Table 1. Characteristics of the study participants and associated RRs and corresponding 95% CIs for breast cancer by menopausal status.
| Premenopausal | Postmenopausal | ||||||||
| Characteristics | n (%) | Breast cancer cases n (%) | RR (95% CI)1 | n (%) | Breast cancercases n (%) | RR (95% CI)1 | |||
| Overall | 40031 (100) | 736 (100) | 27509 (100) | 448 (100) | |||||
| BMI at baseline (kg/m 2 ) | ≤20 | 5032 (12.6) | 105 (14.3) | 1.00 | 2869 (10.4) | 36 (8.0) | 1.00 | ||
| 21–24 | 24959 (62.3) | 459 (62.4) | 0.90 (0.72–1.11) | 16892 (61.4) | 276 (61.6) | 1.18 (0.83–1.68) | |||
| ≥25 | 10040 (25.1) | 172 (23.4) | 0.88 (0.68–1.12) | 7748 (28.2) | 136 (30.4) | 1.24 (0.85–1.80) | |||
| Age at menarche (years) | ≤12 | 14870 (37.2) | 253 (34.4) | 1.00 | 9726 (35.4) | 159 (35.5) | 1.00 | ||
| 13 | 12118 (30.3) | 238 (32.3) | 1.13 (0.95–1.35) | 8112 (29.5) | 136 (30.4) | 0.96 (0.76–1.21) | |||
| ≥14 | 13043 (32.5) | 245 (33.3) | 1.07 (0.89–1.27) | 9671 (35.2) | 153 (34.1) | 0.86 (0.69–1.08) | |||
| Age at first birth (years), total number of children at baseline | No children | 5539 (13.8) | 132 (17.9) | 1.00 | 3041 (11.0) | 49 (10.9) | 1.00 | ||
| <30, 1 child | 3615 (9.0) | 74 (10.0) | 0.84 (0.63–1.12) | 2630 (9.6) | 45 (10.0) | 1.00 (0.67–1.50) | |||
| <30, 2 children | 1563 (36.4) | 265 (36.0) | 0.74 (0.60–0.91) | 10529 (38.3) | 176 (37.3) | 0.90 (0.65–1.24) | |||
| <30, ≥3 children | 10297 (25.7) | 151 (20.5) | 0.58 (0.46–0.73) | 7633 (27.7) | 118 (26.4) | 0.92 (0.66–1.28) | |||
| ≥30, 1 child | 2519 (6.3) | 43 (5.8) | 0.69 (0.49–0.98) | 1280 (4.6) | 31 (6.9) | 1.37 (0.87–2.15) | |||
| ≥30, 2 children | 2789 (6.9) | 60 (8.1) | 0.80 (0.59–1.09) | 1825 (6.6) | 28 (6.2) | 0.95 (0.60–1.52) | |||
| ≥30, ≥3 children | 715 (1.8) | 11 (1.5) | 0.52 (0.28–0.97) | 571 (2.1) | 10 (2.2) | 1.20 (0.61–2.38) | |||
| Ever oral contraceptive use at follow-up | Never | 5868 (14.7) | 100 (13.6) | 1.00 | 2821 (10.2) | 45 (10.0) | 1.00 | ||
| Ever | 24274 (60.6) | 441 (59.9) | 1.00 (0.80–1.25) | 18453 (67.1) | 307 (68.5) | 0.76 (0.55–1.04) | |||
| Unknown2 | 9889 (24.7) | 195 (26.5) | 1.07 (0.72–1.58) | 6235 (22.7) | 96 (21.4) | 0.86 (0.53–1.39) | |||
| Ever hormone therapy use at follow-up | Never | … | … | … | 9523 (34.6) | 118 (26.3) | 1.00 | ||
| Ever | … | … | … | 10603 (38.5) | 223 (49.8) | 1.21 (0.96–1.51) | |||
| Unknown3 | … | … | … | 7383 (26.8) | 107 (23.9) | 1.04 (0.70–1.55) | |||
| History of breast cancer in mother and/or sister(s) at baseline | No | 38215 (95.5) | 674 (91.6) | 1.00 | 26062 (94.7) | 408 (91.1) | 1.00 | ||
| Yes | 1816 (4.5) | 62 (8.4) | 1.90 (1.46–2.46) | 1447 (5.3) | 40 (8.9) | 1.61 (1.16–2.23) | |||
| Smoking status at baseline | Never | 16700 (41.7) | 301 (40.9) | 1.00 | 10908 (39.6) | 160 (35.7) | 1.00 | ||
| Former | 14927 (37.3) | 277 (37.6) | 1.01 (0.85–1.18) | 10309 (37.5) | 187 (41.7) | 1.28 (1.03–1.58) | |||
| Current | 8404 (21.0) | 158 (21.5) | 1.03 (0.85–1.26) | 6292 (22.9) | 101 (22.5) | 1.26 (0.98–1.62) | |||
| Total energy intake at baseline (in kJ) | <5590 | 12652 (31.6) | 233 (31.7) | 1.00 | 10279 (37.4) | 170 (37.9) | 1.00 | ||
| 5590−<7725 | 13787 (34.4) | 272 (37.0) | 1.08 (0.90–1.29) | 9434 (34.3) | 160 (35.7) | 1.09 (0.87–1.35) | |||
| ≥7225 | 13592 (33.9) | 231 (31.4) | 0.94 (0.78–1.13) | 7796 (28.3) | 118 (26.3) | 1.02 (0.80–1.29) | |||
1
Analyses were adjusted for all characteristics presented in the table, as well as height, educational level, alcohol consumption, and personal history of benign disease.
2
Includes women who reported no oral contraceptive use at baseline and did not complete the follow-up questionnaire.
3
Includes women who reported no hormone therapy use at baseline and did not complete the follow-up questionnaire.
Abbreviations: n, number of participants, RR, relative risk.