Table 2.

Multivariable analyses for associations of risk factors with invasive breast cancer among women age <45 years, by ER statusa

All invasive (n = 529)ER (n = 151)ER+ (n = 219)
Person-yearsCasesHR (95% CI)CasesHR (95% CI)CasesHR (95% CI)
Age at menarche (y)
 ≤11124,3851681.00 (reference)531.00 (reference)751.00 (reference)
 12–13221,9872750.86 (0.71–1.04)720.73 (0.51–1.04)1100.79 (0.58–1.06)
 ≥1473,990830.73 (0.56–0.96)250.70 (0.43–1.13)320.67 (0.44–1.01)
Oral contraceptive use
 Never70,615791.00 (reference)191.00 (reference)291.00 (reference)
 Ever, ≥10 years ago116,7191800.93 (0.71–1.21)541.21 (0.71–2.07)741.02 (0.66–1.58)
 Ever, 5–10 years ago63,872821.07 (0.78–1.46)191.03 (0.54–1.97)341.12 (0.67–1.85)
 Ever, within 5 years171,1101881.25 (0.95–1.64)591.53 (0.90–2.62)821.40 (0.90–2.16)
Family history of breast cancer
 No397,0644561.00 (reference)1311.00 (reference)1901.00 (reference)
 Yes25,280732.27 (1.77–2.90)202.11 (1.32–3.39)292.16 (1.46–3.20)
Parity
 Nulliparous163,0061561.11 (0.80–1.55)431.30 (0.69–2.47)680.90 (0.54–1.48)
 1 birth111,3721481.00 (reference)401.00 (reference)721.00 (reference)
 2 births90,2521351.07 (0.84–1.37)401.30 (0.82–2.05)510.80 (0.54–1.16)
 ≥3 births54,927901.21 (0.89–1.64)281.71 (0.98–2.99)280.69 (0.41–1.14)
Age at first birth (years)b
 <2064,929861.00 (reference)221.00 (reference)351.00 (reference)
 20–2476,126921.01 (0.74–1.36)281.42 (0.80–2.50)280.65 (0.39–1.09)
 ≥25110,1131901.44 (1.08–1.94)582.29 (1.32–3.97)851.06 (0.67–1.67)
Years since last birthb
 ≥10 years114,9151861.00 (reference)541.00 (reference)751.00 (reference)
 <10 years132,8171781.43 (1.13–1.81)531.39 (0.90–2.16)721.44 (1.00–2.07)
Lactationb
 Never133,1071991.00 (reference)641.00 (reference)701.00 (reference)
 Ever121,5511740.87 (0.70–1.08)440.61 (0.40–0.92)811.22 (0.87–1.73)
BMI at age 18 (kg/m2)
 <20152,1772161.00 (reference)691.00 (reference)871.00 (reference)
 20–24192,8972590.97 (0.80–1.18)620.68 (0.47–0.99)1111.04 (0.77–1.41)
 ≥2571,914490.51 (0.36–0.72)190.55 (0.31–0.99)210.52 (0.31–0.88)
Current BMI (kg/m2)
 <25147,5591681.00 (reference)401.00 (reference)731.00 (reference)
 25–29.9125,6681741.03 (0.83–1.29)571.47 (0.97–2.25)630.87 (0.62–1.24)
 ≥30145,6161831.04 (0.81–1.33)531.24 (0.76–2.01)831.10 (0.76–1.60)
Waist:hip ratio
 Tertile 1 (<0.76)113,9451231.00 (reference)331.00 (reference)551.00 (reference)
 Tertile 2 (0.76–0.84)102,0181141.00 (0.77–1.29)280.89 (0.54–1.48)531.00 (0.68–1.47)
 Tertile 3 (>0.84)101,9011411.30 (1.01–1.68)501.64 (1.04–2.59)450.88 (0.58–1.33)
Alcohol consumption (drinks)
 <1/week300,5593721.00 (reference)1141.00 (reference)1511.00 (reference)
 1–6/week104,7271300.96 (0.78–1.18)290.69 (0.46–1.05)560.94 (0.69–1.29)
 ≥7/week16,542271.28 (0.86–1.91)50.73 (0.30–1.83)121.39 (0.76–2.55)
Vigorous exercise
 None155,7442111.00 (reference)641.00 (reference)861.00 (reference)
 <5 hours/week215,8572580.96 (0.80–1.16)730.97 (0.69–1.37)1100.93 (0.28–3.02)
 ≥5 hours/week47,631551.00 (0.74–1.35)130.87 (0.47–1.59)231.00 (0.75–1.33)
  • aMultivariable HRs adjusted for age, smoking history (never active or passive, passive only, never active/unknown passive, 1 to <10 packyears, 10+ packyears), menopausal status (premenopausal, hysterectomy only, postmenopausal, missing), and all factors in Table 2.

  • bAmong parous women only. Estimates for age at first birth adjusted for age, smoking history, menopausal status, and all factors in Table 2 except years since last birth; estimates for years since last birth adjusted for age, smoking history, menopausal status, and all factors in Table 2 except age at first birth; estimates for lactation adjusted for age, smoking history, menopausal status, and all factors in Table 2.