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. cases	Person-years of follow-up	Age-adjusted HR (95% CI)	Covariate-adjusted HR* (95% CI)
<26	108	114,797	0.92 (0.75-1.12)	0.91 (0.74-1.12)
26-31	743	700,241	1.00	1.00
32-39	167	165,870	0.95 (0.80-1.12)	0.94 (0.80-1.12)
>40 or too irregular to estimate	78	84,158	0.87 (0.69-1.10)	0.87 (0.69-1.10)
Total	1,096	1,065,066
Ptrend				0.38
Women <40 y
<26	34	66,578	0.95 (0.66-1.37)	0.93 (0.65-1.34)
26-31	212	389,517	1.00	1.00
32-39	36	91,828	0.72 (0.50-1.02)	0.73 (0.51-1.05)
>40 or too irregular to estimate	16	46,637	0.63 (0.38-1.05)	0.68 (0.41-1.13)
Ptrend				0.07
Women ≥40 y
<26	74	48,219	0.90 (0.71-1.15)	0.90 (0.70-1.15)
26-31	531	310,724	1.00	1.00
32-39	131	74,042	1.04 (0.86-1.26)	1.02 (0.85-1.24)
>40 or too irregular to estimate	62	37,521	0.97 (0.74-1.26)	0.94 (0.72-1.23)
Ptrend				0.93

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.