Table 4.

HR and 95% CIs for breast cancer risk by categories of the cumulative number of menstrual cycles before FFTP

No. menstrual cyclesCancer casesPerson-yearsCrude model
Adjusted models
HR (95% CI)HR (95% CI)*HR (95% CI)
Always irregular125,1130.72 (0.37-1.41)0.71 (0.35-1.44)0.72 (0.35-1.46)
≤118309,127111
119-147399,5651.25 (0.78-2.01)1.24 (0.77-1.99)1.21 (0.75-1.96)
148-185256,5681.17 (0.69-1.99)1.15 (0.67-1.96)1.14 (0.66-1.95)
≥186389,3431.25 (0.77-2.01)1.23 (0.76-1.98)1.27 (0.77-2.08)
Ptrend§0.3510.4940.407
  • NOTE: Cox model with follow up as time scale. n = 5,130 (144 breast cancer cases).

  • * Adjusted for age at enrollment, mother or sister with breast cancer (y/n), BMI, more than 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), BMI, more than 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.

  • Reference category.

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