Table 4

RRs and 95% CIs of developing breast cancer among ever-users of OCs, according to different pattern of use, The Women’s Lifestyle and Health Study

Use of OCsStudy populationBreast cancer casesRR (95% CI)
Age-adjustedMultivariatea
Never-users28,1712611.0 (reference)1.00 (reference)
Time since last use (from start of follow-up; yrs)b
 <29,299911.6 (1.2–2.2)1.6 (1.2–2.3)
 2–45,711401.2 (0.8–1.7)1.2 (0.8–1.8)
 5–911,4471011.4 (1.0–1.8)1.4 (1.0–1.8)
 10–1414,4751271.2 (1.0–1.5)1.2 (1.0–1.6)
 15+29,2673481.2 (1.0–1.4)1.3 (1.0–1.5)
P for linear trend within users0.10.1
Time since first use (before start of follow-up; yrs)b
 <581991.9 (1.0–3.6)1.9 (1.0–3.9)
 5–92,388191.3 (0.8–2.2)1.3 (0.8–2.1)
 10–148,727681.3 (1.0–1.8)1.3 (1.0–1.8)
 15–1926,2642041.2 (1.0–1.4)1.2 (1.0–1.5)
 20–2424,5542661.1 (0.9–1.3)1.1 (0.9–1.3)
 25+11,4141731.2 (0.9–1.4)1.2 (0.9–1.5)
P for linear trend within users0.10.2
Age at first use (yrs)b
 <2030,9592291.0 (0.8–1.3)1.1 (0.8–1.4)
 20–2428,8813321.2 (1.0–1.5)1.2 (1.0–1.5)
 25–2910,4771281.1 (0.9–1.4)1.2 (0.9–1.5)
 30+3,849501.2 (0.9–1.7)1.3 (0.9–1.7)
P for linear trend within users0.40.6
Duration of use before age 20 (months)b
 ≤128,801831.3 (1.0–1.7)1.3 (1.0–1.7)
 13–369,886590.9 (0.6–1.2)0.9 (0.6–1.2)
 37+3,342191.0 (0.6–1.6)1.0 (0.6–1.7)
P for linear trend within users before age 200.90.9
Duration of use before first full-term pregnancy (months)b
 ≤128,165871.4 (1.1–1.8)1.4 (1.0–1.8)
 13–6017,1401571.3 (1.0–1.6)1.2 (0.9–1.5)
 61+7,210671.5 (1.1–2.0)1.3 (0.9–1.9)
P for linear trend within users before first full-term pregnancy0.70.8
  • a Multivariate analysis adjusted for: age (continuous variable), parity (0, 1, 2, 3+), age at first birth (−20, 21–24, 25+), age at menarche (continuous variable), use of HRT (ever/never), menopausal status (pre-/postmenopausal), history of breast cancer in first-degree relatives (yes/no), duration of breastfeeding (continuous variable), BMI (continuous variable), region (Sweden and five health regions in Norway), and a term for interaction between BMI and menopausal status.

  • b Adjusted for total duration of use.