Table 3.

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

Menstrual irregularity (ages 18-22)No. of casesPerson-years of follow-upAge-adjusted HR (95% CI)Covariate-adjusted HR* (95% CI)
Regular869838,3911.001.00
Usually irregular140154,8060.89 (0.74-1.07)0.88 (0.74-1.05)
Always irregular115108,6891.01 (0.83-1.23)1.02 (0.84-1.25)
Total1,1231,101,886
Ptrend0.70
Among women <40 y
    Regular249470,8571.001.00
    Usually irregular3988,7480.84 (0.60-1.18)0.86 (0.61-1.21)
    Always irregular2460,8050.75 (0.49-1.14)0.79 (0.52-1.20)
    Ptrend0.20
Among women ≥40 y
    Regular620367,5341.001.00
    Usually irregular10166,0580.91 (0.74-1.12)0.89 (0.72-1.10)
    Always irregular9147,8841.12 (0.90-1.40)1.11 (0.88-1.38)
Ptrend0.75
  • NOTE: Question asks subject to exclude periods around pregnancies or oral contraceptive use; consequently, oral contraceptive users were included in this table. Women who did not answer the question regarding menstrual pattern in 1989 were excluded.

  • * 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.