Retrospective study of impact of parity and breastfeeding on breast density and subsequent postmenopausal breast cancer risk

A retrospective study at Hanyang University College of Medicine (Seoul, South Korea) examined the associations of parity and breastfeeding with postmenopausal breast density, longitudinal changes in breast density, and breast cancer risk. The study enrolled 2.3 million women who underwent mammography in 2009-2010 and had at least one follow-up mammogram through 2016. Parity and breastfeeding were self-reported, and breast density was classified using BI-RADS. Women were followed through 2023. Lower parity and shorter breastfeeding duration were associated with higher baseline breast density and less favorable density trajectories over time, including a greater likelihood of increasing density and a lower likelihood of decreasing density. Compared with women with at least two children who breastfed for > 12 months, women with one child who did not breastfeed had a higher breast cancer risk (RR 1.53; 95% CI 1.43-1.64), with a clear dose-response pattern across parity and breastfeeding categories. Parity showed stronger associations than breastfeeding duration with both breast density and breast cancer risk, and these associations were generally stronger among women aged 65 years or older. Baseline breast density and its longitudinal change mediated approximately 10-15% of the associations between reproductive factors and breast cancer risk. The authors conclude that reproductive history has long-term effects on breast tissue composition and partly explains subsequent breast cancer risk in postmenopausal women (graphical abstract).