There were 1521 women with one or more vaginal births. A total of 3362 vaginal births occurred within Kaiser. An additional 322 births were reported to have occurred outside of Kaiser and did not have labor and delivery records available. Parturition variables from these 322 births were therefore missing, though year of birth was included in the data. Values were missing for < 1% of patient characteristic variables except household income for which was missing for 7.4%. Information was missing for less than 10% of parturition variables except ever induced labor (18.4%), length of first or second stage of labor (15.5%), and type of anesthesia (12.6%). Of the 1577 women with one or more vaginal births, 257 (16.3%) reported urinary incontinence at least weekly but less than daily and 202 women (12.8%) reported daily incontinence. Among women with at least weekly incontinence, stress predominate incontinence was the most commonly reported type (38.3%), followed by urgency predominant (28.8%), mixed (25.9%) and other (7.0%). The comparison group was composed of 818 women with incontinence ≤ monthly (n=410) or no incontinence (n=408) in the past 12 months. Two-hundred and forty-four with monthly incontinence were thus excluded. Twenty women reporting ‘other’ race were not included due to their small number. Thus there were a total of 1257 women included in the analysis.
As seen in , approximately half the subjects self-identified as white/Caucasian, with the remaining half approximately equally divided among Black/African-American, Latino/Hispanic and Asian. The mean age was 56.1± 8.5 years at time of survey with a range from 41 to 73. Approximately one-third of women held a bachelor’s degree and approximately two-thirds were employed outside the home.
Characteristics of participants (n=1257)*
In univariate analysis (), urinary incontinence was significantly less common in Asian and Black women. Incontinence was significantly more common in women who had had a hysterectomy, who had ever smoked, and who were in poor health. There was also a significant linear association between parity and incontinence. While age demonstrated a significant general linear association with urinary incontinence (OR=1.04 per year, p<.001), the spline function provided a better fit to the data and revealed that the association was linear only below age 55 and above age 65, with essentially no association between ages 55 to 65. There was no significant interaction by patient race for any of the associations.
Unadjusted association of participant characteristics, parturition variables, and urinary incontinence (UI).
Age at first and last birth, length of labor stages and greatest birth weight are presented both dichotomized by a priori cut points to allow for comparison with previous studies, and as continuous variables which provided a better fit to the data. Younger age at first birth, dichotomized at 23 years old, showed a significant association with risk of later incontinence (OR=1.7, p<.001). When examined as a continuous variable, age at first birth showed a ‘U-shaped’ association with the lowest risk of later incontinence seen for women who first gave birth in their late 20s (). Ever having a baby weighing 4000 grams or more was also significantly associated with risk of incontinence (OR=1.5, p=.001). When analyzed as a continuous variable, greatest birth weight showed evidence of a threshold effect with an increase in the risk of later urinary incontinence associated with increasing birth weight above about 3200 grams (). Of the remaining parturition variables, only induction of labor was significantly associated with later urinary incontinence.
Figure 1 Relationship between age at first birth and later urinary incontinence. Bubbles indicate percent of women in the analysis group (which excluded women with monthly urinary incontinence) with weekly incontinence for each year of age at first birth. Size (more ...)
Figure 2 Relationship between greatest birth weight later urinary incontinence. Bubbles indicate percent of women in the analysis group (which excluded women with monthly urinary incontinence) with weekly incontinence birth weight category. Size of bubble corresponds (more ...)
In the multivariate model, age, race, current BMI, general health, hysterectomy, age at first birth, ever having been induced and greatest birth weight all remained significantly associated with urinary incontinence (). For induction, there appeared to be a dose-response effect (p=0.011 by test for trend), with a stronger association for women with 2 or more inductions (OR=2.67) than for women with only one induction (OR=1.35).
Adjusted associations of participant characteristics and parturition events and weekly urinary incontinence (UI).
Additional analyses examining the unadjusted associations of parturition variables restricted to women age 55 years or younger, by other thresholds for incontinence (ever in past 12 months, > monthly and > daily) and by type of incontinence (stress, urge, mixed) provided very similar results (data not shown).