Table 3.

HRs and 95% CIs for breast cancer risk by categories of the lifetime cumulative number of menstrual cycles

No. menstrual cyclesCancer casesPerson-yearsCrude model
Adjusted models
HR (95% CI)HR* (95% CI)HR (95% CI)HR (95% CI)
Always irregular156,2031.03 (0.54-1.97)1.091.11 (0.56-2.19)1.11 (0.56-2.19)
≤4152410,2131§1§1§1§
416-453419,2731.88 (1.14-3.11)1.91 (1.15-3.16)1.88 (1.14-3.78)1.89 (1.14-3.12)
454-4904210,2731.75 (1.06-2.88)1.74 (1.05-2.88)1.74 (1.05-2.87)1.74 (1.05-2.87)
≥4914610,7601.82 (1.11-2.97)1.77 (1.08-2.91)1.80 (1.09-2.96)1.80 (1.09-2.96)
Ptrend0.0430.0650.0530.053
  • NOTE: Cox model with follow-up as time scale. n = 6,031 (168 breast cancer cases).

  • * Adjusted for age at enrollment, mother or sister with breast cancer (y/n), body mass index, >1 year of irregular years (y/n), hormone replacement therapy ever (y/n).

  • Adjusted for age at enrollment, mother or sister with breast cancer (y/n), body mass index, >1 year of irregular years (y/n), hormone replacement therapy ever (y/n), ever married (y/n), infertility consult ever (y/n), and educational level.

  • Adjusted for age at enrollment, mother or sister with breast cancer (y/n), body mass index, >1 year of irregular years (y/n), hormone replacement therapy ever (y/n), ever married (y/n), infertility consult ever (y/n), educational level, and parity and age at first childbirth (centered around its mean and multiplied by parity).

  • § Reference category.

  • The irregular category was not included when testing for trend.