Table 1.
Baseline characteristics of the participants by case–control status
| Controls (n = 685) | Cases (n = 414) | |||
|---|---|---|---|---|
| Age, years at mammography | ||||
| Mean (SD) | 59.5 (6.6) | 67.5 (12.7) | ||
| Missing, n | 6 | 2 | ||
| Ethnicity | ||||
| White, n (%) | 520 (76.5) | 370 (90.5) | ||
| Other, n (%) | 160 (23.5) | 39 (9.5) | ||
| Missing, n | 5 | 5 | ||
| Educational level | ||||
| None, n (%) | 15 (2.2) | 10 (3.7) | ||
| Primary, n (%) | 20 (3.0) | 7 (2.6) | ||
| Secondary, n (%) | 297 (43.9) | 186 (68.4) | ||
| University, n (%) | 344 (50.9) | 69 (25.4) | ||
| Missing, n | 9 | 142 | ||
| Body mass index at mammography a , kg/m 2 | ||||
| Mean (SD) | 26.1 (5.6) | 26.4 (4.9) | ||
| Missing, n | 29 | 46 | ||
| Age at menarche, yrs | ||||
| <12, n (%) | 141 (21.0) | 81 (21.3) | ||
| 12, n (%) | 130 (19.4) | 78 (20.5) | ||
| 13, n (%) | 168 (25.0) | 87 (22.8) | ||
| 14+, n (%) | 232 (34.6) | 135 (35.4) | ||
| Missing, n | 14 | 33 | ||
| Menopausal status at mammography b | ||||
| Pre- and perimenopausal, n (%) | 91 (13.3) | 55 (13.3) | ||
| Postmenopausal, n (%) | 591 (86.7) | 358 (86.7) | ||
| Missing, n | 3 | 1 | ||
| Ever use of oral contraceptives | ||||
| Yes, n (%) | 472 (70.5) | 216 (55.2) | ||
| No, n (%) | 198 (29.6) | 175 (44.8) | ||
| Missing, n | 15 | 23 | ||
| Ever use of hormonal therapy | ||||
| Yes, n (%) | 208 (31.2) | 143 (36.8) | ||
| No, n (%) | 459 (68.8) | 246 (63.2) | ||
| Missing, n | 18 | 25 | ||
| Nulliparity | ||||
| No, n (%) | 467 (69.1) | 343 (84.1) | ||
| Yes, n (%) | 209 (30.9) | 65 (15.9) | ||
| Missing, n | 9 | 6 | ||
| Number of children c , n (%) | ||||
| 1 to 2 | 301 (65.4) | 218 (64.9) | ||
| 2 to 3 | 126 (27.4) | 105 (31.3) | ||
| 5+ | 33 (7.2) | 13 (3.9) | ||
| Age at first birth, yrs c , n (%) | ||||
| <20 | 83 (18.1) | 25 (7.7) | ||
| 20 to 30 | 258 (56.3) | 213 (65.7) | ||
| 30+ | 117 (25.6) | 86 (26.5) | ||
| Ever breastfed c , n (%) | ||||
| Yes | 358 (77.2) | 224 (74.7) | ||
| No | 106 (22.8) | 76 (25.3) | ||
aBody mass index estimated from self-reported height and weight as weight/height2 (in kg/m2). bPostmenopausal women defined as those who self-reported natural (that is, cessation of menses for at least 12 months) or surgical menopause, were older than 55 years, or ever used hormone replacement therapy. Due to small numbers pre- (that is, younger than 55 years and still having regular periods) and perimenopausal (that is, younger than 55 years and having irregular periods) women were combined into a single category. cRestricted to ever-parous women.