Table 2.

Reproductive factors in relation to breast cancer subtypesa

 ER+/PR+ER+/PRER/PR
CasesPerson- yearsHR95% CICasesPerson- yearsHR95% CICasesPerson- yearsHR95% CI
Number of births
 0110212,6291.00Reference29212,4951.00Reference56212,5391.00Reference
 1112161,9640.770.56–1.0435161,8350.860.48–1.5364161,8871.040.69–1.56
 2136171,8180.760.57–1.0135171,6490.700.40–1.22107171,7711.541.07–2.21
 ≥399144,7650.530.39–0.7332144,6490.620.35–1.1189144,7491.480.98–1.84
Ptrend = 0.0002Ptrend = 0.08Ptrend = 0.009
Age at first birth
 <20103150,9431.00Reference26150,8081.00Reference82150,9061.00Reference
 20–24106162,4250.900.68–1.1935162,3061.210.72–2.0482162,3870.970.71–1.33
 25–2980100,0931.230.90–1.6825100,0041.540.85–2.7959100,0641.320.92–1.90
 ≥305865,0861.340.93–1.941665,0161.450.72–2.943765,0501.470.94–2.29
Ptrend = 0.05Ptrend = 0.18Ptrend = 0.04
Years since last birth
 ≥15264278,4251.00Reference85278,1211.00Reference184278,2951.00Reference
 10–142665,2080.930.60–1.57865,1790.960.42–2.212957,2141.300.82–2.08
 <1051128,9131.691.08–2.639128,1210.890.34–2.3444128,9011.610.97–2.68
Ptrend = 0.03Ptrend = 0.82Ptrend = 0.08
Lactation, parous women only
 No192269,7251.00Reference56269,4931.00Reference162269,6751.00Reference
 Yes151203,1351.130.91–1.4246202,9601.250.83–1.8995203,0470.780.60–1.03
  <6 mo85112,0081.110.86–1.4427111,9121.300.81–2.0851111,9560.780.56–1.08
  ≥6 mo6691,1271.170.87–1.571991,0481.180.68–2.064491,0910.790.56–1.13

NOTE: HRs were derived from models that adjusted for age, time, geographic region, age at menarche, age at menopause, use of menopausal hormone supplements, use of oral contraceptives, body mass index, physical activity, alcohol intake, cigarette smoking, family history of breast cancer, and each of the other reproductive factors.

  • a18 breast cancer cases were classified as ER/PR+ and were omitted from this table.