Table 2.

Risk factors for second primary CBC among WECARE subjects

FactorBilateral (n)Unilateral (n)Unadjusted (weighted), RR* (95% CI)Adjusted (weighted), RR (95% CI)
Age at menarche
    13+ y3677821.001.00
    <13 y3386111.29 (1.04-1.60)1.26 (1.01-1.58)
Menopausal status (as of reference date)
    Postmenopausal <452094551.001.00
    Premenopausal1242730.99 (0.71-1.36)0.78 (0.55-1.10)
    Postmenopausal 45+3746651.32 (1.01-1.73)1.20 (0.91-1.58)
Gravidity§
    Never pregnant871571.001.00
    Incomplete pregnancies only46681.30 (0.78-2.18)1.23 (0.73-2.09)
    At least one full-term pregnancy5751,1720.93 (0.68-1.29)0.92 (0.66-1.28)
Age at first FTP (as of reference date)
    <20 y771701.001.00
    20-24 y2364611.28 (0.89-1.84)1.13 (0.78-1.66)
    25-29 y1593631.15 (0.79-1.68)0.96 (0.65-1.43)
    Nulliparous or 30+ y2364031.53 (1.06-2.21)1.22 (0.77-1.92)
        Trend P value0.76
Number of FTPs (as of reference date)
    Nulliparous1332251.001.00
    11212041.17 (0.81-1.68)1.14 (0.78-1.66)
    22705450.92 (0.69-1.24)0.92 (0.68-1.25)
    31282630.72 (0.51-1.02)0.73 (0.51-1.04)
    4+561600.48 (0.31-0.75)0.46 (0.29-0.72)
        Trend P value0.001
Breastfeeding (as of reference date)
Never3366081.001.00
Ever3727890.79 (0.64-0.99)0.82 (0.63-1.07)
Duration of breastfeeding (as of reference date)
    Never3366081.001.00
    1 to 3 mo1021790.97 (0.71-1.34)1.02 (0.71-1.45)
    4 to 6 mo942110.79 (0.57-1.10)0.75 (0.52-1.08)
    7 to 23 mo1323050.76 (0.57-1.02)0.77 (0.55-1.08)
    24+ mo43940.66 (0.43-1.03)0.82 (0.50-1.34)
        Trend P value0.09
  • * Rate ratios are adjusted for the counter-matching sampling.

  • Rate ratios are adjusted for the counter-matching sampling and for the following covariates: age at first diagnosis, age at menarche, age at menopause, number of full-term pregnancies, family history of breast cancer, histology of the first primary, stage of the first primary, first primary treatment history and radiation therapy.

  • Reference date is the date of contralateral breast cancer diagnosis for the cases and the corresponding date for controls.

  • § Rate ratios are adjusted for all the covariates listed above except for the number of full-term pregnancies.