Women who experience at least one full-term pregnancy in their lifetime develop four types of breast lobules. (A lobule is a unit of breast tissue consisting of a milk duct and glands.) During adolescence, the majority of lobules are Type 1 and Type 2, which are immature and cancer susceptible. Throughout the first trimester of pregnancy, the number of Type 1 and 2 lobules rapidly increases. As a result, breasts have more sites for cancer to start. In the second trimester, the breast lobules start maturing into Type 4 lobules, which are cancer resistant. By the end of the third trimester, 85% of the breast has fully matured. Only 15% of the lobules remain immature and cancer-susceptible, leaving fewer opportunities for cancer to start. After birth and weaning, Type 4 lobules regress to Type 3. There is evidence of permanent changes in the genes of Type 3 lobules which provide life-long cancer resistance.
A premature delivery before 32 weeks doubles the risk of breast cancer because it leaves the breast with more places for cancer to start. In the same way, abortion also stops the progression of breast lobule development. This prevents the development of Type 4 and subsequently Type 3 cancer-resistant lobules. Induced abortion of a normal pregnancy results in increased risk of breast cancer for the mother because more Type 1 and 2 lobules remain.
Approximately 90% of miscarriages occur in the first trimester. However, the vast majority of natural miscarriages in the f irst trimester do not increase the risk of breast cancer. In these cases, pregnancy hormones are lower than those of a normal pregnancy due to either a fetal or ovarian abnormality. Therefore, a breast may not have grown more Type 1 and 2 lobules (sites where cancer starts) in response to pregnancy hormones, or at least very few.
"Reproductive Breast Cancer Risks and Breast Lobule Maturation." Reproductive Breast Cancer Risks Brochure. Breast Cancer Prevention Institute, 2007. Web