Table 1. Characteristics of 783 premenopausal and 436 postmenopausal women by quartile of percent mammographic density, NHS and NHSII, mean (SD) or N (%).

	Premenopausal				Postmenopausal
	Percent mammographic density				Percent mammographic density
Q1 (<24 %) N = 195	Q2 (24–40 %) N = 196	Q3 (40–55 %) N = 196	Q4 (55+ %) N = 196	Q1 (<8 %) N = 109	Q2 (8–18 %) N = 109	Q3 (18–33 %) N = 109	Q4 (33+ %) N = 109
Mean (SD)
Age (year)	48.2 (4.0)	47.6 (4.0)	46.5 (4.2)	45.8 (4.2)	62.3 (5.6)	61.4 (5.8)	60.9 (6.4)	58.3 (7.8)
BMI (kg/m2)	29.1 (6.3)	25.4 (4.1)	23.8 (3.5)	22.4 (2.9)	29.9 (5.7)	27.0 (3.9)	25.1 (4.1)	23.4 (3.3)
Age at menarche	12.1 (1.3)	12.4 (1.3)	12.7 (1.4)	12.6 (1.4)	12.5 (1.3)	12.4 (1.4)	12.7 (1.5)	12.7 (1.4)
Parity (among parous)	2.7 (1.1)	2.7 (1.0)	2.6 (1.0)	2.3 (0.9)	3.8 (1.6)	3.8 (1.8)	3.5 (1.5)	3.3 (1.7)
Age at first birth (among parous)	24.8 (3.8)	25.9 (3.9)	26.2 (3.6)	25.8 (3.8)	25.1 (3.1)	25.2 (3.3)	25.6 (3.3)	25.8 (3.2)
Alcohol (gm/day)	4.3 (8.6)	4.8 (7.2)	4.8 (7.3)	4.6 (8.6)	5.5 (9.4)	6.3 (10.4)	5.8 (9.1)	6.9 (9.5)
N (%)
Nulliparous	18 (9.2)	14 (7.1)	21 (10.7)	29 (14.8)	4 (3.7)	4 (3.7)	8 (7.3)	15 (13.8)
History of BBD	80 (41.0)	87 (44.4)	104 (53.1)	118 (60.2)	36 (33.0)	39 (35.8)	38 (34.9)	52 (47.7)
Family history of breast cancer	13 (6.7)	23 (11.7)	16 (8.2)	13 (6.6)	16 (14.7)	14 (12.8)	16 (14.7)	17 (15.6)
Smoking
Never	107 (54.9)	103 (52.6)	117 (59.7)	127 (64.8)	49 (45.0)	39 (35.8)	59 (54.1)	60 (55.1)
Past	71 (36.4)	72 (36.7)	65 (33.2)	54 (27.6)	52 (47.7)	56 (51.4)	42 (38.5)	34 (31.2)
Current	17 (8.7)	21 (10.7)	14 (7.1)	15 (7.7)	8 (7.3)	14 (12.8)	8 (7.3)	15 (13.8)
Past PMH use	–	–	–	–	34 (31.2)	41 (37.6)	41 (37.6)	37 (33.9)