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 irregularity	No. of cases	Person-years of follow-up	Age-adjusted HR (95% CI)	Covariate-adjusted HR* (95% CI)
Regular	645	498,469	1.00	1.00
Usually irregular	36	32,619	0.88 (0.63-1.24)	0.89 (0.63-1.24)
Always irregular†	50	37,576	0.95 (0.71-1.26)	0.98 (0.73-1.31)
Total	731	568,664
Ptrend				0.70
Among women <40 y
Regular	112	195,806	1.00	1.00
Usually irregular	11	13,870	1.48 (0.80-2.76)	1.56 (0.83-2.92)
Always irregular†	6	12,825	0.79 (0.35-1.80)	0.82 (0.36-1.87)
Ptrend				0.89
Among women ≤40 y
Regular	533	302,663	1.00	1.00
Usually irregular	25	18,749	0.75 (0.50-1.12)	0.75 (0.50-1.12)
Always irregular†	44	24,751	0.97 (0.71-1.32)	1.01 (0.74-1.38)
Ptrend				0.66

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.