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Table 3 Adjusted RRs for the association between pregnancy-related factors and the subtypes of 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

  ER+ ER- ER+/PR+ ER-/PR-
  Cases No. Adjusted RR (95% CI) Cases No. Adjusted RR (95% CI) Cases No. Adjusted RR (95% CI) Cases No. Adjusted RR (95% CI)
All women         
Ever pregnant         
   No 382 Reference 56 Reference 318 Reference 50 Reference
   Yes 1,722 0.87 (0.78 to 0.97) 283 0.99 (0.74 to 1.31) 1,333 0.81 (0.71 to 0.91) 267 1.04 (0.77 to 1.41)
P for homogeneitya 0.42     0.12   
Number of full-term pregnancyb         
   1 272 0.93 (0.80 to 1.09) 33 0.78 (0.51 to 1.20) 218 0.90 (0.75 to 1.06) 32 0.85 (0.55 to 1.33)
   2 640 0.88 (0.77 to 1.00) 121 1.15 (0.84 to 1.58) 485 0.80 (0.69 to 0.92) 111 1.19 (0.85 to 1.66)
   3 443 0.85 (0.74 to 0.98) 66 0.88 (0.62 to 1.26) 352 0.82 (0.70 to 0.95) 66 0.99 (0.68 to 1.43)
   ≥ 4 267 0.76 (0.65 to 0.89) 45 0.90 (0.61 to 1.34) 206 0.71 (0.59 to 0.85) 40 0.90 (0.59 to 1.37)
   P trend   0.0004   0.72   < 0.0001   0.85
P for homogeneitya 0.33     0.17   
Ever pregnant, incomplete pregnancies only or outcome of pregnancies unknown 100   18   72   18  
Parous women only c, d         
Number of full-term pregnancies         
   1 272 Reference 33 Reference 218 Reference 32 Reference
   2 638 0.96 (0.83 to 1.11) 121 1.51 (1.01 to 2.25) 484 0.91 (0.77 to 1.08) 111 1.39 (0.92 to 2.10)
   3 443 0.95 (0.80 to 1.12) 65 1.18 (0.75 to 1.86) 352 0.95 (0.79 to 1.15) 65 1.17 (0.74 to 1.85)
   ≥ 4 267 0.85 (0.70 to 1.04) 45 1.24 (0.75 to 2.07) 206 0.84 (0.68 to 1.05) 40 1.08 (0.64 to 1.83)
   P trend   0.13   0.99   0.25   0.74
P for homogeneitya   0.57     0.87   
Age at first full-term pregnancy, years         
   < 21 152 Reference 22 Reference 120 Reference 21 Reference
   21 to 24 527 1.04 (0.87 to 1.25) 97 1.37 (0.86 to 2.18) 413 1.04 (0.85 to 1.28) 90 1.31 (0.81 to 2.12)
   25 to 29 660 1.21 (1.01 to 1.45) 98 1.31 (0.82 to 2.10) 501 1.18 (0.97 to 1.45) 93 1.28 (0.79 to 2.07)
   30 to 34 204 1.17 (0.94 to 1.46) 39 1.71 (0.99 to 2.95) 166 1.23 (0.96 to 1.57) 37 1.66 (0.95 to 2.90)
   ≥ 35 77 1.37 (1.02 to 1.83) 8 1.21 (0.52 to 2.81) 60 1.36 (0.98 to 1.89) 7 1.07 (0.44 to 2.60)
   P trend   0.004   0.23   0.009   0.30
   P for homogeneitya   0.96     0.92   
Duration of breastfeeding, months         
   Never 499 Reference 80 Reference 379 Reference 75 Reference
   < 6 433 0.98 (0.86 to 1.11) 68 0.97 (0.70 to 1.34) 346 1.03 (0.89 to 1.19) 63 0.96 (0.69 to 1.34)
   6 to 11 296 1.06 (0.92 to 1.23) 52 1.15 (0.81 to 1.63) 229 1.08 (0.91 to 1.27) 50 1.16 (0.81 to 1.67)
   12 to 23 259 1.04 (0.89 to 1.21) 42 1.02 (0.69 to 1.50) 205 1.08 (0.90 to 1.29) 39 1.02 (0.69 to 1.52)
   ≥ 24 133 1.02 (0.83 to 1.25) 22 1.00 (0.61 to 1.66) 101 1.00 (0.80 to 1.27) 21 1.06 (0.64 to 1.78)
   P trend   0.52   0.78   0.54   0.65
   P for homogeneitya   0.98     0.88   
  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 subtypes of 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: RR, relative risk; CI, confidence interval; ER, estrogen receptor; PR, progesterone receptor; HT, hormone therapy.