Table 1.
Baseline characteristics of the participants by status in the UK and Norwegian studiesa
| UK case control study | Norwegian cohort study | |||
|---|---|---|---|---|
| Controls (n = 685) | Cases (n = 414) | Non-cases (n = 61,059) |
Cases (n = 657) |
|
| Age at mammography | ||||
| Mean (SD) | 59.5 (6.6) | 67.5 (12.7) | 56.9 (5.74) | 57.7 (5.43) |
| Number | 679 | 412 | 61,059 | 657 |
| BMIb | ||||
| Mean (SD) | 26.1 (5.6) | 26.4 (4.9) | 25.6 (4.2) | 25.8 (4.1) |
| Number | 656 | 368 | 54,345 | 589 |
| Ethnicity (UK)/Country of birth (Norway), n (%) | ||||
| White/Norway | 520 (76.5) | 370 (89.4) | 56,234 (93.8) | 612 (94.2) |
| Non-white/Outside Norway | 160 (23.5) | 39 (9.6) | 3693 (6.2) | 38 (5.8) |
| Missing | 5 | 5 | 1132 | 7 |
| Family history of BC, n (%) | ||||
| No | N/A | N/A | 45,168 (77.1) | 447 (70.0) |
| Yes | N/A | N/A | 13,390 (22.9) | 192 (30.0) |
| Missing | 2501 | 18 | ||
| Menopausal statusc, n (%) | ||||
| Pre- + peri-menopausal | 91 (13.3) | 55 (13.3) | 14,776 (25.2) | 141 (22.1) |
| Post-menopausal | 591 (86.7) | 358 (86.7) | 43,856 (74.8) | 496 (77.9) |
| Missing | 3 | 1 | 2427 | 20 |
| Parity, n (%) | ||||
| Nulliparous | 209 (30.9) | 65 (15.9) | 4946 (8.5) | 57 (9.0) |
| Parous | 467 (69.1) | 343 (84.1) | 53,563 (91.5) | 577 (91.0) |
| Missing | 9 | 6 | 2550 | 23 |
| Age at menarche in years, n (%) | ||||
| <13 | 271 (53.9) | 159 (54.1) | 16,764 (40.9) | 186 (41.9) |
| 14+ | 232 (46.1) | 135 (45.9) | 24,202 (59.1) | 258 (58.1) |
| Missing | 14 | 33 | 4107 | 43 |
| Hormone therapy use, n (%) | ||||
| No | 459 (68.8) | 246 (63.2) | 34,150 (66.2) | 305 (55.6) |
| Yes | 208 (31.2) | 143 (36.8) | 17,418 (33.8) | 244 (44.4) |
| Missing | 18 | 25 | 9491 | 108 |
| Educational level, n (%) | ||||
| None/primary school | 35 (5.2) | 17 (6.2) | ||
| Lower secondary | 13,772 (23.3) | 164 (25.9) | ||
| Secondary or higher | 641 (94.8) | 225 (93.8) | 45,457 (76.7) | 470 (74.1) |
| Missing | 9 | 142 | 1830 | 23 |
| Breastfeeding among parous women, n (%) | ||||
| Yes | 358 (76.7) | 224 (74.7) | 46,107 (99.9) | 497 (100) |
| Missing | 3 | 43 | 9929 | 103 |
Abbreviations: BC breast cancer, BMI body mass index, N/A data not available, SD standard deviation
aPercentages calculated without missing values
bBMI estimated from self-reported height and weight as weight/height2 (in kg/m2)
cPost-menopausal women defined as those who self-reported natural (cessation of menses for at least 12 months) or surgical menopause, were older than 55 years, or had ever used hormone therapy. Owing to small numbers, pre-menopausal (younger than 55 years and still having regular periods) and peri-menopausal (younger than 55 years and having irregular periods) women were combined into a single category