Table 2.

Case–control ORs of breast cancer risk factors by age among premenopausal women in the AMBER Consortium

<40 years (N = 1,775)≥40 years (N = 4,961)
Risk factorControls N (%)Cases N (%)OR (95% CI)aControls N (%)Cases N (%)OR (95% CI)aPheterogeneityb
BMI (kg/m2)
 <25.0466 (35.0)149 (35.1)1.0955 (25.4)283 (24.6)1.00.8
 25–29.9368 (27.7)119 (28.1)0.92 (0.66–1.28)1,172 (31.2)376 (32.7)0.99 (0.81–1.21)
 ≥30.0497 (37.3)156 (36.8)0.71 (0.51–0.98)1,628 (43.4)491 (42.7)0.91 (0.75–1.10)
 Trend testP = 0.0005P = 0.2
 Missing1644214
WHR
 <0.77441 (37.5)104 (26.3)1.01,166 (34.3)1,166 (34.3)1.00.06
 0.77–0.83341 (29.0)128 (32.4)1.14 (0.81–1.59)952 (28.0)952 (28.0)1.02 (0.83–1.25)
 ≥0.84394 (33.5)163 (41.3)1.46 (1.04–2.05)1,285 (37.8)1,285 (37.8)1.11 (0.91–1.35)
 Trend testP = 0.003P = 0.9
 Missing1713339477
Age at menarche
 <13 years803 (59.8)251 (58.6)1.02,038 (53.9)654 (56.4)1.00.3
 ≥13 years540 (40.2)177 (41.4)0.97 (0.76–1.24)1,746 (46.1)506 (43.6)0.85 (0.74–0.98)
 Missing40134
Parity
 Nulliparous516 (38.3)111 (26.9)1.0872 (23.0)222 (19.1)1.00.2
 1–2 births644 (47.8)224 (52.3)1.19 (0.85–1.66)2,081 (54.8)654 (56.2)1.09 (0.90–1.34)
 ≥3 births187 (13.9)93 (21.7)1.26 (0.78–2.03)844 (22.2)288 (24.7)0.97 (0.75–1.25)
Age at first live birthc
 <25 years481 (59.0)202 (63.9)1.01,633 (56.9)547 (58.8)1.00.3
 ≥25 years334 (41.0)114 (36.1)1.03 (0.74–1.43)1,238 (43.1)383 (41.2)1.18 (0.99–1.41)
 Missing165412
Age at last live birthc
 <30 years483 (59.6)198 (62.9)1.01,436 (50.5)453 (48.9)1.00.1
 ≥30 years327 (40.4)117 (37.1)0.89 (0.63–1.25)1,408 (49.5)474 (51.1)1.03 (0.86–1.23)
 Missing2128115
Time since last birthc
 <10 years515 (63.6)202 (64.1)1.0521 (18.3)183 (19.7)1.00.3
 ≥10 years295 (36.4)113 (35.9)1.14 (0.82–1.57)2,323 (81.7)744 (80.3)0.86 (0.69–1.07)
 Missing2128115
Breastfeeding durationc
 Parous, never388 (47.7)177 (56.2)1.01,444 (50.3)491 (52.7)1.00.1
 <3 months114 (14.0)30 (9.5)0.70 (0.43–1.16)315 (11.0)96 (10.3)1.08 (0.82–1.42)
 ≥3 months312 (38.3)108 (34.3)0.83 (0.58–1.17)1,111 (38.7)344 (37.0)0.94 (0.78–1.13)
 Missing1725511
Oral contraceptive use
 Never177 (13.2)74 (17.3)1.0539 (14.2)232 (20.0)1.00.7
 Ever1,167 (86.8)354 (82.7)1.22 (0.86–1.72)3,254 (85.8)929 (80.0)1.18 (0.97–1.44)
 Missing3043
OC use duration
 Never/<1 year413 (30.7)124 (29.0)1.01,296 (35.2)410 (35.2)1.01.0
 1–4 years473 (35.1)135 (31.6)1.13 (0.82–1.55)1,165 (30.7)332 (28.5)1.02 (0.85–1.23)
 ≥5 years460 (34.2)168 (39.3)1.42 (1.04–1.93)1,334 (35.2)422 (36.3)1.22 (1.03–1.46)
 Missing1120
OC use recency
 Never177 (13.2)74 (17.3)1.0539 (14.2)232 (20.0)1.00.4
 <10 years840 (62.5)254 (59.3)1.16 (0.69–1.95)1,065 (28.1)329 (28.3)1.48 (1.08–2.03)
 ≥10 years327 (24.3)100 (23.4)0.90 (0.56–1.47)2,189 (57.7)600 (51.7)1.13 (0.87–1.45)
 Missing3043
Family history of breast cancer
 No1,276 (94.7)363 (84.8)1.03,472 (91.4)996 (85.6)1.00.005
 Yes71 (5.3)65 (15.2)3.10 (2.08–4.63)325 (8.6)168 (14.4)1.57 (1.26–1.94)
  • aAdjusted for age, study site, index year, geographic location, education level, and confounders, by model. BMI: WHR, parity; WHR: BMI, parity; parity: age at first live birth; age at last live birth: parity, age at first birth; time since last birth: parity, age at first live birth, age at last live birth; breastfeeding duration: BMI, parity, age at first live birth, age at last live birth; oral contraceptive use/duration/recency: parity, age at first live birth, age at last live birth (OC use duration and recency models also adjusted for the other).

  • bLikelihood ratio tests assessed age-related heterogeneity in risk factor associations by comparing the estimated log-likelihood of adjusted models to 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.