However, after she completes a full-term pregnancy, 70% of a woman's breast lobules
have become mature, cancer-resistant Type 3 lobules.
On the one hand, aborting a pregnancy increases the likelihood of breast cancer because it short-circuits the natural maturation process by which cancer-vulnerable breast lobules
(which increase massively during pregnancy) are replaced by cancer-resistant breast lobules
During the "susceptibility window," nearly all of the breast lobules
consist of cancer-susceptible Type 1 and 2 lobules, where 95% of all breast cancers originate.
That is because by the end of a 40-week pregnancy, 85% of a woman's breast lobules
have become mature, cancer-resistant lobules known as Type 4 lobules.
Abundant breast lobules were present in the adjacent breast tissue in all cases in this subgroup, but these showed little evidence of active lactational change.
Although abundant normal breast lobules were present around the tumors, no histologic lactational change was noted.