Reproductive and hormonal factors associated with mammographic breast density by age (United States)

We determined the association of certain reproductive and hormonal factors with breast density over decades of life.

Subjects were women age 20-79 years who had a screening mammogram between 1 June 1996 and 1 August 1997, in Seattle, Washington. Women with increased breast density (upper two categories of BI-RADS terminology) (n = 14,432) were compared to those with fatty breasts (lower two categories (n = 14,552). Unconditional logistic regression was used with adjustment for age at mammogram, parity, age at first birth, menopausal status, current use of hormone replacement therapy (HRT), and body mass index.

The association of nulliparity with density was evident for women at all ages (odds ratio (OR) and 95% confidence interval (CI) = 1.5 (1.3-1.7) and 1.6 (1.4-1.9) for women age < or = 45 and > 65, respectively). Older age at first birth was more strongly associated with density among women > 55 than among younger women. The association of current use of HRT with density, but not of former use, increased with age when compared to never users (OR = 1.4 (1.2-1.7) and 2.2 (2.0-2.5) for women age 46-55 and > 65, respectively).

Results suggest that pregnancy at an early age has a permanent beneficial association with density, while HRT has a transitory adverse association.