Age at first full-term pregnancy and other reproductive factors are associated with mammographic breast density in postmenopausal women: a study in Flanders, Belgium

This study evaluated the association between age at first full-term pregnancy (FFTP) and mammographic breast density (MBD) in postmenopausal women. 1,034 women, age 50-69y, 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 BI-RADS density classification. For statistical analysis we used the piecewise linear regression (PLR) model. Average age at FFTP was 26.1 years. Among women with a FFTP >25.7 years, each year increase in FFTP was associated with 1.17% increase in GLAND (95%CI: 0.20% to 2.46%, p = 0.041) and 1.45% increase in VBD (95%CI: 0.18% to 2.75%, p = 0.026). Similarly, the odds of a higher BI-RADS classification increased by 5.0% (95%CI: 0.0% to 11.0%, p = 0.059) for each year increase in FFTP age after 25.7 years. For every year delay in age at menarche, there was a 2.48% higher GLAND (95%CI: 0.43 % to 4.57%, p = 0.017) and 2.45% higher VBD (95%CI: 0.38 % to 4.56%, p = <0.020). Ever use of oral contraceptive resulted in 12.24% lower in GLAND (95%CI: -20.90% to -2.63%, p = 0.014) and 13.48% lower 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, while ever use of oral contraceptive was associated with lower MBD at postmenopause.