Table 2.

Associations^a of Mammographic Percent Density With Reproductive Factors Among 24,840 Women in the Research Program on Genes, Environment and Health Who Underwent Full-Field Digital Mammography for Breast Cancer Screening, Kaiser Permanente Northern California, 2003–2013

Reproductive Factor	Percent Density, %b		Dense Area, cm2 c
	Estimated Difference	95% CI	Estimated Difference	95% CI
Menopausal status
Premenopausal	3.50	0.62, 6.38	4.20	0.69, 7.71
Postmenopausal	0.00	Referent	0.00	Referent
Parity
0 (nulliparous)	1.16	0.60, 1.72	1.35	0.55, 2.14
1	0.32	−0.09, 0.73	0.89	−0.07, 1.86
2	0.00	Referent	0.00	Referent
≥3	−1.11	−2.87, 0.65	−2.15	−4.70, 0.40
Age at first birth, years
<20	−0.54	−1.00, −0.09	−0.88	−1.52, −0.23
20–24	0.00	Referent	0.00	Referent
25–29	0.25	−0.54, 1.04	0.17	−0.55, 0.90
30–34	0.75	0.25, 1.25	0.50	−0.21, 1.20
35–39	1.11	0.39, 1.82	1.20	0.18, 2.22
≥40	1.89	−0.15, 3.92	2.45	0.47, 4.43
Age at menarche, years
<10	−1.19	−2.15, −0.22	−0.58	−1.97, 0.81
10–11	−0.46	−0.84, −0.09	−0.74	−1.26, −0.22
12–13	0.00	Referent	0.00	Referent
14–15	0.87	0.47, 1.27	0.57	0.01, 1.13
≥16	1.69	0.91, 2.48	1.83	0.72, 2.95

Abbreviation: CI, confidence interval.

^a Combined effects for Hologic (Hologic, Inc., Marlborough, Massachusetts) and General Electric (General Electric Company, Boston, Massachusetts) full-field digital mammography machines.

^b Parameter estimates from the model for batch-corrected square root of percent density were transformed to estimate the difference in percent density. The model included age, body mass index, menopause, menopausal hormone therapy, family history of breast cancer, parity, age at first birth, and age at menarche.

^c Parameter estimates from the model for cube root of dense area were transformed to estimate the difference in dense area. The model included age, body mass index, menopause, menopausal hormone therapy, family history of breast cancer, parity, age at first birth, and age at menarche.