      • Kusek J.W.
      • Burgio K.L.
      • Diokno A.C.
      • Foldspang A.
      • et al.
      Proceedings of the National Institute of Diabetes and Digestive and Kidney Diseases International Symposium on epidemiologic issues in urinary incontinence in women.
      thus it is important to explore risk factors for incontinence in younger women. Moreover, several potentially important predictors of incontinence have not been investigated sufficiently in any age group. For example, although limited findings are suggestive of a positive association,
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      • Hannestad Y.S.
      • Rortveit G.
      • Daltveit A.K.
      • Hunskaar S.
      Are smoking and other lifestyle factors associated with female urinary incontinence? The Norwegian EPINCONT study.
      • Bump R.C.
      • McClish D.K.
      Cigarette smoking and urinary incontinence in women.
      the relation between incontinence and cigarette smoking, which is a common behavior, has not been well examined. Likewise, observed racial differences in incontinence prevalence in limited studies suggest that additional analysis of race as a potential modifier of incontinence risk is warranted.
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      Studies of the association between hysterectomy and incontinence have reported conflicting results,
      • Griffith-Jones M.D.
      • Jarvis G.J.
      • McNamara H.M.
      Adverse urinary symptoms after total abdominal hysterectomy: Fact or fiction?.
      • Carlson K.J.
      • Miller B.A.
      • Fowler Jr., F.J.
      The Maine Women's Health study: I, outcomes of hysterectomy.
      • Brown J.S.
      • Sawaya G.
      • Thom D.H.
      • Grady D.
      Hysterectomy and urinary incontinence: a systematic review.
      which indicate a need for further study. Finally, although the role of elevated body mass index (BMI) in the development of incontinence has been explored in numerous studies,
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      information about the role of type 2 diabetes mellitus, one of its potential sequelae, is scarce. Thus, we examined these potential risk factors for urinary incontinence in a large cohort of women aged 37 to 54 years from the Nurses' Health Study II.

      Material and methods

      Study population

      The Nurses' Health Study II began in 1989 when 116,671 female, registered nurses completed a mailed questionnaire. At enrollment, study participants were 25 to 42 years of age and lived in 14 states. Follow-up questionnaires are mailed biennially to update information on lifestyle and health; information on urinary incontinence was first requested in 2001. Follow-up remains high, at 90% to date (including a small proportion of known deaths). The study was approved by the Institutional Review Board of Brigham and Women's Hospital.

      Information on urinary incontinence

      In 2001, study participants were asked, “During the last 12 months, how often have you leaked or lost control of your urine?”; response categories were never, less than once a month, once a month, 2 to 3 times a month, approximately once a week, and almost every day. Women who reported that they lost urine were then asked, “When you lose your urine, how much usually leaks?”; response categories were a few drops, enough to wet underwear, enough to wet outer clothing, and enough to wet the floor. Self-reported frequency and quantity of urinary incontinence have been shown to be highly reproducible in a similar population of nurses.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      We created 2 case groups: (1) occasional urine loss, which was defined as leaking 1 to 3 times per month, and (2) frequent urine loss, which was defined as leaking at least once a week. We further defined severe incontinence as frequent urine loss of at least enough to wet the underwear. This definition was based on a validated severity index, which correlates well with pad weights.
      • Sandvik H.
      • Hunskaar S.
      • Seim A.
      • Hermstad R.
      • Vanvik A.
      • Bratt H.
      Validation of a severity index in female urinary incontinence and its implementation in an epidemiological survey.
      Control subjects were defined as women who reported never leaking urine in the past 12 months or leaking less than once a month and only a few drops. In a given analysis, women who did not meet the relevant case/control definition were not included in that analysis; for example, in analyses of occasional urine loss, women with frequent urine loss were not included as cases or control subjects but were included in separate analyses of frequent urine loss.

      Information on risk factors

      In general, the risk factor information was based on data that were collected through the 1999 questionnaire. Although this is a cross-sectional study, we attempted to reduce the likelihood of reverse causation by imposing a short lag period between the report of risk factors and of incontinence; we chose a 2-year lag period because we also did not want to assess risk factors that were too far remote from outcome status. Parity, a history of oral contraceptive use, and a history of smoking were derived with data from all questionnaires between 1989 and 1999. BMI was calculated with self-reported height from 1989 and self-reported weight in 1999. Information on race/ethnicity was obtained in 1989, when participants were asked to “mark their major ancestry,” with response options of Southern European/Scandinavian/other white, black, Hispanic, Asian, and other ancestry.
      In addition, on each questionnaire, participants were asked whether they had received a physician-diagnosis of diabetes mellitus or whether they had undergone a hysterectomy. Validation studies have established that these nurses' self-reports of health and medical conditions, including type 2 diabetes mellitus, are highly valid.
      • Carey V.J.
      • Walters E.E.
      • Colditz G.A.
      • Soloman C.G.
      • Willett W.C.
      • Rosner B.A.
      • et al.
      Body fat distribution and risk of non-insulin-dependent diabetes mellitus in women: the Nurses' Health study.

      Data analysis

      Of the 101,294 women who returned the 2001 questionnaire, we restricted analyses to women who answered the incontinence questions (n = 86,006), which were included only on initial questionnaire mailings (after the initial mailing, the questionnaire is abbreviated to encourage participation), and who provided information on parity (n = 83,355), which is a key risk factor and potential confounder. Thus, the sample for analysis included 83,355 women. Characteristics of participants that were analyzed were quite similar to those of the entire cohort (eg, the mean age was 44.8 years in both the study sample and cohort; 8.7% of the population that was analyzed currently smoked compared with 8.6% in the entire cohort). Separate multivariate logistic regression models were used to calculate odds ratios and 95% CIs for each of the incontinence case definitions. In all models, the primary risk factors that were examined were included: age (<40, 40-44, 45-49, 50-54 years), race/ethnicity (white, black, Hispanic, Asian, and other race/ethnicity), BMI (<22, 22-24, 25-29, ≥30 kg/m2), type 2 diabetes mellitus (yes/no), hysterectomy (yes/no), parity (0, 1, 2, ≥3 births), oral contraceptive use (never/former/current), and smoking (never/former/current). In addition, we adjusted for the following potential confounding factors in all models: stroke (yes/no), significant functional limitations (yes/no; defined as significant limitations in climbing 1 flight of stairs, walking 1 block, or bathing/dressing), and menopausal status (premenopausal, postmenopausal/never used hormone therapy, formerly used hormone therapy, currently use hormone therapy).

      Results

      The mean age of the study population was 44.8 years (Table I). Self-reported incontinence was highly prevalent: 43% of women reported leaking urine at least once a month. Black women and Asian women reported urinary incontinence less frequently than white women. Overall, of women who leaked urine at least once a month, 6% reported leaking enough to wet their outer clothing or the floor.
      Table ICharacteristics of the Nurses' Health Study II population
      Race/ethnicity (n)
      CharacteristicTotal (n)WhiteBlackHispanicAsianOther/missing
      Leaked urine in past 12 mo
       Never or <1 per mo48,648 (57%)44,715 (56%)773 (64%)651 (55%)920 (68%)1589 (58%)
       1-3 Times per month14,959 (17%)13,918 (18%)164 (14%)218 (19%)194 (14%)465 (17%)
       1/Wk to daily22,063 (26%)20,566 (26%)270 (22%)305 (26%)237 (18%)685 (25%)
      Quantity of urine leaked
      Quantity of urine leaked includes only the women who reported leaking at least once a month.
       A few drops13,074 (41%)12,208 (41%)126 (35%)164 (36%)165 (47%)411 (40%)
       Enough to wet underwear17,075 (53%)15,867 (53%)225 (62%)265 (58%)174 (49%)544 (53%)
       Enough to wet outer clothing/floor1,836 (6%)1,718 (6%)13 (4%)26 (6%)15 (4%)64 (6%)
      Mean age (y)44.844.845.944.745.044.3
      BMI (kg/m2)
       <2219,998 (23%)18,602 (23%)90 (7%)218 (18%)449 (33%)639 (23%)
       22-2421,892 (25%)20,303 (26%)189 (16%)267 (23%)447 (33%)686 (25%)
       25-2921,689 (25%)19,982 (25%)385 (32%)341 (29%)274 (20%)707 (26%)
       ≥3018,748 (22%)17,316 (22%)470 (39%)291 (25%)103 (8%)568 (21%)
      Type 2 diabetes mellitus5,539 (6%)5,021 (6%)117 (10%)116 (10%)130 (10%)155 (6%)
      Hysterectomy13,181 (15%)12,103 (15%)329 (27%)186 (16%)130 (10%)433 (16%)
      Parity
       None15,371 (18%)14,157 (18%)253 (21%)223 (19%)302 (22%)436 (16%)
       1 Birth11,724 (14%)10,725 (13%)273 (22%)168 (14%)218 (16%)340 (12%)
       2 Births32,906 (38%)30,546 (38%)397 (33%)427 (36%)483 (35%)1053 (38%)
       ≥3 Births23,354 (27%)21,753 (27%)225 (18%)284 (24%)274 (20%)818 (30%)
      Oral contraceptive use
       Never10,738 (12%)9,712 (12%)126 (10%)150 (13%)386 (28%)364 (13%)
       Former62,124 (72%)57,658 (73%)927 (76%)832 (71%)750 (55%)1957 (71%)
       Current11,627 (14%)10,818 (14%)121 (10%)166 (14%)145 (11%)377 (14%)
      Cigarette smoking
       Never56,156 (65%)51,458 (65%)864 (71%)855 (72%)1154 (85%)1825 (66%)
       Former22,121 (26%)20,834 (26%)227 (19%)265 (22%)152 (11%)643 (23%)
       Current7,522 (9%)7,018 (9%)122 (10%)57 (5%)52 (4%)273 (10%)
      Percentages may not equal 100% because of missing values.
      Quantity of urine leaked includes only the women who reported leaking at least once a month.
      After adjustment for potential confounding factors, there was a highly significant trend of increasing prevalence of incontinence with increasing age (Table II). For example, women who were aged 50 to 54 years had almost twice the odds of severe incontinence compared with women who were aged <40 years. In addition, black women (odds ratio [OR], 0.49; 95% CI, 0.40-0.60) and Asian women (OR, 0.57; 95% CI, 0.46-0.72) were at significantly reduced odds of severe incontinence compared with white women. Hispanic women did not significantly differ from white women in their prevalence of incontinence.
      Table IIOdds of urinary incontinence according to potential risk factors
      Occasional incontinence
      Occasional incontinence defined as leaking 1 to 3 times per month; frequent incontinence defined as at least once per week; severe incontinence defined as frequent and at least enough to wet the underwear.
      Frequent incontinence
      Occasional incontinence defined as leaking 1 to 3 times per month; frequent incontinence defined as at least once per week; severe incontinence defined as frequent and at least enough to wet the underwear.
      Severe incontinence
      Occasional incontinence defined as leaking 1 to 3 times per month; frequent incontinence defined as at least once per week; severe incontinence defined as frequent and at least enough to wet the underwear.
      VariableCases
      Case numbers may not add up because some participants were missing data for given risk factors.
      OR
      Adjusted for age, race/ethnicity, BMI, parity, use of oral contraceptives, cigarette smoking, diabetes mellitus, hysterectomy, stroke, functional limitations, menopausal status, and postmenopausal hormone therapy.
      (95% CI)
      Cases
      Case numbers may not add up because some participants were missing data for given risk factors.
      OR
      Adjusted for age, race/ethnicity, BMI, parity, use of oral contraceptives, cigarette smoking, diabetes mellitus, hysterectomy, stroke, functional limitations, menopausal status, and postmenopausal hormone therapy.
      (95% CI)
      Cases
      Case numbers may not add up because some participants were missing data for given risk factors.
      OR
      Adjusted for age, race/ethnicity, BMI, parity, use of oral contraceptives, cigarette smoking, diabetes mellitus, hysterectomy, stroke, functional limitations, menopausal status, and postmenopausal hormone therapy.
      (95% CI)
      Age (y)
       <402,270Reference3,174Reference1,293Reference
       40-444,6641.20 (1.13,1.28)6,4751.16 (1.10,1.22)3,1251.34 (1.24,1.44)
       45-494,9491.29 (1.21,1.37)7,6851.32 (1.26,1.40)4,1461.69 (1.57,1.81)
       50-542,6231.38 (1.28,1.48)4,1051.35 (1.27,1.44)2,3541.81 (1.66,1.97)
      P for trend<.0001<.0001<.0001
      Race/ethnicity
       White13,512Reference20,011Reference10,186Reference
       Black1590.58 (0.48,0.69)2540.56 (0.48,0.65)1280.49 (0.40,0.60)
       Hispanic2071.09 (0.92,1.28)2861.01 (0.87,1.17)1591.10 (0.91,1.33)
       Asian1840.78 (0.67,0.92)2240.71 (0.61,0.83)870.57 (0.46,0.72)
      BMI (kg/m2)
       <222,8810.80 (0.76,0.85)3,6230.78 (0.74,0.82)1,4760.70 (0.66,0.76)
       22-243,574Reference4,703Reference2,150Reference
       25-293,9051.26 (1.20,1.33)5,6511.37 (1.31,1.44)2,9311.52 (1.43,1.62)
       ≥303,5471.80 (1.70,1.91)6,5442.43 (2.31,2.55)3,8883.10 (2.91,3.30)
      P for trend<.0001<.0001<.0001
      Type 2 diabetes mellitus9871.08 (1.00,1.17)1,6781.18 (1.10,1.26)9901.30 (1.20,1.41)
      Hysterectomy2,2951.23 (1.13,1.33)4,1471.43 (1.33,1.54)2,3731.59 (1.45,1.73)
      Parity
       None2,142Reference3,538Reference1,587Reference
       1 Birth2,0381.43 (1.33,1.53)3,0051.26 (1.19,1.34)1,5601.48 (1.36,1.60)
       2 Births6,0381.58 (1.49,1.67)8,7831.41 (1.35,1.48)4,5661.67 (1.57,1.79)
       ≥3 Births4,2881.59 (1.50,1.69)6,1131.41 (1.34,1.49)3,2051.69 (1.58,1.82)
      Oral contraceptive use
       Never1,735Reference2,465Reference1,252Reference
       Former10,9541.18 (1.12,1.26)16,3021.21 (1.15,1.28)8,5561.20 (1.12,1.29)
       Current1,7801.04 (0.96,1.12)2,5911.13 (1.05,1.21)1,0710.97 (0.89,1.07)
      Cigarette smoking
       Never9,629Reference13,399Reference6,707Reference
       Former3,7431.00 (0.96,1.05)5,9151.12 (1.08,1.16)3,0121.11 (1.05,1.17)
       Current1,1150.91 (0.85,0.98)2,0971.20 (1.13,1.28)1,1881.34 (1.25,1.45)
      Occasional incontinence defined as leaking 1 to 3 times per month; frequent incontinence defined as at least once per week; severe incontinence defined as frequent and at least enough to wet the underwear.
      Case numbers may not add up because some participants were missing data for given risk factors.
      Adjusted for age, race/ethnicity, BMI, parity, use of oral contraceptives, cigarette smoking, diabetes mellitus, hysterectomy, stroke, functional limitations, menopausal status, and postmenopausal hormone therapy.
      BMI was associated strongly with incontinence (Table II). In a comparison of obese women (BMI, ≥30 kg/m2) with those women with a BMI of 22 to 24 kg/m2, the odds of occasional incontinence were almost 2-fold higher (OR, 1.80; 95% CI, 1.70-1.91), which rose to a 3-fold higher odds of severe incontinence (OR, 3.10; 95% CI, 2.91-3.30). Women with a BMI <22 kg/m2 had significantly reduced odds of incontinence compared with women with a BMI of 22 to 24 kg/m2 (OR, 0.70-0.80 across case definitions). After being adjusted for BMI and many other potential confounding factors, type 2 diabetes mellitus was associated with a modest but statistically significant increase in the odds of frequent incontinence (OR, 1.18; 95% CI, 1.10-1.26) and severe incontinence (OR, 1.30; 95% CI, 1.20-1.41).
      Parity, as expected, was associated positively with incontinence (Table II). Women with 2 live births had as much as 67% increased odds compared with nulliparous women. In addition, hysterectomy was associated with a significant increase in incontinence (OR, 1.59 for severe incontinence). We found modest, but significantly higher, odds of incontinence for women who had used oral contraceptives in the past, although no consistent association was observed for current users of oral contraceptives.
      Finally, both former and current cigarette smoking were associated positively with frequent and with severe incontinence (Table II), with a somewhat stronger relation among women who were currently smoking (OR, 1.34; 95% CI, 1.25-1.45).

      Comment

      Among the >80,000 participants of the Nurses' Health Study II who were aged 37 to 54 years, 43% reported leaking urine at least once per month during the previous year. After multivariate adjustment, the prevalence of incontinence was higher in white women than in Asian or black women. In addition, the prevalence of incontinence increased with increasing age and BMI and was higher in parous women as compared with nulliparous women. We found that current smoking, diabetes mellitus, and hysterectomy appeared to elevate the odds of frequent or severe incontinence modestly in these middle-aged women.
      Overall, in studies of young and middle-aged women that used similar definitions of incontinence to ours, prevalence reports are generally consistent with those that we observed. Chiarelli et al
      • Chiarelli P.
      • Brown W.
      • McElduff P.
      Leaking urine: prevalence and associated factors in Australian women.
      found a 36% prevalence of any self-reported leakage during the past year among 14,070 Australian women who were aged 45 to 50 years. In a review of 13 general population studies, Hunskaar et al
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      reported a 30% to 40% prevalence of incontinence among middle-aged women. Indeed, although our nurse-participants likely have better access to healthcare and health knowledge than those in many other studies (as reflected in a lower prevalence of smoking in our subjects compared with the general population

      Behavioral Risk Factor Surveillance System (BRFSS) [database on the Internet]. Atlanta (GA): National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention [updated 2004 Dec 20; cited 2005 Jan]. Available from: http://www.cdc.gov/brfss/.

      ), the prevalence of many health characteristics (such as obesity, hysterectomy

      Behavioral Risk Factor Surveillance System (BRFSS) [database on the Internet]. Atlanta (GA): National Center for Chronic Disease Prevention and Health Promotion, Centers for Disease Control and Prevention [updated 2004 Dec 20; cited 2005 Jan]. Available from: http://www.cdc.gov/brfss/.

      and type 2 diabetes mellitus
      • Narayan K.M.
      • Boyle J.P.
      • Thompson T.J.
      • Sorensen S.W.
      • Williamson D.F.
      Lifetime risk for diabetes mellitus in the United States.
      ) are quite similar to national estimates.
      It is interesting to note that the overall prevalence of incontinence in these middle-aged women is somewhat higher than the prevalence of 34.1% that we previously reported in a study of older nurses (aged 50-75 years).
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      Our findings are consistent with the existing literature, which reports that the prevalence of incontinence peaks in mid life and then declines somewhat before rising again at older ages.
      • Sandvik H.
      • Hunskaar S.
      • Seim A.
      • Hermstad R.
      • Vanvik A.
      • Bratt H.
      Validation of a severity index in female urinary incontinence and its implementation in an epidemiological survey.
      The explanation for such a pattern is unclear. It may be physiologic or may be due to a cohort effect. For example, perhaps younger generations are more willing to discuss certain health issues and thus report their incontinence more often than older women.
      After adjusting for various risk factors, we found a lower prevalence of frequent urine leaking among black and Asian women relative to white women. Data on incontinence prevalence across racial groups are scarce, and the heterogeneity that may exist within racial categories could limit the validity of comparisons across studies. Nevertheless, 2 studies have reported a higher prevalence of incontinence in middle-aged white women compared with black women. In the Study of Women's Health Across the Nation (SWAN),
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      black women were 70% less likely than white women to report any incontinence (multivariate-adjusted relative risk, 0.31; 95% CI, 0.23-0.40); among 486 women aged 45 to 53 years, Burgio et al
      • Burgio K.L.
      • Matthews K.A.
      • Engel B.T.
      Prevalence, incidence and correlates of urinary incontinence in healthy, middle-aged women.
      observed a higher prevalence of incontinence in white compared with black women (32.0% vs 17.9%; P < .01, without adjustment for risk factor differences). In addition, in our previous study among older nurses (50-75 years), we also found significantly lower prevalence of urinary incontinence among black women.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      Because incontinence was self-reported in all these studies, it is possible that observed differences in prevalence are mediated by differences in reporting urinary symptoms among various racial groups. However, we believe that the likelihood of differential reporting by race is lessened in our cohort of health professionals with comparable education and socioeconomic status. Moreover, structural and functional differences in the urethra and its support systems have been demonstrated between black and white women.
      • Graham C.A.
      • Mallett V.T.
      Race as a predictor of urinary incontinence and pelvic organ prolapse.
      • Howard D.
      • Delancey J.O.
      • Tunn R.
      • Ashton-Miller J.A.
      Racial differences in the structure and function of the stress urinary continence mechanism.
      • Baragi R.V.
      • Delancey J.O.
      • Caspari R.
      • Howard D.H.
      • Ashton-Miller J.A.
      Differences in pelvic floor area between African American and European American women.
      In addition, our finding of a 43% (95% CI, 28%-54%) decreased prevalence of severe leaking in Asian women compared with white women is comparable to the 42% (95% CI, 14%-61%) decreased prevalence of incontinence in Japanese women in the SWAN study
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      and is generally consistent with the lower prevalence that was found in our previous study of older nurses.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      We did not observe significant differences in incontinence between white and Hispanic women, as found by Sampselle et al
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      and as we found in our cohort of older nurses.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      Given the paucity of epidemiologic or biologic data on incontinence in different racial or ethnic groups, it is difficult to conclude the reason that findings on race/ethnicity may vary across studies. Further research is needed.
      Higher BMI generally is considered an incontinence risk factor,
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      and a positive association between BMI and incontinence has been observed in other cross-sectional studies of middle-aged women. Among 14,070 women who were aged 45 to 50 years in the Women's Health Australia project, obese women (BMI, 30-40 kg/m2) had an increased risk (relative risk, 2.05; 95% CI 1.70-2.46) of any incontinence compared with a BMI of <20 kg/m2.
      • Chiarelli P.
      • Brown W.
      • McElduff P.
      Leaking urine: prevalence and associated factors in Australian women.
      These results are consistent with our finding of a 2.43-fold (95% CI, 2.31-2.55) increase in the prevalence of frequent incontinence in obese women. In another study, a 5-unit increase in BMI was associated with a 30% (95% CI, 10%-60%) increased risk of severe incontinence among 48-year-old women.
      • Kuh D.
      • Cardozo L.
      • Hardy R.
      Urinary incontinence in middle aged women: childhood enuresis and other lifetime risk factors in a British prospective cohort.
      In the SWAN study, Sampselle et al
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      observed a 5% (95% CI, 4%-7%) increased risk of any incontinence with each 1-unit increase in BMI. It is possible that episodes of urine leaking led to increases in BMI by dissuading some women from being physically active. However, there is evidence that BMI has both chronic and acute effects on urinary function. Noblett et al
      • Noblett K.L.
      • Jensen J.K.
      • Ostergard D.R.
      The relationship of body mass index to intra-abdominal pressure as measured by multichannel cystometry.
      found strong correlations between BMI and intra-abdominal pressure (r = .76; P < .0001) and intravesical pressure (r = .71; P < .0001), which suggests that obesity may cause a chronic state of increased pressure that stresses the pelvic floor. Furthermore, massive weight loss in morbidly obese women has been associated with a decrease in stress incontinence (61.2% prevalence before bariatric surgery vs 11.6% prevalence after weight loss stabilization; P < .001).
      • Deitel M.
      • Stone E.
      • Kassam H.A.
      • Wilk E.J.
      • Sutherland D.J.
      Gynecologic-obstetric changes after loss of massive excess weight following bariatric surgery.
      Type 2 diabetes mellitus has not been studied well as a possible incontinence risk factor. However, in the SWAN study,
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      those women with diabetes mellitus had a 53% (95% CI, 12%-110%) increased risk of any incontinence; among women older than 60 years, diabetes mellitus has been associated with a 30% to 70% increased risk of incontinence.
      • Brown J.S.
      • Nyberg L.M.
      • Kusek J.W.
      • Burgio K.L.
      • Diokno A.C.
      • Foldspang A.
      • et al.
      Proceedings of the National Institute of Diabetes and Digestive and Kidney Diseases International Symposium on epidemiologic issues in urinary incontinence in women.
      Moreover, several possible mechanisms suggest that diabetes mellitus may affect continence adversely. Hyperglycemia has been associated with increased urine volume and detrusor overactivity; the microvascular complications of diabetes mellitus may damage innervation of the bladder, alter detrusor muscle function, or predispose to impaired bladder sensation and overflow incontinence.
      • Brown J.S.
      • Nyberg L.M.
      • Kusek J.W.
      • Burgio K.L.
      • Diokno A.C.
      • Foldspang A.
      • et al.
      Proceedings of the National Institute of Diabetes and Digestive and Kidney Diseases International Symposium on epidemiologic issues in urinary incontinence in women.
      Given the increasing prevalence of type 2 diabetes mellitus in the United States, further research on this issue is warranted.
      Our finding of a modest increased odds of frequent leaking in women with a previous hysterectomy is similar to that of Miller et al,
      • Miller Y.D.
      • Brown W.J.
      • Russell A.
      • Chiarelli P.
      Urinary incontinence across the lifespan.
      who reported higher incontinence severity scores in women with a history of hysterectomy than without hysterectomy (adjusted OR, 1.49; 95% CI, 1.01-2.22) among incontinent women aged 45 to 50 years. However, this is a controversial issue, and hysterectomy could be a marker of pelvic floor dysfunction. Indeed, study results are conflicting. Two prospective studies reported no association between hysterectomy and incontinence in women under age 60 years but time since hysterectomy was <2 years and neither study controlled for potential confounders.
      • Griffith-Jones M.D.
      • Jarvis G.J.
      • McNamara H.M.
      Adverse urinary symptoms after total abdominal hysterectomy: Fact or fiction?.
      • Carlson K.J.
      • Miller B.A.
      • Fowler Jr., F.J.
      The Maine Women's Health study: I, outcomes of hysterectomy.
      In a meta-analysis of 11 observational studies, Brown et al
      • Brown J.S.
      • Sawaya G.
      • Thom D.H.
      • Grady D.
      Hysterectomy and urinary incontinence: a systematic review.
      found a significant increase in the odds of incontinence after hysterectomy in women aged ≥60 years (summary OR, 1.6; 95% CI, 1.4-1.8), but not in women younger than age 60 years (summary OR, 1.1; 95% CI, 1.0-1.4), which suggests that hysterectomy may have long-term, but not short-term, effects on continence.
      Childbearing is an established incontinence risk factor.
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      In our study, the first 2 births seemed to account for most of the effect; the odds ratios for ≥3 births compared with none were nearly identical to the odds ratios for 2 births. Our findings are comparable with the SWAN study, which found a 62% (95% CI, 31%-101%) increased risk of any leakage in parous women compared with nulliparous women aged 42 to 52 years.
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      In the Women's Health Australia project, Chiarelli et al
      • Chiarelli P.
      • Brown W.
      • McElduff P.
      Leaking urine: prevalence and associated factors in Australian women.
      observed similar odds of leaking urine among women with 1 childbirth (adjusted OR,1.58; 95% CI, 1.29-1.93) and women with 2 childbirths (adjusted OR, 1.66; 95% CI, 1.41-1.95). Additionally, as expected,
      • Brown J.S.
      • Nyberg L.M.
      • Kusek J.W.
      • Burgio K.L.
      • Diokno A.C.
      • Foldspang A.
      • et al.
      Proceedings of the National Institute of Diabetes and Digestive and Kidney Diseases International Symposium on epidemiologic issues in urinary incontinence in women.
      the odds ratios in these younger women were somewhat higher than those that we had found in our cohort of older nurses.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      We did not collect information on mode of delivery. Cesarean delivery may avoid trauma to the muscles and connective tissue of the pelvic floor and damage to the pudendal and pelvic nerves that are associated with vaginal delivery
      • Hunskaar S.
      • Arnold E.P.
      • Burgio K.
      • Diokno A.C.
      • Herzog A.R.
      • Mallett V.T.
      Epidemiology and natural history of urinary incontinence.
      ; consequently, the risks we observed for parity may underestimate true relations.
      Current smoking appeared to increase frequent or severe leaking urine in our cohort by 20% and 34%, respectively. Similarly, Sampselle et al
      • Sampselle C.M.
      • Harlow S.D.
      • Skurnick J.
      • Brubaker L.
      • Bondarenko I.
      Urinary incontinence predictors and life impact in ethnically diverse perimenopausal women.
      reported a 38% (95% CI, 4%-82%) increased risk of moderate/severe incontinence among current smokers relative to never smokers in the SWAN study. After adjustment for multiple variables, Hannestad et al
      • Hannestad Y.S.
      • Rortveit G.
      • Daltveit A.K.
      • Hunskaar S.
      Are smoking and other lifestyle factors associated with female urinary incontinence? The Norwegian EPINCONT study.
      observed a 40% (95% CI, 20%-60%) increased risk of severe incontinence in current smokers relative to never smokers among 27,936 women who were aged 20 to 64 years in the Norwegian Epidemiology of Incontinence study. There are numerous potential pathways through which cigarette smoking may affect continence
      • Bump R.C.
      • McClish D.K.
      Cigarette smoking and urinary incontinence in women.
      : smoker's cough may result in damage to the urethral sphincter mechanism; decreases in collagen synthesis that are associated with smoking may weaken pelvic support structures; and smoking-related diseases (such as vascular disease, asthma, and obstructive pulmonary disease) may have indirect or direct effects on bladder and urethra function.
      Some limitations should be considered. All information on urine leaking was self-reported. However, we found that the responses of a similar group of nurses to these questions on leaking urine were highly reliable.
      • Grodstein F.
      • Fretts R.
      • Lifford K.
      • Resnick N.
      • Curhan G.
      Association of age, race, and obstetric history with urinary symptoms among women in the Nurses' Health study.
      In addition, several studies in educated populations have established the high validity of self-reported incontinence compared with clinical assessment.
      • Diokno A.C.
      • Brock B.M.
      • Herzog A.R.
      • Bromberg J.
      Medical correlates of urinary incontinence in the elderly.
      • Herzog A.R.
      • Fultz N.H.
      Prevalence and incidence of urinary incontinence in community-dwelling populations.
      Moreover, self-reported severity of incontinence, based on the frequency and amount of urine leakage, has been found to correlate highly with pad weights.
      • Sandvik H.
      • Hunskaar S.
      • Seim A.
      • Hermstad R.
      • Vanvik A.
      • Bratt H.
      Validation of a severity index in female urinary incontinence and its implementation in an epidemiological survey.
      We did not have information on type of incontinence; thus, we could not explore whether risk factors vary for stress, urge, or mixed incontinence. However, the odds ratios reported here represent the average effects of suspected risk factors on stress and urge incontinence and thus have broad public health importance. Most importantly, in this cross-sectional study, we did not have information on incident incontinence, which prevented the establishment of temporal relations between potential risk factors and incontinence. For example, it is possible that incontinence leads to high BMI (eg, if women with incontinence stop exercising and gain weight) rather than that high BMI leads to incontinence. To partially address this issue, we identified potential risk factors from information that was reported 2 years before the reports of incontinence; nonetheless, this would likely address this problem only modestly because many of the women may have longstanding incontinence (ie, substantially >2 years before their reports). However, in the analysis of some risk factors (eg, age, race/ethnicity, parity), this limitation would not be relevant.
      Overall, although urinary incontinence is relatively common in middle-aged women, there have been little available data on incontinence risk factors in this age group. In this cross-sectional analysis of women aged 37 to 54 years, we found that age, race/ethnicity, BMI, diabetes mellitus, smoking, parity, and hysterectomy appear to be associated with incontinence. Future studies are needed to further explore risk factors for incontinence.

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