Table.
Distribution of Person-Years Among 889 944 Women in 15 Prospective Cohort Studies, by Exposure Characteristics*
| Variable | Person-Years, n (%)† | Cases of Breast Cancer, n | Stage, n‡ | Type of Breast Cancer, n§ | |||
|---|---|---|---|---|---|---|---|
| 0 | I–III | IV | ER-Positive | ER-Negative | |||
| Total | 9 625 727 (100) | 18 826 | 2319 | 9428 | 247 | 8508 | 2758 |
| Time since most recent birth | |||||||
| Nulliparous | 1 680 909 (17.5) | 3033 | 413 | 1446 | 38 | 1396 | 394 |
| 0–2.9 y | 402 357 (4.2) | 219 | 22 | 125 | 5 | 84 | 57 |
| 3.0–4.9 y | 317 723 (3.3) | 308 | 41 | 173 | 5 | 126 | 58 |
| 5.0–9.9 y | 1 082 408 (11.2) | 1437 | 193 | 753 | 18 | 608 | 216 |
| 10.0–14.9 y | 1 555 448 (16.2) | 2780 | 346 | 1461 | 39 | 1256 | 388 |
| 15.0–19.9 y | 1 839 722 (19.1) | 4185 | 497 | 2152 | 41 | 1917 | 583 |
| 20.0–24.9 y | 1 624 570 (16.9) | 4017 | 495 | 1964 | 48 | 1820 | 597 |
| 25.0–29.9 y | 882 071 (9.2) | 2208 | 231 | 1045 | 42 | 1001 | 350 |
| ≥30.0 y | 240 519 (2.5) | 639 | 81 | 309 | 11 | 300 | 115 |
| Age at first birth | |||||||
| 13–23 y | 3 149 633 (39.1) | 5562 | 598 | 2882 | 81 | 2295 | 921 |
| 24–25 y | 1 493 666 (18.5) | 2851 | 344 | 1428 | 44 | 1260 | 422 |
| 26–29 y | 2 026 933 (25.2) | 4246 | 534 | 2154 | 54 | 2000 | 612 |
| 30–39 y | 1 327 752 (16.5) | 3001 | 407 | 1455 | 30 | 1482 | 395 |
| 40–49 y | 55 027 (0.7) | 130 | 22 | 61 | 0 | 73 | 13 |
| Missing | 1848 (0) | 3 | 1 | 2 | 0 | 2 | 1 |
| Parity | |||||||
| 0 | 1 680 909 (17.5) | 3033 | 413 | 1446 | 38 | 1396 | 394 |
| 1 | 1 569 982 (16.3) | 3190 | 404 | 1451 | 40 | 1412 | 479 |
| 2 | 3 628 391 (37.7) | 7548 | 904 | 3664 | 99 | 3427 | 1111 |
| 3 | 1 890 292 (19.6) | 3639 | 422 | 1994 | 46 | 1669 | 541 |
| 4 | 590 884 (6.1) | 1015 | 130 | 604 | 19 | 429 | 165 |
| 5 | 169 499 (1.8) | 264 | 30 | 174 | 4 | 122 | 45 |
| 6–10 | 95 770 (1.0) | 137 | 16 | 95 | 1 | 53 | 23 |
| Family history of breast cancer | |||||||
| Yes | 1 051 985 (12.4) | 3587 | 666 | 2099 | 37 | 1983 | 545 |
| No | 7 087 732 (83.5) | 11 737 | 1451 | 6736 | 181 | 5168 | 1691 |
| Missing | 346 339 (4.1) | 1536 | 118 | 210 | 11 | 632 | 279 |
| Breastfeeding status (parous women only) | |||||||
| Yes | 5 007 247 (72.9) | 10 096 | 1368 | 5582 | 128 | 5020 | 1491 |
| No | 1 337 496 (19.5) | 2924 | 418 | 1661 | 43 | 1460 | 595 |
| Missing | 519 487 (7.6) | 879 | 109 | 613 | 36 | 390 | 182 |
ER = estrogen receptor.
*
Percentages may not sum to 100 due to rounding.
†
Calculated for studies that provided information on the characteristic of interest and that reflect changes in time-updated exposures, including time since most recent birth, parity, family history, and breastfeeding status.
‡
Available for 11 994 women.
§
Available for 11 266 women.