Table 2.

Risk of breast cancer (HR) associated with age at menarche and duration of ovarian activity

Whole cohort
BRCA1 carriers
BRCA2 carriers
Pyrs*BCHR (95% CI)HR (95% CI)HR (95% CI)
Age at menarche
    ≤11 y10,940145111
    12-14 y42,7275540.89 (0.68-1.15)0.80 (0.61-1.05)1.09 (0.62-1.92)
    ≥15 y10,2521420.96 (0.68-1.36)0.86 (0.60-1.25)1.22 (0.61-2.43)
    Unknown903121.22 (0.53-2.82)0.95 (0.39-2.29)1.00 (0.05-21.7)
Duration of breast mitotic activity before first full-term pregnancy§
    <10 y43,646253111
    10-20 y17,3004500.94 (0.74-1.20)0.89 (0.70-1.14)1.26 (0.71-2.23)
    ≥20 y2,0431180.82 (0.54-1.24)0.72 (0.46-1.14)1.68 (0.65-4.38)
    Unknown1,833320.71 (0.32-1.61)0.54 (0.22-1.33)0.66 (0.07-6.60)
Lifetime duration of breast mitotic activity menstrual cycles§
    <20 y49,46372111
    20-30 y8,1113480.92 (0.63-1.34)1.07 (0.71-1.60)0.51 (0.15-1.69)
    ≥30 y3,1962211.00 (0.63-1.61)1.16 (0.71-1.92)0.42 (0.11-1.65)
    Unknown4,0521120.99 (0.62-1.58)1.11 (0.68-1.81)0.48 (0.11-2.04)
  • * Number of person-years of observation in the specified cohort.

  • Number of breast cancer cases occurring in the specified cohort.

  • Estimated hazard ratio, stratified by birth cohort and country group and adjusted for number of children and oophorectomy.

  • § Defined as in Statistical Methods.

  • Estimated hazard ratio, stratified by birth cohort and country group and adjusted for number of children, oophorectomy and HRT use.