Our mouse model data are not in conflict with the epidemiological observation that a first pregnancy before age 22 greatly reduces breast cancer risk (MacMahon et al., 1970), because at this young age, the chance of having already
accumulated precancerous cells is small.
However, when old
cells return to a stem
cell - like status, they can carry with them all of the mutations that have
accumulated to date, predisposing some of those
cells to developing into
precancerous lesions.