Table 3.
Case-control ORs of breast cancer risk factors by age among premenopausal women in Carolina Breast Cancer Study phases I and II (n = 1904)
| Age <40 years | Age ≥40 years | Test for heterogeneity | |||||
|---|---|---|---|---|---|---|---|
| Risk factor | Control subjects, n (%) | Cases, n (%) | OR (95 % CI)a | Control subjects, n (%) | Cases, n (%) | OR (95 % CI)a | χ2 test, df (p value)b |
| BMI, kg/m2 | |||||||
| < 25.0 | 93 (41.9) | 154 (49.2) | 1.0 | 255 (40.0) | 316 (44.3) | 1.0 | 0.13, 2 (0.9) |
| 25–29.9 | 58 (26.1) | 71 (22.7) | 0.72 (0.45–1.13) | 184 (28.9) | 181 (25.4) | 0.87 (0.65–1.14) | |
| ≥ 30.0 | 71 (32.0) | 88 (28.1) | 0.73 (0.47–1.13) | 198 (31.1) | 217 (30.4) | 0.98 (0.74–1.29) | |
| Missing | 12 | 6 | |||||
| WHR | |||||||
| < 0.77 | 89 (41.4) | 135 (44.3) | 1.0 | 235 (36.8) | 231 (32.4) | 1.0 | 2.47, 2 (0.3) |
| 0.77–0.83 | 68 (31.6) | 98 (32.1) | 0.94 (0.61–1.43) | 209 (32.7) | 245 (34.4) | 1.42 (1.08–1.86) | |
| ≥ 0.84 | 58 (27.0) | 72 (23.6) | 0.86 (0.54–1.36) | 195 (30.5) | 237 (33.2) | 1.58 (1.18–2.11) | |
| Missing | 7 | 8 | 10 | 7 | |||
| Parity | |||||||
| Nulliparous | 49 (22.1) | 70 (22.4) | 1.0 | 76 (11.7) | 106 (14.7) | 1.0 | 3.55, 2 (0.2) |
| 1–2 births | 133 (59.9) | 180 (57.5) | 1.05 (0.67–1.63) | 383 (59.0) | 420 (58.3) | 0.82 (0.58–1.14) | |
| ≥ 3 births | 40 (18.0) | 63 (20.1) | 1.29 (0.74–2.27) | 190 (29.3) | 194 (26.9) | 0.81 (0.55–1.18) | |
| History of breastfeedingc | |||||||
| Never | 77 (44.5) | 145 (60.2) | 1.0 | 343 (60.2) | 370 (60.6) | 1.0 | 9.01, 1 (0.003) |
| Ever | 96 (55.5) | 96 (39.8) | 0.48 (0.31–0.75) | 227 (39.8) | 241 (39.4) | 0.90 (0.70–1.16) | |
| Lifetime breastfeeding durationc | |||||||
| Never | 86 (49.7) | 148 (61.4) | 1.0 | 346 (60.7) | 378 (61.9) | 1.0 | 6.36, 2 (0.04) |
| > 0–3 months | 25 (14.5) | 33 (13.7) | 0.79 (0.43–1.45) | 69 (12.1) | 65 (10.6) | 0.86 (0.58–1.25) | |
| ≥ 4 months | 62 (35.8) | 60 (24.9) | 0.50 (0.30–0.81) | 155 (27.2) | 168 (27.5) | 0.87 (0.65–1.15) | |
| Time since last term pregnancyc | |||||||
| < 10 years | 119 (68.8) | 169 (70.1) | 1.0 | 76 (13.3) | 108 (17.7) | 1.0 | 4.44, 2 (0.1) |
| 10–19 years | 50 (28.9) | 66 (27.4) | 1.02 (0.64–1.63) | 302 (53.0) | 283 (46.3) | 0.71 (0.50–1.01) | |
| ≥ 20 years | 4 (2.3) | 6 (2.5) | 1.14 (0.30–4.27) | 192 (33.7) | 220 (36.0) | 0.96 (0.64–1.43) | |
| Age at first live birthc | |||||||
| < 26 years | 103 (59.5) | 146 (60.6) | 1.0 | 386 (67.7) | 414 (67.8) | 1.0 | 0.00, 1 (1.0) |
| ≥ 26 years | 70 (40.5) | 95 (39.4) | 1.03 (0.68–1.58) | 184 (32.3) | 197 (32.2) | 0.93 (0.71–1.20) | |
| Age at last live birthc | |||||||
| < 30 years | 102 (59.0) | 148 (61.4) | 1.0 | 344 (60.4) | 362 (59.2) | 1.0 | 0.01, 1 (0.9) |
| ≥ 30 years | 71 (41.0) | 93 (38.6) | 1.01 (0.67–1.54) | 226 (35.6) | 249 (40.8) | 0.99 (0.78–1.26) | |
| Age at menarche | |||||||
| < 13 years | 111 (50.2) | 170 (54.3) | 1.0 | 323 (49.8) | 393 (54.6) | 1.0 | 0.01, 1 (0.9) |
| ≥ 13 years | 110 (49.8) | 143 (45.7) | 0.85 (0.59–1.20) | 326 (50.2) | 327 (45.4) | 0.83 (0.66–1.03) | |
| Missing | 1 | ||||||
| Oral contraceptive use | |||||||
| Never | 33 (14.9) | 36 (11.5) | 1.0 | 112 (17.3) | 125 (17.4) | 1.0 | 0.67, 1 (0.4) |
| Ever | 189 (85.1) | 276 (88.5) | 1.32 (0.78–2.22) | 534 (82.7) | 593 (82.6) | 0.93 (0.70–1.25) | |
| Missing | 1 | 3 | 2 | ||||
BMI body mass index, WHR waist-to-hip ratio
aAdjusted for matching factors race and age
bLikelihood ratio tests were performed to assess age-related heterogeneity in risk factor associations by comparing the estimated log-likelihood of adjusted models with that of the adjusted model including a multiplicative interaction term for age and the corresponding risk factor (e.g., BMI × age). Statistically significant heterogeneity by age was defined with α = 0.1
cAmong parous women