Table 2.

HRs of the association between current menstrual cycle length (1993) and premenopausal breast cancer incidence among participants of the Nurses' Health Study II between 1993 and 2001

Current menstrual cycle length (d)No. of casesPerson-years of follow-upAge-adjusted HR (95% CI)Covariate-adjusted HR* (95% CI)
<2612690,6630.98 (0.80-1.19)0.98 (0.80-1.19)
26-31477379,4181.001.00
>31 or too Irregular to estimate9884,2031.07 (0.86-1.33)1.09 (0.88-1.37)
Total701554,284
Ptrend0.45
Women <40 y
    <261328,7980.82 (0.46-1.48)0.80 (0.44-1.43)
    26-3189159,5501.001.00
    >31 or too irregular to estimate2543,1041.05 (0.67-1.64)1.08 (0.69-1.69)
    Ptrend0.41
Women ≥40 y
    <2611361,8651.00 (0.81-1.24)1.00 (0.81-1.24)
    26-31388219,8681.001.00
    >31 or too irregular to estimate7341,0991.07 (0.83-1.38)1.09 (0.85-1.41)
    Ptrend0.62
  • NOTE: Excluding women who used oral contraceptives >20 months since 1991 and women who did not answer the question regarding menstrual cycle length in 1993.

  • * Adjusted for age, family history of breast cancer, history of benign breast disease, height, current body mass index, body mass index at age 18, age at menarche, age at first birth, parity, alcohol use, physical activity, current and past oral contraceptive use.