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Table 2 Adjusted RRs for the association between pregnancy-related factors and breast CIS and invasive breast cancer

From: Pregnancy-related factors and the risk of breast carcinoma in situand invasive breast cancer among postmenopausal women in the California Teachers Study cohort

  Breast CIS Invasive breast cancer
  Observed Person-years Cases No. Adjusted RR (95% CI) Observed Person-years Cases No. Adjusted RR (95% CI)
All women       
Ever pregnant       
   No 90,296 116 Reference 92,927 493 Reference
   Yes 444,276 508 0.85 (0.69 to 1.04) 457,417 2335 0.91 (0.83 to 1.00)
   P for homogeneitya    0.55    
Number of full-term pregnanciesb       
   1 68,527 91 1.01 (0.77 to 1.33) 70,615 355 0.95 (0.83 to 1.09)
   2 165,472 188 0.84 (0.67 to 1.06) 170,385 878 0.94 (0.84 to 1.04)
   3 112,458 131 0.84 (0.65 to 1.08) 115,629 591 0.88 (0.78 to 0.99)
   ≥ 4 74,458 70 0.69 (0.51 to 0.93) 76,634 373 0.82 (0.72 to 0.94)
   P trend    0.008    0.003
   P for homogeneitya    0.26    
   Ever pregnant, incomplete pregnancies only or outcome of pregnancies unknown 23,361 28   24,154 138  
Parous women only c, d       
Number of full-term pregnancies       
   1 68,338 90 Reference 70,426 355 Reference
   2 164,995 188 0.92 (0.70 to 1.20) 169,889 875 1.00 (0.88 to 1.14)
   3 112,082 131 0.93 (0.69 to 1.26) 115,253 590 0.95 (0.82 to 1.10)
   ≥ 4 74,291 70 0.77 (0.54 to 1.11) 76,467 373 0.90 (0.77 to 1.07)
   P trend    0.22    0.15
   P for homogeneitya    0.61    
Age at first full-term pregnancies, years       
   < 21 46,965 44 Reference 48,165 203 Reference
   21 to 24 146,568 166 1.17 (0.84 to 1.63) 150,692 727 1.07 (0.92 to 1.25)
   25 to 29 158,816 183 1.18 (0.84 to 1.66) 163,749 888 1.22 (1.05 to 1.43)
   30 to 34 51,266 50 0.99 (0.65 to 1.50) 52,784 281 1.22 (1.01 to 1.47)
   ≥ 35 16,091 36 2.18 (1.36 to 3.49) 16,644 94 1.27 (0.99 to 1.65)
   P trend    0.09    0.002
   P for homogeneitya    0.82    
Duration of breastfeeding, months       
   Never 127,851 151 Reference 131,753 688 Reference
   < 6 113,869 132 0.97 (0.77 to 1.22) 117,111 571 0.94 (0.84 to 1.05)
   6-11 73,115 82 0.93 (0.71 to 1.22) 75,315 402 1.05 (0.93 to 1.19)
   12-23 67,172 79 1.03 (0.77 to 1.36) 69,112 352 1.02 (0.90 to 1.17)
   ≥ 24 37,699 35 0.87 (0.59 to 1.29) 38,744 180 0.99 (0.84 to 1.18)
   P trend    0.70    0.56
   P for homogeneitya    0.56    
  1. RRs are from multivariable Cox proportional hazards regression models using age (in days) as the time metric and stratified by age (in years) with the adjustment for race, family history of breast cancer in a first degree relative, age at menarche, HT use, body mass index. aHomogeneity in risk estimates between breast CIS and invasive breast cancer. bCompared with nulligravid women. cAdditionally, number of full-term pregnancies, age at first full-term pregnancy, and duration of breastfeeding mutually adjusted among parous women. dAdditionally excluded parous women who were missing information on breastfeeding or age at first full-term pregnancy. Abbreviations: CIS, carcinoma in situ; RR, relative risk; CI, confidence interval; HT, hormone therapy.