Mammographic density is a heritable quantitative trait and is a strong risk factor for breast cancer in middle-aged and older women. However, little is known about the development of mammographic density in early life. We used MRI to measure the water content of the breast, which provides a measurement of the fibro-glandular content of breast tissue with similar accuracy to mammography, but without the attendant exposure to radiation.

Between December, 2003, and December, 2007, we recruited 400 young women, aged 15–30 years, and their mothers. We used MRI scans to measure daughters’ breast water and fat, and on the same day obtained blood for hormone assays in the follicular phase of the menstrual cycle for each young woman. Mothers underwent mammography (n=356), and a random sample (n=100) also consented to have a breast MRI scan.

In mothers, per cent water—as measured by MRI—was strongly correlated with per cent mammographic density (r=0·85). Per cent water in daughters (median 44·8%) was significantly higher than in mothers (median 27·8%; p<0·0001), and was independently inversely associated with both their age (p=0·04) and weight (p<0·0001), and positively associated with their height (p<0·0001) and their mothers’ per cent mammographic density (p<0·0001). Serum growth hormone concentrations, adjusted for covariates, were positively associated with per cent breast water (p=0·001) in a subgroup of young women (n=280) who had not used oral contraceptives within 6 months.

Per cent breast water was greatest during the ages when women are most susceptible to breast carcinogens, and was associated with weight, height, and mother’s breast-tissue characteristics, and with serum concentrations of growth hormone: a breast mitogen that also mediates postnatal somatic growth. Mammographic density in middle age might partly be the result of genetic factors that affect growth and development in early life.

Canadian Breast Cancer Research Alliance.