Table 1.

HRs of the association between menstrual cycle length at ages 18 to 22 years and premenopausal breast cancer incidence among participants of the Nurses' Health Study II between 1989 and 2001

Menstrual cycle length (d)No. casesPerson-years of follow-upAge-adjusted HR (95% CI)Covariate-adjusted HR* (95% CI)
<26108114,7970.92 (0.75-1.12)0.91 (0.74-1.12)
26-31743700,2411.001.00
32-39167165,8700.95 (0.80-1.12)0.94 (0.80-1.12)
>40 or too irregular to estimate7884,1580.87 (0.69-1.10)0.87 (0.69-1.10)
Total1,0961,065,066
Ptrend0.38
Women <40 y
    <263466,5780.95 (0.66-1.37)0.93 (0.65-1.34)
    26-31212389,5171.001.00
    32-393691,8280.72 (0.50-1.02)0.73 (0.51-1.05)
    >40 or too irregular to estimate1646,6370.63 (0.38-1.05)0.68 (0.41-1.13)
    Ptrend0.07
Women ≥40 y
    <267448,2190.90 (0.71-1.15)0.90 (0.70-1.15)
    26-31531310,7241.001.00
    32-3913174,0421.04 (0.86-1.26)1.02 (0.85-1.24)
    >40 or too irregular to estimate6237,5210.97 (0.74-1.26)0.94 (0.72-1.23)
    Ptrend0.93
  • NOTE: Excluding women who used oral contraceptives >10 months every year from ages 18 to 22 years and women who did not answer the question about menstrual cycle length in 1989.

  • * 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, and current and past oral contraceptive use.