Age at First Full-term Pregnancy and Other Reproductive Factors Are Associated with Mammographic Breast Density in Postmenopausal Women: A Study in Flanders, Belgium.

ABSTRACT: This study evaluated the association between age at first full-term pregnancy (FFTP) and mammographic breast density (MBD) in postmenopausal women. A total of 1,034 women, ages 50 to 69 years, were recruited from the Flemish (Belgium) population–based breast cancer screening program. Participants completed a questionnaire on lifestyle and reproductive factors. From mammography, we assessed the percentage glandular tissue of the total breast volume (GLAND), the volumetric breast density (VBD), and the Breast Imaging Reporting and Data System (BI-RADS) density classification. For statistical analysis, we used the piecewise linear regression model. The average age at FFTP was 26.1 years. Among women with an FFTP > 25.7 years, an each year increase in FFTP age was associated with a 1.17% increase in GLAND [95% confidence interval (CI), 0.20%–2.46%; P = 0.041] and a 1.45% increase in VBD (95% CI, 0.18%–2.75%; P = 0.026). Similarly, the odds of a higher BI-RADS classification increased by 5.0% (95% CI, 0.0%–11.0%; P = 0.059) for each year increase in FFTP age after 25.7 years. For every year delay in age at menarche, a 2.48% higher GLAND (95% CI, 0.43%–4.57%; P = 0.017) and a 2.45% higher VBD (95% CI, 0.38%–4.56%; P ≤ 0.020) were observed. Ever use of oral contraceptive resulted in a 12.24% decrease in GLAND (95% CI, −20.90% to −2.63%; P = 0.014) and a 13.48% decrease in VBD (95% CI, −22.1% to −3.91%; P = 0.007). MBD is significantly higher when FFTP takes place after 25.7 years. Later age at menarche is associated with higher MBD, whereas ever use of oral contraceptive is associated with lower MBD at postmenopause.

SIGNIFICANCE: We consider our findings to be highly novel, and to the best of our knowledge, our study is one of the first to investigate associations between age at FFTP and three markers of MBD (GLAND, VBD, and BI-RADS), uncovering that MBD is significantly higher when FFTP occurs after 25.7 years.