American Journal of Epidemiology
Copyright ª 2006 by the Johns Hopkins Bloomberg School of Public Health
All rights reserved; printed in U.S.A.
Vol. 165, No. 3
DOI: 10.1093/aje/kwk018
Advance Access publication November 28, 2006
Original Contribution
Factors Associated with Prevalent and Incident Urinary Incontinence in a Cohort
of Midlife Women: A Longitudinal Analysis of Data
Study of Women’s Health Across the Nation
L. Elaine Waetjen1, Shanmei Liao2, Wesley O. Johnson3, Carolyn M. Sampselle4, Barbara
Sternfield5, Siobán D. Harlow6, and Ellen B. Gold7 for the Study of Women’s Health Across the
Nation
1
Department of Obstetrics and Gynecology, University of California, Davis, CA.
Department of Statistics, University of California, Davis, CA.
3
Department of Statistics, University of California, Irvine, CA.
4
School of Nursing, University of Michigan, Ann Arbor, MI.
5
Kaiser Permanente Department of Research, Oakland, CA.
6
Department of Epidemiology, School of Public Health, University of Michigan, Ann Arbor, MI.
7
Department of Public Health Sciences, University of California, Davis, CA.
2
Received for publication February 1, 2006; accepted for publication July 5, 2006.
To compare the characteristics of and baseline factors associated with prevalent and incident urinary incontinence in a diverse cohort of midlife women, the authors analyzed the baseline and first five annual follow-up visits
of the Study of Women’s Health Across the Nation (SWAN), 1995–2001. From responses to annual questionnaires, the authors defined prevalent incontinence as at least monthly incontinence reported at baseline and
incident incontinence as at least monthly incontinence first reported over follow-up. They used multiple logistic
regression for their comparison. The mean age of their cohort at baseline was 45.8 (standard deviation: 2.7) years.
Prevalent incontinence was 46.7%, and the average incidence was 11.1% per year. Most women reported stress,
but a higher proportion developed urge incontinence (15.9% vs. 7.6% at baseline). African Americans (29.5%) and
Hispanics (27.5%) had the lowest prevalence of incontinence; African Americans (11.6%) and Caucasians (13.4%)
had the highest average annual incidence. Parity, diabetes, fibroids, and poor social support were associated with
prevalent incontinence, while high body mass index, high symptom sensitivity, and poor health were associated
with incident incontinence. In midlife women, incident incontinence is mild with different characteristics and baseline risk factors; overweight women have a higher risk of developing incontinence.
cohort studies; incidence; middle aged; prevalence; urinary incontinence; women
Abbreviations: CI, confidence interval; OR, odds ratio; SWAN, Study of Women’s Health Across the Nation.
The prevalence of urinary incontinence in women peaks
in midlife, between the ages of 45 and 55 years (1), yet the
epidemiology of incontinence in this age group is understudied relative to research in women aged 60 years or more.
Incontinence that develops in midlife is not well character-
ized, and little is known about factors that may predispose
women to the different clinical types of incontinence during
this time.
Most large, community-based, epidemiologic studies of female incontinence have been cross-sectional or retrospective
Correspondence to Dr. L. Elaine Waetjen, Department of Obstetrics and Gynecology, 4860 Y Street, Suite 2500, Sacramento, CA 95817
(e-mail: [email protected]).
309
Am J Epidemiol 2007;165:309–318
310 Waetjen et al.
and conducted predominantly in older women of European
descent (1–5). Additionally, most have assessed risk factors
for prevalent incontinence in these populations with uncertainty as to the duration of incontinence. Yet, incontinence
that develops in midlife may have unique characteristics and
risk factors.
The purpose of this study was to compare prevalent stress,
urge, and mixed incontinence with incontinence that developed over 5 years of follow-up in a racially and ethnically
diverse cohort of midlife women. We used data from the
Study of Women’s Health Across the Nation (SWAN), a
community-based, prospective cohort study of women from
five different racial/ethnic groups, to determine whether risk
factors for prevalent incontinence such as parity, high body
mass index, and diabetes were also associated with incident
incontinence. We also examined whether factors associated
with prevalent and incident stress, urge, and mixed incontinence varied by race/ethnicity.
MATERIALS AND METHODS
This analysis used data from the baseline and first five
annual follow-up visits from the prospective cohort of
SWAN, 1995–2001. SWAN is a multicenter, multiethnic, and
multidisciplinary prospective cohort study of the menopausal
transition. Details of the study’s methodology have been
published previously (6). At seven clinical sites (Boston,
Massachusetts; Chicago, Illinois; the Detroit area, Michigan;
Los Angeles, California; Newark, New Jersey; Pittsburgh,
Pennsylvania; and Oakland, California), 16,065 communitybased women, aged 40–55 years, were identified by random
digit dialing, snowball, and/or list-based sampling and interviewed for a cross-sectional screening survey. From this large
sample, each of the sites recruited about 450 women to include a Caucasian group and one designated minority group
(African American at four sites and Chinese, Japanese, and
Hispanic at one site each) for a total longitudinal cohort of
3,302 women. Eligibility criteria for the SWAN longitudinal
cohort were age 42–52 years and self-identification as one of
the five racial/ethnic groups to be studied. The exclusion
criteria were inability to speak English, Spanish, Japanese,
or Cantonese; no menstrual period in more than 3 months
before enrollment; hysterectomy and/or bilateral oophorectomy prior to the onset of the study; and current use of oral
contraceptives, estrogens, progestins, or luteinizing hormone
agonists. The institutional review boards at each clinical site
and the coordinating center approved the study protocol, and
all participants gave written, informed consent. For this analysis, to compare factors associated with prevalent and incident incontinence in the same cohort, we excluded 599
women who dropped out of the study before year 5. We
considered women to have dropped out of the study when
they were missing 3 or more consecutive years of outcome
data.
Covariates
At baseline, participants completed both interviewadministered and self-administered questionnaires so that
demographic, medical, lifestyle, quality of life, and other
information could be obtained. Race and ethnicity were selfdefined as Black or African American, non-Hispanic Caucasian, Chinese or Chinese American, Japanese or Japanese
American, and Hispanic (including Central American, Cuban or Cuban American, Dominican, Mexican or Mexican
American, Puerto Rican, South American, Spanish, or other
Hispanic). Socioeconomic status was assessed by income and
level of difficulty in paying for basics. Interviewers obtained
medical history, obstetric history, gynecologic history including self-reported diagnosis of fibroids, smoking history
and medication use, educational level attained, and general
health status. SWAN used an adaptation of the 36-Item
Short-Form Health Survey (SF-36) (7) to assess quality
of life, the Center for Epidemiologic Studies Depression
(CES-D) Scale (8) to evaluate depressive symptoms, the
Baeke questionnaire (9) to assess physical activity, and the
Medical Outcomes Study (MOS) Social Support Survey
(10) and Life Stressors and Social Resources Inventory
(LISRES) (11) to estimate social support. Finally, at year 1,
women responded to a series of questions assessing their
sensitivity to physical sensations (heat, cold, noise, hunger);
the responses were combined into a Symptom Sensitivity
Scale (12).
Each woman underwent weight, height, and waist circumference measurements by certified staff that used calibrated scales and a stadiometer. We calculated body mass
index (weight (kg)/(height (m)2) and measured waist circumference in centimeters.
Outcomes
The self-administered questionnaire assessed incontinence at baseline and at each annual follow-up visit. Women
were asked the question: ‘‘In the past year (or since your last
study visit), have you ever leaked even a small amount of
urine involuntarily?’’ The frequency of incontinence was
recorded as ‘‘almost daily/daily’’ (daily), ‘‘several days per
week’’ (weekly), ‘‘less than one day per week’’ (monthly),
‘‘less than once a month,’’ or ‘‘none.’’ We classified the responses indicating weekly or daily incontinence as ‘‘frequent’’
and the responses indicating monthly as ‘‘occasional.’’ We
defined ‘‘any incontinence’’ as at least monthly incontinence.
Because we considered incontinence that occurred less than
once a month not to be clinically significant with a higher
misclassification rate, we combined this category with none
as ‘‘no incontinence’’ or no regular incontinence.
We categorized type of incontinence as ‘‘stress’’ if participants reported at least monthly leakage with ‘‘coughing,
laughing, sneezing, jogging, jumping, with physical activity, or picking up an object from the floor’’ or as ‘‘urge’’ if
participants reported at least monthly leakage ‘‘when you
have the urge to void and can’t reach the toilet fast enough.’’
Affirmative responses to both circumstances of at least
monthly incontinence in the same visit were categorized
as ‘‘mixed.’’ We classified incontinence that did not fit these
three types as ‘‘other.’’
We defined prevalent incontinence as incontinence occurring at least monthly that was reported by women at baseline. Prevalence was calculated as the number of women
reporting incontinence (by frequency and type) at baseline
Am J Epidemiol 2007;165:309–318
Prevalent and Incident Urinary Incontinence
divided by the total baseline cohort. Women who had no
incontinence at baseline but reported at least monthly incontinence at any of the five annual follow-up visits were
considered to have incident incontinence at their first report.
We calculated cumulative incidence as the the number of
women reporting new-onset incontinence in the 5 years of
follow-up divided by the number of women at risk during
those 5 years. For the average annual incidence, we then
divided this by five. Finally, we defined our reference group
narrowly: Only women who did not report incontinence occurring at least monthly at baseline and over all 5 years of
follow-up were considered to have no incontinence. For
169 women who were missing incontinence data at one of
the first five visits, we imputed values for incontinence frequency and type by randomly assigning the individual participant’s reported frequency and type from either the
previous or subsequent year to the missing year.
Statistical analysis
After univariate exploration of our variables, we performed bivariate analyses. Variables associated with incontinence in the literature or with p values of 0.10 or less in our
data were entered into multivariable analyses by use of a
forward stepwise process. Additionally, because race, parity, and body mass index (3, 13–15) have all been associated
with incontinence, we forced these variables into most of
our models.
Because we wanted to compare baseline factors associated with prevalent and first reported incident incontinence
within the same cohort, we used multiple logistic regression
for our analyses of the dichotomous outcomes: any incontinence versus none; frequent (daily or weekly) incontinence
versus none. Our primary models to evaluate factors associated with at least monthly incontinence by type used the
following outcomes, each compared with none: urge, stress,
or mixed incontinence. When sample size permitted, we
developed separate models for prevalent and incident incontinence. We also developed separate models for each racial/
ethnic group to study whether factors associated with incontinence varied by race/ethnicity. Because of small numbers in the Chinese, Japanese, and Hispanic cohorts,
prevalent and incident factors associated with the individual
types of incontinence could not be evaluated separately in
these three models. We assessed model fit with the HosmerLemenshow goodness-of-fit test.
To explore the robustness of our results, we developed
other models to compare with our primary ones reported
here. For urge and stress symptoms, we developed secondary
models comparing the following: urge and mixed incontinence (any urge symptoms) versus stress incontinence
and none (no urge symptoms), and stress and mixed incontinence (any stress symptoms) versus urge incontinence and
none (no stress symptoms). Additionally, because the distributions of body mass index were significantly different
among racial/ethnic groups, we explored a number of methods to handle the differences in distributions. First, we entered body mass index by each race/ethnicity as interaction
terms into our models. Second, we ran our models categorizing body mass index by quantiles specific to each racial/
Am J Epidemiol 2007;165:309–318
311
ethnic group to evaluate body mass index independent of
race. Finally, we removed data from all women in the top
5 percent of the body mass index distributions. Although we
observed no large differences in our point estimates in each
of these methods, model fit by the Hosmer-Lemenshow
goodness-of-fit test was generally better when all women
remained in the analysis. For all our analyses, we used
SAS, version 9.1, software (SAS Institute, Inc., Cary, North
Carolina).
RESULTS
Study sample
Of the 3,302 women in the cohort at baseline, 599 (18.1
percent) either dropped out of SWAN or had not completed
the incontinence questions at baseline and in all 5 years of
follow-up. Table 1 displays the characteristics of the baseline participants and compares the women who remained in
the study at year 5 with those women who dropped out of the
study or were missing incontinence data. As expected, we
found a number of differences between the follow-up and
drop-out cohorts. Women who remained in the study were
more likely to be Caucasian, to have gone to college, and to
be financially secure. Our follow-up cohort was also thinner
and had lower parity. Importantly, we also found differences
in incontinence reporting. Women who remained in the
study were more likely to report any incontinence and more
likely to report frequent incontinence. However, the distribution of incontinence type did not vary between follow-up
and drop-out cohorts.
Prevalent and incident incontinence
The prevalence and cumulative incidence by frequency
and type of incontinence for all racial/ethnic groups are
displayed in tables 2 and 3. At least monthly incontinence
prevalence was 46.7 percent overall and 15.3 percent for incontinence occurring several days per week or more. The
average 1-year incidence of at least monthly incontinence
was 11.1 percent per year, but only 1.2 percent per year was
reported to be frequent. African-American and Hispanic
women had the lowest prevalence of incontinence, while
African-American (11.6 percent) and Caucasian (13.4 percent) women had the highest average annual incidence.
Therefore, African-American women had the highest proportional difference in reporting incident compared with
prevalent incontinence at baseline. Incontinence that developed over the 5 years was less frequent (5.8 vs. 15.3 percent
had frequent incontinence) and less bothersome (3.2 vs. 4.8
on a Likert scale). Although stress incontinence was the
most frequent type of both prevalent and incident incontinence, women of all racial/ethnic groups reported twice
as much urge incontinence at follow-up than at baseline
(15.9 vs. 7.6 percent at baseline). For both prevalent and
incident incontinence, Japanese and Hispanic women were
the most likely to report stress incontinence, while AfricanAmerican women were the most likely to report urge
incontinence.
312 Waetjen et al.
TABLE 1. Baseline characteristics and differences between women who followed up
and dropped out* of the Study of Women’s Health Across the Nation over 5 years
between 1995 and 2001
Baseline
characteristics
Age, years (mean (SDz))
Baseline
cohort
(n ¼ 3,301)
Follow-up
cohort
(n ¼ 2,702)
Drop-out
cohort
(n ¼ 599)
45.8 (2.7)
45.8 (2.7)
45.6 (2.7)
Caucasian
1,550
47.0
1,313
48.6
237
39.6
Black
934
28.3
709
26.2
225
37.6
Chinese
250
7.6
237
8.8
13
2.2
Japanese
281
8.5
266
9.8
15
2.5
Hispanic
286
8.7
177
6.6
109
18.2
p valuey
0.04
Race
<0.001
Menopausal status
Premenopausal
1,498
46.5
1,201
45.4
297
49.6
Early perimenopausal
1,726
53.5
1,446
54.6
280
46.7
High school or less
819
25.1
594
22.2
225
37.6
College or higher
2,451
74.9
2,087
77.8
364
60.8
0.12
Education
0.00
Marital
Single
439
13.5
352
13.2
87
14.5
Married, living as
2,148
66.1
1,813
68.0
335
55.9
Separated/divorced/widowed
660
20.3
500
18.8
160
26.7
>25th percentile
2,214
67.1
1,849
68.4
365
60.9
25th percentile
1,087
32.9
853
31.6
234
39.1
0.01
Social support
0.05
General health status
Excellent
694
21.4
599
22.5
95
15.9
Very good
1,179
36.3
1,010
37.9
169
28.2
Good
945
29.1
744
27.9
201
33.6
Fair
368
11.3
265
9.9
103
17.2
Poor
62
1.9
48
1.8
14
2.3
<0.001
Difficulty paying for basics
Not at all hard
1,968
60.0
1,693
63.0
275
45.9
Somewhat hard
1,005
30.7
784
29.2
221
36.9
Very hard
306
9.3
209
7.8
97
16.2
<0.001
Table continues
Factors associated with prevalent and incident
incontinence
In our multivariable analyses, we found some differences
in the factors associated with any or frequent prevalent incontinence compared with any or frequent incident incontinence (table 4). For example, compared with Caucasians,
African-American, Chinese, Japanese, and Hispanic women
had lower odds of reporting frequent prevalent incontinence,
while we observed no significant trend in the reporting of
any or frequent incident incontinence by racial/ethnic group.
Parity, diabetes, uterine fibroids, depressive symptoms, and
poor social support were significantly associated with prevalent incontinence but not incident incontinence. We found a
significant interaction between education and both Chinese
and Japanese ethnicity. A lower level of education was associated with less prevalent incontinence (for Chinese: odds
ratio (OR) ¼ 0.33, 95 percent confidence interval (CI):
0.17, 0.69; for Japanese: OR ¼ 0.48, 95 percent CI: 0.25,
0.97). Being either a current or past smoker was associated
with increased odds of incident incontinence in AfricanAmerican women but not in other racial/ethnic groups.
Factors associated with frequent prevalent or incident incontinence are similar to those for any incontinence, but the odds
ratios tended to be higher for frequent incontinence.
Diabetes was the strongest risk factor that we identified for
prevalent incontinence. The estimated probability that a parous woman with a normal body mass index had incontinence
Am J Epidemiol 2007;165:309–318
Prevalent and Incident Urinary Incontinence
313
TABLE 1. Continued
Baseline
characteristics
Body mass index, kg/m2
(mean (SD))
Baseline
cohort
(n ¼ 3,301)
28.2 (7.2)
Follow-up
cohort
(n ¼ 2,702)
27.9 (7.2)
Drop-out
cohort
(n ¼ 599)
p valuey
28.9 (7.2)
0.01
85.6 (16.1)
87.7 (15.9)
0.01
1.9 (1.4)
2.5 (1.3)
0.01
7.5 (1.8)
0.06
0.00
Waist circumference, cm
(mean (SD))
86.4 (16.1)
Parity (mean (SD))
2.0 (1.4)
Total physical activity, score
(mean (SD))
7.7 (1.8)
7.7 (1.8)
Smoking history
Never smoked
1,904
57.7
1,601
59.3
303
50.6
Current smoker
828
25.1
694
25.7
134
22.4
Past smoker
569
17.2
407
15.1
162
27.0
Medical conditions
Diabetes
163
5.0
126
5.0
37
6.2
0.20
Hypertension
642
19.8
506
19.6
136
22.7
0.09
Fibroids
670
20.7
546
21.3
124
20.7
0.60
Depression
804
24.4
613
23.5
191
31.9
0.01
Quality of life (mean (SD))
Physical role
75.0 (36.7)
75.4 (36.3)
74.0 (34.9)
0.20
Vitality
54.7 (20.6)
54.9 (20.2)
53.5 (21.1)
0.06
Social functioning
79.8 (22.5)
80.5 (21.9)
77.2 (24.6)
0.00
1,804
1,439
365
0.00
Incontinence frequency
None
54.7
53.2
60.9
Occasional (monthly to
weekly)
996
30.2
850
31.5
146
24.4
Frequent (>weekly to daily)
496
15.1
413
15.3
83
13.9
Stress
1,007
55.4
871
55.6
136
47.7
Urge
302
16.3
249
15.9
53
18.6
Mixed
456
24.6
373
23.8
83
29.1
Other
86
4.7
73
4.7
13
4.6
Incontinence type
Symptom sensitivity
(mean (SD))
10.19 (3.6)
10.14 (3.58)
10.42 (3.72)
0.25
0.14
* Reported as number and percent, except where noted otherwise.
y Comparison of follow-up cohort and drop-out cohort only, using the Kolmogorov-Smirnov
test.
z SD, standard deviation.
at baseline increased with diabetes for each racial/ethnic group,
but it was highest in Caucasians (without diabetes: p ¼ 0.51;
with diabetes: p ¼ 0.68) and lowest in Hispanics (without
diabetes: p ¼ 0.17; wth diabetes: p ¼ 0.29). Although the odds
of having incontinence at baseline increased with waist circumference, a high body mass index was associated with developing
incontinence during follow-up. The estimated probability of a
parous woman’s developing incident incontinence with a body
mass index of 30 or more was 0.41 for Caucasians, 0.50 for
African Americans, 0.39 for Chinese, 0.31 for Japanese, and
0.30 for Hispanics, while for a normal body mass index of 19–
24.9, it was 0.33 for Caucasians, 0.42 for African Americans,
0.31 for Chinese, 0.24 for Japanese, and 0.23 for Hispanics.
Am J Epidemiol 2007;165:309–318
Factors associated with stress, urge, and mixed
prevalent and incident incontinence
Factors associated with prevalent and incident incontinence differed when analyzed by type of incontinence
(table 5). Although parity was associated with increased
odds of prevalent stress incontinence, it was not associated
with incident stress incontinence. Diabetes and waist circumference also were associated with increased odds of
prevalent stress incontinence, but body mass index was associated with incident stress incontinence. Prevalent urge
incontinence was negatively associated with higher education (college or more compared with high school or less)
314 Waetjen et al.
TABLE 2. Prevalence* of at least monthly incontinence reported at baseline by racial/ethnic group, Study of Women’s Health Across
the Nation, 1995–2001
Incontinence
characteristics
All women
(n ¼ 2,702)
Caucasian
(n ¼ 1,263)
African
American
(n ¼ 719)
Chinese
(n ¼ 249)
Japanese
(n ¼ 264)
Hispanic
(n ¼ 207)
Prevalent incontinence
1,263
46.7
722
57.1
284
29.5
87
34.9
113
42.8
57
27.5
No incontinence
1,439
53.3
541
42.9
435
60.5
162
65.1
151
57.2
150
72.5
Occasional (monthly to weekly)
850
31.4
473
37.4
188
26.1
70
28.1
85
32.2
34
16.4
Frequent (>weekly/daily)
413
15.3
249
19.7
96
13.3
17
6.8
28
10.6
23
11.1
p value
0.04
Incontinence frequency
0.6
Incontinence typey
Stress
681
25.2
402
31.8
94
13.1
58
23.3
82
31.1
44
21.2
<0.01
Urge
205
7.6
98
7.7
85
11.8
8
3.2
10
3.8
3
1.4
0.02
Mixed
335
12.4
196
15.5
93
12.9
18
7.2
18
6.8
10
4.8
0.60
Other
42
1.6
26
2.1
12
1.7
3
1.2
3
1.1
0
0
0.98
Bothersomenessz (mean (SD§))
4.8 (2.9)
4.6 (2.9)
4.8 (3.1)
3.9 (2.5)
4.7 (2.6)
7.9 (2.6)
<0.01
* Reported as frequency and percent, except where noted otherwise.
y p values are from chi-square tests evaluating differences in proportions of each incontinence type across racial/ethnic groups.
z Likert’s scale from 0 to 10 among women reporting any incontinence.
§ SD, standard deviation.
and had a strong positive association with diabetes. Incident
urge incontinence was associated with increasing waist
circumference. Prevalent mixed incontinence was associated with predictors of both stress and urge incontinence,
as well as with unique factors. In particular, low social support, depressive symptoms, and a high symptom sensitivity
score were associated with prevalent mixed incontinence
symptoms. Incident mixed incontinence was associated
with diabetes, a high body mass index, and a high symptom
sensitivity score. Race/ethnicity categories could not be
forced into this mixed incontinence model because of poor
model fit.
The proportional increase in African-American women
reporting incident incontinence can largely be explained in
our multivariable models by their higher odds of developing
urge incontinence, not stress incontinence. Chinese, Japanese, and Hispanic women had less stress, urge, or mixed
incontinence compared with Caucasian women; although
not statistically significant, they tended to have less incident
incontinence of all types as well (table 5).
TABLE 3. Cumulative rates* of first reported at least monthly incident incontinence by racial/ethnic group over 5 years, Study of
Women’s Health Across the Nation, 1995–2001
Incontinence
characteristics
All women
(n ¼ 1,439)
Caucasian
(n ¼ 541)
African
American
(n ¼ 435)
Chinese
(n ¼ 162)
Japanese
(n ¼ 151)
Hispanic
(n ¼ 150)
p value
Incident incontinence
801
55.7
353
65.2
253
58.2
71
43.8
73
48.3
51
34.0
No incontinence
638
44.3
188
34.8
182
41.8
91
56.2
78
51.7
99
66.0
0.07
Occasional (monthly to weekly)
717
49.8
328
60.6
221
50.8
65
40.1
65
43.0
38
25.3
Frequent (>weekly/daily)
84
5.8
25
4.6
32
7.4
6
3.7
8
5.3
13
8.7
Stress
364
25.2
179
33.1
66
15.2
40
24.7
47
31.1
33
Urge
230
15.9
92
17.0
101
23.2
14
8.6
5
3.3
7
4.7
0.27
Mixed
171
11.9
64
11.8
70
16.1
16
9.8
11
7.2
11
7.3
0.87
Other
36
2.5
18
3.3
16
3.6
1
0.6
0
0
0
0
Incontinence frequency
0.68
Incontinence typey
Bothersomenessz (mean (SD§))
3.2 (2.8)
2.8 (2.6)
3.3 (2.9)
2.8 (2.5)
3.7 (2.8)
22
5.7 (2.7)
0.02
0.67
<0.01
* Reported as frequency and rate/100 per 5 years.
y p values are from chi-square tests evaluating differences in rates of each incontinence type across racial/ethnic groups.
z Likert’s scale from 0 to 10 among women reporting any incontinence at annual follow-up in years 1–3.
§ SD, standard deviation.
Am J Epidemiol 2007;165:309–318
Prevalent and Incident Urinary Incontinence
315
TABLE 4. Adjusted* odds ratios for any and frequent prevalent and incident urinary incontinence, Study of Women’s Health Across
the Nation, 1995–2001
Prevalent incontinence
Any (at least monthly)
Characteristic
Odds
ratio
95%
confidence
interval
Incident incontinence
Frequent (>weekly/daily)
Odds
ratio
Any (at least monthly)
95%
confidence
interval
Odds
ratio
95%
confidence
interval
Frequent (>weekly/daily)
95%
confidence
interval
Odds
ratio
Race/ethnicity
Caucasian
Referent
Referent
Referent
Referent
African American
0.36
0.27, 0.48
0.31
0.21, 0.46
1.33
0.89, 1.99
1.66
0.67, 4.14
Chinese
2.35
0.76, 7.33
0.26
0.14, 0.47
0.81
0.50, 1.30
1.28
0.31, 5.22
Japanese
0.24
0.03, 2.17
0.62
0.09, 0.29
0.64
0.36, 1.13
2.15
0.62, 7.48
Hispanic
0.18
0.11, 0.30
0.16
0.11, 0.35
0.63
0.36, 1.13
2.96
1.06, 8.18
1.05
1.02, 1.07
1.12
1.06, 1.19
Social support
>25th percentile
Referent
25th percentile
Referent
1.39
1.06, 1.81
1.87
1.32, 2.66
Body mass index, kg/m2
(per unit increase)
1.02
0.97, 1.07
1.02
0.96, 1.04
Waist circumference, cm
(per cm increase)
1.03
1.01, 1.05
1.05
1.02, 1.07
Parity
Nulliparous
Parous
Referent
Referent
Referent
1.43
1.00, 2.03
1.70
1.05, 2.73
2.34
1.21, 4.55
3.10
1.43, 6.74
1.33
0.99, 1.81
Referent
1.31
0.88, 1.96
1.25
0.96, 1.61
2.57
0.58, 11.48
Medical conditions
Diabetes
Fibroids
1.31
1.00, 1.72
Depressive symptoms
1.28
1.02, 1.61
General health status
Good to excellent
Referent
Fair to poor
Symptom severity score
Age
1.03
Hosmer-Lemenshow
goodness of fit
0.99, 1.06
1.06
0.52
2.99
1.35, 6.62
1.09
1.00, 1.19
1.01, 1.12
0.62
0.55
0.39
* All factors with values for odds ratios and 95% confidence intervals are included in each multiple logistic regression model.
Factors associated with incontinence by race
We had sufficient numbers to evaluate factors associated
with incontinence type by race/ethnicity in women with
prevalent incontinence for our two largest groups, Caucasian and African-American women. Interestingly, parity
was significantly associated with prevalent stress and mixed
incontinence in Caucasian women (for stress: OR ¼ 1.85, 95
percent CI: 1.18, 2.92; for mixed: OR ¼ 2.51, 95 percent CI:
1.35, 4.69) but not in African-American women (for stress:
OR ¼ 0.46, 95 percent CI: 0.11, 1.91; for mixed: OR ¼ 0.51,
95 percent CI: 0.11, 2.41). Although low social support was
important only in the reporting of mixed incontinence for
Caucasian women (OR ¼ 2.49, 95 percent CI: 1.41, 4.38),
African-American women with low social support were
more likely to report stress and mixed incontinence (for
stress: OR ¼ 2.32, 95 percent CI: 1.21, 4.45; for mixed:
OR ¼ 2.35, 95 percent CI: 1.19, 4.66). Finally, self-reported
Am J Epidemiol 2007;165:309–318
diagnosis of uterine fibroids was associated with urge incontinence in African-American women only (OR ¼ 1.95,
95 percent CI: 1.07, 3.54), while poor health status was
associated with stress and urge incontinence in Caucasian
women only (for stress: OR ¼ 4.49, 95 percent CI: 1.03,
19.64; for urge: OR ¼ 9.27, 95 percent CI: 1.72, 49.97).
DISCUSSION
This study is among the first to examine the prevalence
and incidence of urinary incontinence in a racially/ethnically
diverse, community-based sample of midlife women followed longitudinally over 5 years. Nearly half of the women
reported having at least monthly incontinence at baseline,
while half of those who did not have incontinence at baseline reported developing at least monthly incontinence over
the 5 years of observation. The characteristics of and factors
Prevalent incontinence
Stress
Risk factor
Odds
ratio
95%
confidence
interval
Incident incontinence
Urge
Odds
ratio
95%
confidence
interval
Mixed
Odds
ratio
95%
confidence
interval
Stress
Odds
ratio
95%
confidence
interval
Urge
Odds
ratio
95%
confidence
interval
Mixed
Odds
ratio
95%
confidence
interval
1.09
1.04, 1.13
Race/ethnicity
Caucasian
Referent
Referent
Referent
Referent
Referent
African American
0.26
0.19, 0.36
0.98
0.65, 1.48
0.35
0.23, 0.53
0.44
0.28, 0.68
1.91
1.22, 2.99
Chinese
0.47
0.31, 0.70
0.43
0.22, 0.84
0.38
0.21, 0.70
0.68
0.39, 1.17
0.22
0.07, 0.72
Japanese
0.77
0.55, 1.12
0.37
0.18, 0.74
3.19
0.55, 18.29
0.88
0.52, 1.48
0.37
0.14, 0.97
Hispanic
0.27
0.17, 0.42
0.10
0.03, 0.34
0.12
0.05, 0.27
0.45
0.25, 0.81
0.33
0.11, 0.95
Education
College or more
Referent
High school or less
0.72
0.51, 1.03
Social support
>25th percentile
Referent
25th percentile
1.95
1.32, 2.89
1.42
0.96, 2.09
1.04
0.97, 1.11
1.06
1.03, 1.10
1.03
1.01, 1.06
1.04
1.01, 1.07
1.44
0.87, 2.40
0.92
0.51, 1.68
Body mass index, kg/m2
(per unit increase)
0.99
0.95, 1.04
Waist circumference, cm
(per cm increase)
1.04
1.02, 1.06
1.03
1.00, 1.06
Referent
Parity
Nulliparous
Parous
Referent
1.91
Referent
1.31, 2.79
0.80
0.50, 1.29
2.16
1.26, 3.71
Medical conditions
None
Am J Epidemiol 2007;165:309–318
Diabetes
Referent
2.11
Referent
1.09, 4.09
Referent
Referent
3.62
1.45, 9.01
Fibroids
1.53
1.08, 2.27
1.46
0.99, 2.15
Depressive symptoms
1.42
1.01, 2.01
1.45
1.04, 2.03
1.06
1.01, 1.11
Symptom sensitivity score
3.02
1.12, 8.10
1.07
1.01, 1.14
Health status
Good to excellent
Referent
Poor to fair
Hosmer-Lemenshow
goodness of fit
0.42
0.61
0.31
0.41
0.40
* All factors with values for odds ratios and 95% confidence intervals are included in each multiple logistic regression model.
0.18, 0.99
0.92
0.29
316 Waetjen et al.
TABLE 5. Adjusted* odds ratios for at least monthly prevalent and incident stress, urge, and mixed urinary incontinence, Study of Women’s Health Across the Nation,
1995–2001
Prevalent and Incident Urinary Incontinence
associated with prevalent and incident incontinence differed.
Although Caucasians reported the highest incontinence
prevalence and incidence, African-American women had
the highest proportional difference between reporting incident and prevalent incontinence; they also had the highest
proportion of urge incontinence.
Incident urinary incontinence that developed in this
racially/ethnically diverse longitudinal cohort of midlife
women was infrequent, occurring less than weekly, compared with the prevalent incontinence reported at baseline.
Although stress incontinence symptoms were more frequently reported, the proportion of women reporting urge
incontinence symptoms was greater in women with incident
compared with prevalent incontinence.
Incontinence described as occurring less than weekly is
responsible for the midlife prevalence peak of incontinence
in previous epidemiologic studies (1). Whether we captured
an early stage of incontinence that will worsen over time or
whether incontinence that develops in midlife is more likely
to remain mild or even to resolve is not known at this time
and will be the subject of future investigations in SWAN.
The prevalence of urge incontinence is low in premenopausal women (1) and increases with age (3, 15, 16); the
timing of changes in that distribution has been unclear. Our
study suggests that this rate begins to change in midlife.
Some factors associated with incident incontinence appeared to be unique. Most important, while parity was associated with increased odds of prevalent stress and mixed
incontinence, it was not an important factor in the development of any type of incident incontinence over the 5 years of
follow-up. This novel observation suggests that the impact
of parity on incontinence presents in the reproductive years,
while incontinence that develops in midlife is not related to
child-bearing, but rather may be related to other factors to be
explored in future studies, such as weight gain and change in
hormone status. We also found that, although a higher waist
circumference was associated with prevalent incontinence at
baseline, higher body mass index was associated with incident incontinence. While this may be, in part, an artifact
of our model, it may also relate to the increased proportion of
urge symptoms in incident incontinence. We found that
a higher waist circumference was associated with stress incontinence; perhaps greater central obesity increases stress
incontinence symptoms because of increased intraabdominal pressure, but it has no impact on urge incontinence symptoms for which simply being overweight is a risk factor.
Some factors associated with prevalent stress, urge, and
mixed incontinence differed between the two largest racial/
ethnic groups: Caucasians and African Americans. In particular, incontinence of any type in African Americans was
not associated with parity, while parity was an important
factor in Caucasian women. We found that urge incontinence
in African Americans was associated with self-reported fibroids, while incontinent Caucasians, but not African Americans, were more likely to report a fair-to-poor health status.
African Americans have been reported to have a lower
prevalence of incontinence (14, 15, 17), but they report
a higher proportion of urge incontinence (18, 19) compared
with other racial/ethnic groups. We found that a higher proportion of African-American midlife women reported inciAm J Epidemiol 2007;165:309–318
317
dent than prevalent incontinence. One possible explanation
for this finding is the association between urge incontinence
and fibroids, which are more prevalent in African-American
women in this age group (20, 21). Because urinary urgency
is thought to be a symptom of fibroids, the association between fibroids and urge incontinence may simply reflect
a higher rate of diagnostic studies in African-American
women, rather than a direct effect of fibroids. On the other
hand, the space-occupying effects of fibroids on the bladder
and urethra may indeed be a cause of urge incontinence.
Another explanation for the higher reporting of incident incontinence is that the African-American women in our sample had the highest average body mass index compared with
the other racial/ethnic groups.
Our study also has important limitations. First, as expected, African-American and Hispanic women, women
with less education, and women with a lower socioeconomic status were more likely to have dropped out of SWAN.
Women who had higher body mass indexes and who reported poorer health, factors associated with incontinence,
were also more likely to be lost to follow-up. Most important, women who reported any and frequent incontinence
were more likely to have stayed in SWAN. These differences, though relatively small in absolute numbers, limit
the generalizability of our study. In particular, we have likely
overestimated the prevalence and incidence of incontinence
in midlife women. However, our estimates of factors associated with prevalent incontinence are similar to those previously reported in an analysis of the entire baseline cohort
in SWAN (13). Second, we defined prevalent incontinence
as incontinence that was reported to be at least once per
month and present at baseline. We do not have information
to determine the duration of this incontinence, which may
have been present for only a few months or for many years.
Third, some of our analyses were limited by small numbers.
For example, because of small sample sizes, we were unable
to evaluate factors associated with the different types of incontinence in our Chinese, Japanese, and Hispanic cohorts.
That we identified fewer factors associated with incident
incontinence may be, in part, explained by smaller numbers
in this group. Fourth, because our incontinence questions
have not been validated in diverse populations, reporting
bias for incontinence based on varying sociocultural interpretation of these questions may have occurred. For example, our finding that poor social support was associated with
urge incontinence in African Americans may reflect psychosocial circumstances that lead to the higher reporting of urge
incontinence in this group. Finally, although of greatest interest is the effect of the menopausal transition and other
time-varying factors on the natural history of incontinence
in midlife, the first 5 years of SWAN do not provide sufficient numbers of women in each stage of the menopausal
transition to allow a robust analysis of this effect.
Nonetheless, SWAN has provided a unique opportunity
to describe and evaluate incident incontinence in a racially/
ethnically diverse, community-based cohort of women over
time and had a number of advantages. This longitudinal
cohort has had good retention (80 percent) over the first
5 years of follow-up. The same incontinence questions were
asked on an annual basis; these questions, which allowed
318 Waetjen et al.
classification of incontinence by clinical type (stress, urge,
and mixed), were similar to those that have been used widely
in other epidemiologic studies and that have been associated
with different risk factors (3, 15). Finally, although selfreport reflects the symptoms rather than the diagnosis of
stress and urge incontinence, the experience of incontinence
is of more direct clinical and public health importance than
the presence or absence of urodynamic abnormalities.
Our study evaluating the baseline factors associated with
incontinence in the cohort of midlife women provides novel
and clinically relevant epidemiologic information about urinary incontinence that develops in this age group. First reported incident incontinence is infrequent, with a higher
proportion of the urge type, and childbearing does not appear
to be an important risk factor. Overweight women appear to
be at a higher risk of developing incontinence in midlife.
ACKNOWLEDGMENTS
SWAN has grant support from the National Institutes
of Health, Department of Health and Human Services,
through the National Institute on Aging, the National Institute of Nursing Research, and the National Institutes of
Health Office of Research on Women’s Health (grants
NR004061, AG012505, AG012535, AG012531, AG012539,
AG012546, AG012553, AG012554, AG012495). The supplemental funding source for this secondary analysis from
the University of California Davis Health System Research
Award is gratefully acknowledged.
Clinical Centers: University of Michigan, Ann Arbor,
Michigan—MaryFran Sowers, Principal Investigator;
Massachusetts General Hospital, Boston, Massachusetts—
Robert Neer, Principal Investigator 1995–1999; Joel
Finkelstein, Principal Investigator 1999–present; Rush University, Rush University Medical Center, Chicago, Illinois—
Lynda Powell, Principal Investigator; University of California,
Davis/Kaiser, California—Ellen Gold, Principal Investigator; University of California, Los Angeles, California—Gail
Greendale, Principal Investigator; University of Medicine
and Dentistry–New Jersey Medical School, Newark, New
Jersey—Gerson Weiss, Principal Investigator 1995–2004;
Nanette Santoro, Principal Investigator 2004–present; and
the University of Pittsburgh, Pittsburgh, Pennsylvania—
Karen Matthews, Principal Investigator. NIH Program
Office: National Institute on Aging, Bethesda, Maryland—
Marcia Ory 1994–2001; Sherry Sherman 1994–present;
National Institute of Nursing Research, Bethesda,
Maryland—Program Officers. Central Laboratory: University of Michigan, Ann Arbor, Michigan—Daniel McConnell
(Central Ligand Assay Satellite Services).
Conflict of interest: none declared.
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