Table 2. Association between mammographic density and breast cancer risk, overall and by menopausal status.

	Mammographic parenchymal composition	Subjects N	Breast cancer event N	IR per 1000 PY	Crude HR (95% CI)	Adjusted HR (95% CI)
Overall	25%<	1933840	7626	0.76	1(reference)	1(reference)
	25-50%	1848284	11546	1.27	1.66(1.62,1.71)	1.55(1.51,1.60)
	51-75%	1780992	16130	1.88	2.45(2.38,2.51)	2.14(2.08,2.21)
	76-100%	802406	9166	2.33	3.05(2.95,3.14)	2.59(2.50,2.69)
Premenopausal	25%<	133834	858	1.17	1(reference)	1(reference)
	25-50%	302196	2216	1.37	1.17(1.08,1.26)	1.19(1.10,1.28)
	51-75%	700600	7377	2.05	1.75(1.63,1.87)	1.77(1.65,1.90)
	76-100%	495971	6278	2.50	2.13(1.99,2.29)	2.17(2.02,2.34)
Postmenopausal	25%<	1800006	6768	0.73	1(reference)	1(reference)
	25-50%	1546088	9330	1.25	1.71(1.65,1.76)	1.61(1.56,1.66)
	51-75%	1080392	8753	1.75	2.39(2.31,2.46)	2.16(2.09,2.24)
	76-100%	306435	2888	2.04	2.77(2.66,2.90)	2.51(2.40,2.63)

Abbreviation: IR, incidence rate; PY, person-years; HR (95% CI), hazard ratio (95% confidence interval)

Adjusted for age (continuous), height (continuous), BMI (continuous), hormone therapy (never, <2 years, <5 and ≥2 years, ≥5 years, unknown), parity (0, 1, ≥2 births), duration of breast feeding (none, <6 months, <12 and ≥6 months, ≥12 months, unknown), use of oral contraceptives (never, <1 year, ≥1 year, unknown), age at menarche (<14, ≥14 years), alcohol consumption (never drinker, mild to moderate, heavy), smoking status (never, current, or past), regular exercise (binary), and baseline menopausal status.