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Table 2 Multivariable-adjusteda hazard ratios (HRs) and 95% confidence intervals for breast cancer overall and by ER status in relation to parity and breastfeeding: Nurses’ Health Study (1976–2012) and Nurses’ Health Study II (1989–2013)

From: Parity, breastfeeding, and breast cancer risk by hormone receptor status and molecular phenotype: results from the Nurses’ Health Studies

  All ER+ ER Phet by
  Cases PY HR (95% CI) Cases PY HR (95% CI) Cases PY HR (95% CI) subtype
Parous
 Nulliparous 1498 742,502 1 (ref.) 979 742,976 1 (ref.) 209 743,672 1 (ref.)  
 Parous 10,954 4,540,107 0.84 (0.80–0.89) 7256 4,543,555 0.82 (0.77–0.88) 1769 4,548,669 0.98 (0.84–1.13) 0.03
Breastfeeding
 Never breastfed (parous + nulliparous) 5101 2,100,966 1 (ref.) 3271 2,102,676 1 (ref.) 835 2,104,928 1 (ref.)  
 Ever breastfed 7351 3,181,643 0.94 (0.91–0.98) 4964 3,183,855 0.99 (0.94–1.04) 1143 3,187,412 0.83 (0.75–0.92) 0.002
 ≤ 6 months 3242 1,255,908 0.93 (0.88–0.97) 2177 1,256,886 0.96 (0.90–1.01) 517 1,258,482 0.84 (0.75–0.95)  
 7–11 months 1203 535,074 0.96 (0.90–1.03) 819 535,428 1.03 (0.95–1.11) 180 535,998 0.81 (0.68–0.95)  
 ≥ 12 months 2906 1,390,661 0.96 (0.91–1.01) 1968 1,391,541 1.02 (0.96–1.09) 446 1,392,932 0.83 (0.73–0.94)  
    Ptrend = 0.72    Ptrend = 0.22    Ptrend = 0.05 0.02
Parity/breastfeeding
 Nulliparous 1498 742,502 1 (ref.) 979 742,976 1 (ref.) 209 743,672 1 (ref.)  
 Parous women, never breastfed 3603 1,358,464 0.88 (0.83–0.94) 2292 1,359,700 0.83 (0.77–0.90) 626 1,361,257 1.11 (0.94–1.31) < 0.001b
 1 child 518 210,088 0.92 (0.83–1.02) 332 210,263 0.88 (0.78–1.00) 91 210,483 1.15 (0.90–1.48) 0.002c
 2 children 1386 516,819 0.93 (0.86–1.00) 870 517,287 0.86 (0.78–0.94) 250 517,868 1.20 (0.99–1.45)  
 ≥ 3 children 1699 631,558 0.81 (0.76–0.87) 1090 632,150 0.77 (0.70–0.84) 285 632,906 1.03 (0.85–1.25)  
 Parous women, ever breastfed 7351 3,181,643 0.83 (0.78–0.88) 4964 3,183,855 0.82 (0.76–0.88) 1143 3,187,412 0.92 (0.79–1.08)  
 1 child 666 313,938 0.95 (0.86–1.04) 437 314,149 0.93 (0.83–1.04) 97 314,457 0.97 (0.76–1.23)  
 2 children 2493 1,166,539 0.85 (0.79–0.91) 1694 1,167,283 0.85 (0.78–0.92) 362 1,168,492 0.87 (0.73–1.03)  
 ≥3 children 4192 1,701,166 0.79 (0.74–0.84) 2833 1,702,423 0.78 (0.72–0.84) 684 1,704,463 0.95 (0.81–1.12)  
Among parous women
 Parityd
  1 child 1184 524,026 1 (ref.) 769 524,412 1 (ref.) 188 524,940 1 (ref.)  
  2 children 3879 1,683,357 1.02 (0.95–1.09) 2564 1,684,570 1.02 (0.94–1.11) 612 1,686,360 1.02 (0.86–1.21)  
  3 children 3165 1,259,779 1.03 (0.96–1.10) 2120 1,260,791 1.03 (0.94–1.13) 502 1,262,311 1.06 (0.88–1.27)  
  ≥ 4 children 2726 1,072,945 0.92 (0.85–1.00) 1803 1,073,782 0.91 (0.82–1.00) 467 1,075,059 1.08 (0.89–1.31)  
    Ptrend = 0.0001    Ptrend = 0.0001    Ptrend = 0.43 0.02
 Breastfeedingd
  Never breastfed 3603 1,358,464 1 (ref.) 2292 1,359,700 1 (ref.) 626 1,361,257 1 (ref.)  
  Ever breastfed 7351 3,181,643 0.95 (0.91–0.99) 4964 3,183,855 0.99 (0.94–1.05) 1143 3,187,412 0.82 (0.74–0.91) < 0.001
  ≤ 6 months 3242 1,255,908 0.93 (0.89–0.98) 2177 1,256,886 0.96 (0.91–1.02) 517 1,258,482 0.84 (0.75–0.95)  
  7–11 months 1203 535,074 0.95 (0.89–1.02) 819 535,428 1.02 (0.94–1.10) 180 535,998 0.79 (0.67–0.94)  
  ≥ 12 months 2906 1,390,661 0.97 (0.92–1.02) 1968 1,391,541 1.04 (0.97–1.11) 446 1,392,932 0.80 (0.70–0.92)  
    Ptrend, incl never = 0.70    Ptrend, incl never = 0.05(+)    Ptrend, incl never = 0.02 0.002
    Ptrend, among ever = 0.10    Ptrend, among ever = 0.02(+)    Ptrend, among ever = 0.35  
  1. aAdjusted for age, height (< 1.60, 1.60 to < 1.65, 1.65 to < 1.70, 1.70 to < 1.75, 1.75+ meters), BMI at age 18 (< 20, 20–21.9, 22–23.9, 24–26.9, 27+ kg/m2), weight change since age 18 (continuous, kg), age at menarche (< 12, 12, 13, 14, 15+ years), menopausal status (premenopausal, postmenopausal or unknown), age at natural menopause (continuous), HT use (never, past, current E only, current E + P, current other), alcohol consumption (non-drinker, < 5, 5–10, 10–15, 15+ g/day), total physical activity (< 3, 3–9, 9–18, 18–27, 27+ MET-h/day), family history of breast cancer (yes/no), history of benign breast disease (yes/no)
  2. bPhet for nulliparous and parous ever vs. never breastfed
  3. cPhet for nulliparous and number of children by ever vs. never breastfed
  4. dAmong parous women only, additionally adjusted for age at first birth (continuous, years), and parity and breastfeeding mutually adjusted