Age, mammographic density and the incidence of breast cancer. (a) Baseline percentage mammographic density in women from three mammographic screening programmes according to those who developed breast cancer 1 to 8 years later (cases) or remained free from breast cancer (control individuals). Average percentage density in the baseline mammogram declined with increasing age at enrolment, both in women who eventually developed breast cancer and in those who remained free from disease. At all ages, percentage density was greater in those who developed breast cancer. Data from Boyd and coworkers . (b) On the left is shown a log-log plot of the age-specific incidence of breast cancer. Adapted from Pike and coworkers . To the right is shown the Pike model of breast tissue ageing. 'b' represents a one time increase in risk associated with first full-term pregnancy (FFTP). See Pike and coworkers . LMP, last menstrual period.