Table 1.
General characteristics of the study population
| No. (%) (n=6,481) | |
|---|---|
| Age, mean±SD (yr) | 55.01±11.0 |
| Age at menarche (yr) | |
| ≤ 13 | 546 (8.4) |
| 14-16 | 3,605 (55.6) |
| ≥ 17 | 2,279 (35.2) |
| Missing | 51 (0.8) |
| No. of live births | |
| 0 | 286 (4.4) |
| 1 | 699 (10.8) |
| ≥ 2 | 5,471 (84.4) |
| Missing | 25 (0.4) |
| Breast feeding (mo) | |
| Never | 774 (11.9) |
| < 12 | 2,128 (32.8) |
| ≥ 12 | 3,578 (55.2) |
| Missing | 1 (0.0) |
| Age at menopause (yr) | |
| Premenopausal | 83 (1.3) |
| < 45 | 259 (4.0) |
| 45-54 | 3,490 (53.9) |
| ≥ 55 | 411 (6.3) |
| Missing | 2,238 (34.5) |
| Hormone replacement therapy | |
| Never | 5,433 (83.8) |
| Ever | 720 (11.1) |
| Missing | 328 (5.1) |
| Oral contraceptive use (mo) | |
| Never | 5,224 (80.6) |
| < 12 | 622 (9.6) |
| ≥ 12 | 320 (4.9) |
| Missing | 315 (4.9) |
| Past history of benign disease | |
| Yes | 528 (8.1) |
| No | 5,459 (84.2) |
| I do not know | 459 (7.1) |
| Missing | 35 (0.5) |
| Breast density | |
| Almost entirely fat | 1,212 (18.7) |
| Scattered fibroglandular densities | 1,862 (28.7) |
| Heterogeneously dense | 2,212 (34.1) |
| Extremely dense | 1,195 (18.4) |
SD, standard deviation.