Parity and breastfeeding are contributing factors for geographical differences in breast cancer risk

Urbanization has emerged as one of the main determinants of the rising breast cancer incidence in Asia, but understanding the link is hindered by the lack of population-based prospective cohorts, especially in low- and middle-income countries. Given that mammographic density (MD) is one of the strongest breast cancer risk factors and that it is associated with known lifestyle and reproductive factors, we explored using MD to delineate factors associated with differences in breast cancer risk between women living in urban and rural areas.

Using data from a cross-sectional study of 9,417 women living in urban or rural areas recruited through hospital- or community-based opportunistic mammography screening programs, we conducted regression and mediation analyses to identify factors contributing to the differences in MD between urban and rural populations across Asian ethnic subgroups.

Consistent with higher risk of breast cancer, age-and-BMI-adjusted percent and absolute MD measurements were significantly higher in women living in urban areas compared to those in rural areas. Mediation analyses showed that differences observed were partly explained by higher parity (7-9%) and breastfeeding (2-3%) among women living in rural areas. Notably, the effect of parity (number of children) was similar in Chinese and Malay women (16-17% and 7-8%, respectively), but not observed in Indian women. Hormonal use, smoking, and physical activity did not predict MD nor mediate the observed association.

Higher MD among women living in urban compared to rural areas is partially attributable to parity and breastfeeding practices, a significant proportion of attributable risk remains unknown.