Table 4.

HRs of the association between current menstrual irregularities in 1993 and premenopausal breast cancer incidence among participants of the Nurses' Health Study between 1993 and 2001

Current menstrual irregularityNo. of casesPerson-years of follow-upAge-adjusted HR (95% CI)Covariate-adjusted HR* (95% CI)
Usually irregular3632,6190.88 (0.63-1.24)0.89 (0.63-1.24)
Always irregular5037,5760.95 (0.71-1.26)0.98 (0.73-1.31)
Among women <40 y
    Usually irregular1113,8701.48 (0.80-2.76)1.56 (0.83-2.92)
    Always irregular612,8250.79 (0.35-1.80)0.82 (0.36-1.87)
Among women ≥40 y
    Usually irregular2518,7490.75 (0.50-1.12)0.75 (0.50-1.12)
    Always irregular4424,7510.97 (0.71-1.32)1.01 (0.74-1.38)
  • NOTE: Excluding all women who used oral contraceptives >20 months since 1991 and women who did not answer the question regarding menstrual pattern 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.

  • Women with no periods were combined with always irregular women.