HRs and 95% CIs for breast cancer risk by categories of the lifetime cumulative number of menstrual cycles
| No. menstrual cycles | Cancer cases | Person-years | Crude model | Adjusted models | ||||
|---|---|---|---|---|---|---|---|---|
| HR (95% CI) | HR* (95% CI) | HR† (95% CI) | HR‡ (95% CI) | |||||
| Always irregular | 15 | 6,203 | 1.03 (0.54-1.97) | 1.09 | 1.11 (0.56-2.19) | 1.11 (0.56-2.19) | ||
| ≤415 | 24 | 10,213 | 1§ | 1§ | 1§ | 1§ | ||
| 416-453 | 41 | 9,273 | 1.88 (1.14-3.11) | 1.91 (1.15-3.16) | 1.88 (1.14-3.78) | 1.89 (1.14-3.12) | ||
| 454-490 | 42 | 10,273 | 1.75 (1.06-2.88) | 1.74 (1.05-2.88) | 1.74 (1.05-2.87) | 1.74 (1.05-2.87) | ||
| ≥491 | 46 | 10,760 | 1.82 (1.11-2.97) | 1.77 (1.08-2.91) | 1.80 (1.09-2.96) | 1.80 (1.09-2.96) | ||
| Ptrend∥ | 0.043 | 0.065 | 0.053 | 0.053 | ||||
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.