. Author manuscript; available in PMC: 2008 Apr 14.

Published in final edited form as: Cancer Epidemiol Biomarkers Prev. 2005 Jun;14(6):1509–1513. doi: 10.1158/1055-9965.EPI-05-0051

Table 2.

HRs of the association between current menstrual cycle length (1993) and premenopausal breast cancer incidence among participants of the Nurses' Health Study II between 1993 and 2001

Current menstrual cycle length (d)	No. of cases	Person-years of follow-up	Age-adjusted HR (95% CI)	Covariate-adjusted HR^* (95% CI)
<26	126	90,663	0.98 (0.80-1.19)	0.98 (0.80-1.19)
26-31	477	379,418	1.00	1.00
>31 or too Irregular to estimate	98	84,203	1.07 (0.86-1.33)	1.09 (0.88-1.37)
Total	701	554,284
P_trend				0.45
Women <40 y
<26	13	28,798	0.82 (0.46-1.48)	0.80 (0.44-1.43)
26-31	89	159,550	1.00	1.00
>31 or too irregular to estimate	25	43,104	1.05 (0.67-1.64)	1.08 (0.69-1.69)
P_trend				0.41
Women ≥40 y
<26	113	61,865	1.00 (0.81-1.24)	1.00 (0.81-1.24)
26-31	388	219,868	1.00	1.00
>31 or too irregular to estimate	73	41,099	1.07 (0.83-1.38)	1.09 (0.85-1.41)
P_trend				0.62

NOTE: Excluding women who used oral contraceptives >20 months since 1991 and women who did not answer the question regarding menstrual cycle length 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.