Table 2. Combined oral contraceptive utilization and fibroid incidence in 1,308 fibroid-free women who enrolled in SELF in 2010–2012 (Detroit, MI, USA).
| Incident Fibroid Cases | Risk Ratios (95% CI) | Risk Differences (95% CI) | Incident Fibroid Cases | |||
|---|---|---|---|---|---|---|
| COC Use | n (%) | Age-Adjusted | Fully Adjusted* | Fully Adjusted* | n (%)* | |
| Unweighted | MVR | IPW/SMR | MVR | IPW/SMR | IPW/SMR | |
| Among all participants (N = 1,308) | ||||||
| Never (N = 395) | 77 (19) | Ref. | Ref. | Ref. | Ref. | 178 (23)† |
| Ever (N = 913) | 144 (16) | 0.77 (0.60, 0.99) | 0.78 (0.60, 1.00)† | 0.76 (0.60, 0.98) | -0.05 (-0.11, 0)† | 140 (18)† |
| Among ever-users (N = 913) | ||||||
| Age at first use (years) | ||||||
| < 17 (N = 320) | 42 (13) | Ref. | Ref. | Ref. | Ref. | 116 (15) |
| ≥ 17 (N = 593) | 102 (17) | 1.25 (0.90, 1.74) | 1.25 (0.89, 1.76) | 1.20 (0.86, 1.67) | 0.04 (-0.02, 0.10) | 150 (19) |
| Duration of use (years) | ||||||
| < 1 (N = 278) | 34 (12) | Ref. | Ref. | Ref. | Ref. | 117 (15) |
| 1–1.99 (N = 164) | 29 (18) | 1.49 (0.95, 2.33) | 1.49 (0.94, 2.38) | 1.43 (0.92, 2.22) | 0.07 (-0.01, 0.16) | 168 (23) |
| 2–4.99 (N = 229) | 34 (15) | 1.18 (0.76, 1.82) | 1.10 (0.69, 1.74) | 1.11 (0.72, 1.70) | 0.01 (-0.06, 0.09) | 126 (17) |
| ≥ 5 (N = 242) | 47 (19) | 1.48 (0.99, 2.21) | 1.30 (0.83, 2.04) | 1.23 (0.81, 1.86) | 0.05 (-0.03, 0.12) | 157 (20) |
| Years since last use‡ | ||||||
| Current user (N = 216) | 36 (17) | Ref. | Ref. | Ref. | Ref. | 155 (20) |
| 1–2 (N = 134) | 25 (19) | 1.18 (0.75, 1.85) | 1.23 (0.72, 2.10) | 1.23 (0.79, 1.92) | 0.05 (-0.07, 0.17) | 174 (25) |
| 3–4 (N = 95) | 16 (17) | 1.01 (0.60, 1.70) | 0.96 (0.50, 1.85) | 1.09 (0.64, 1.84) | -0.01 (-0.14, 0.12) | 154 (19) |
| ≥ 5 (N = 441) | 64 (15) | 0.77 (0.53, 1.12) | 0.87 (0.55, 1.37) | 0.91 (0.62, 1.33) | -0.03 (-0.12, 0.06) | 129 (17) |
| Characteristics of useठ| ||||||
| Short/past (N = 256) | 37 (14) | Ref. | Ref. | Ref. | Ref. | 136 (18) |
| Short/recent (N = 172) | 25 (15) | 1.25 (0.78, 2.00) | 0.97 (0.58, 1.62) | 1.09 (0.68, 1.74) | -0.01 (-0.1, 0.09) | 123 (18) |
| Long/past (N = 185) | 27 (15) | 0.96 (0.61, 1.51) | 0.92 (0.55, 1.54) | 0.86 (0.55, 1.35) | -0.01 (-0.1, 0.08) | 124 (17) |
| Long/recent (N = 273) | 52 (19) | 1.39 (0.95, 2.04) | 1.19 (0.77, 1.83) | 1.13 (0.76, 1.68) | 0.03 (-0.05, 0.12) | 166 (22) |
*Weighted (IPW/SMR) or Adjusted (MVR) for age in years (continuous), age at menarche (<11 years), Depo-Provera duration of and years since last use (never use, short/past, long/past, short/recent, long/recent), total implant and H-IUD duration of use (≥24 months), years since last birth (<5 years, 5–9.99 years, ≥10 years and no birth), parity (nulliparous or never pregnant, 1 birth, 2 births, ≥3 births), BMI (≥30, remaining values according to tertile), education (Bachelor’s degree or higher). Weighted risk ratios, risk differences, and incident fibroid case counts are shown in the columns corresponding to this footnote.
A total of n = 198 women were censored, including n = 136 COC users. Inverse probability of censoring weights were applied to upweight individuals most likely to have been censored who remained in the study. The model for probability of censoring included the exposure of interest (e.g., duration of use), all covariates used in the IPW/SMR models, with the addition of annual household income, baseline employment status, smoking history, and history of heavy gushing type bleeding.
†The weighted estimate for Ever-use was weighted according to the covariate distribution of COC users; i.e., standardized morbidity ratio [SMR] weighting was employed for this exposure.
‡Excludes n = 27 participants who used both pill and mini-pill, for whom years since last COC use could not be distinguished.
§Duration of use was characterized as short (< 2 years) or long (≥ 2 years). Years since last use was characterized as recent (< 5 years) or past (≥ 5 years).
Abbreviations: SELF, Study of Environment, Lifestyle, and Fibroids; CI, confidence interval; MVR, multivariable logistic regression; IPW, inverse probability weighting; SMR, standardized morbidity ratio; H-IUD, hormonal intrauterine device; BMI, body mass index.