Table 1. Odds Ratios for Breast Cancer According to Risk Factors, Collaborative Breast Cancer Study, 1988-2008.

Characteristic	Cases (n=23,959) %	Controls (n=28,304) %	Odds Ratioa	95% CIa	Effect Modification P-value
					with Ageb	with Menopausal Statusc
First degree family history of breast cancer
None	78.2	85.8	1		0.01	0.001
1	17.3	11.5	1.61	1.53-1.69
2	2.2	0.9	2.44	2.10-2.84
≥ 3	0.3	0.1	3.04	1.97-4.69
Unknown	2.0	1.7
None	78.2	85.8	1		0.001	0.001
1, diagnosed < 50 yr	5.0	3.1	1.74	1.59-1.90
1, diagnosed 50-59 yr	4.0	2.7	1.61	1.46-1.78
1, diagnosed ≥ 60 yr	7.9	5.4	1.55	1.44-1.66
≥ 2, diagnosed <50 yr	1.4	0.6	2.62	2.16-3.17
≥ 2, diagnosed ≥50 yr	1.1	0.5	2.49	2.02-3.07
Unknown	2.5	2.0
Benign breast disease	n=14,144 h	n=15,830 h
Never	72.0	79.7	1		0.14	0.15
Ever	26.0	18.5	1.61	1.53-1.71
Unknown	2.0	1.8
Age at menarche
≤ 12	43.9	41.5	1		0.42	0.75
13	27.2	27.2	0.94	0.90-0.98
14	15.1	16.0	0.88	0.84-0.93
≥ 15	11.4	13.2	0.80	0.76-0.85
Unknown	2.4	2.2
Parity
Nulliparous	13.5	12.0	1		0.0001	0.11
Parous	85.7	87.4	0.84	0.80-0.89
Unknown	0.8	0.7
Parity
0	13.5	11.9	1		0.06	0.33
1	11.2	10.1	0.99	0.92-1.07
2	29.2	27.2	0.94	0.90-1.00
≥ 3	45.4	50.2	0.75	0.71-0.80
Unknown	0.8	0.7
Age at first full term pregnancy (years)
< 22	29.8	34.5	1.00		0.30 d	0.26 d
22-24	23.2	24.3	1.12	1.07-1.17
25-29	22.5	20.6	1.30	1.24-1.36
≥ 30	10.2	8.0	1.52	1.43-1.63
Nulliparous	13.5	12.0	1.37	1.29-1.45
Unknown	0.9	0.7
Breastfeeding (parous women only)
Never	49.4	49.3	1		0.94	0.87
Ever	50.6	50.7	0.97	0.94, 1.01
Breastfeeding duration (parous women only)
Never	49.4	49.3	1		0.75	0.04
≤ 12 months	36.3	35.8	0.98	0.94-1.03
> 12 months	14.1	14.7	0.95	0.89-1.00
Unknown	0.2	0.2
Oral contraceptive usee
Never	54.9	54.7	1.00		<0.0001	0.19
Ever	44.1	44.3	1.05	1.01-1.10
Unknown	1.1	1.0
Body mass index (kg/m2)
< 18.5 (underweight)	1.8	2.4	0.79	0.70-0.90	<0.0001	<0.0001
18.5 –24.9 (normal)	46.8	48.9	1
25 –29.9 (overweight)	30.0	29.3	1.02	0.98-1.06
≥ 30 (obese)	19.2	17.2	1.09	1.03-1.14
Unknown	2.2	2.2
Smoking statusg
Never	49.0	49.1	1		0.03	0.38
Former	30.3	28.8	1.04	1.00-1.08
Current	19.8	21.3	0.96	0.91-1.00
Unknown	0.9	0.8
Recent alcohol consumption (drinks/week)	n=18,895 h	n=23,028 h
Never	10.2	11.7	1.00		0.0002	0.0004
Former	9.4	8.8	1.16	1.07-1.27
< 7	63.0	64.8	1.11	1.04-1.19
7-13	9.6	8.8	1.25	1.15-1.37
≥ 14	6.0	4.2	1.65	1.48-1.83
Unknown	1.8	1.8

^aOdds ratios are age-adjusted for age, state of residence, and study period.

^bAge defined continuously (per year)

^cExcludes women with unknown menopausal status

^dParous women only

^eOdds ratio adjusted for residuals of time since last use regressed on age (instead of adjusted for age).

^fExcludes women who never used oral contraceptive.

^gOdds ratios are age-adjusted for age, state of residence, study period, body mass index, and alcohol consumption.

^hSample sizes are shown when a reduced number of women are included in the analysis.