| 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 | Â | Â | Â | Â | Â | Â |
Ever pregnant | Â | Â | Â | Â | Â | Â |
   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 | Â | Â | Â | Â | Â | Â |
   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 | Â | Â | Â | Â | Â | Â |
Number of full-term pregnancies | Â | Â | Â | Â | Â | Â |
   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 | Â | Â | Â | Â | Â | Â |
   < 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 | Â | Â | Â | Â | Â | Â |
   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 |  |  |  |