According to this hypothesis, an early FFTP [first full-term pregnancy] would provide the greatest protection against breast cancer by drastically reducing, early on, the presence of undifferentiated and hence vulnerable breast cells, thereby decreasing the risk of subsequent transformation ... Other types of pregnancies, however, might increase the risk of breast cancer. If a woman's first pregnancy resulted in a first trimester abortion, the dramatic rise in undifferentiated cells that takes place during the first trimester would not be followed by the marked differentiation occurring during the second and third trimesters. The consequent sharp increase in the number of vulnerable cells would thus elevate breast cancer risk.
PIP: A theory may explain why breast cancer rates have been increasing over several decades: abortion before the first live birth stops the processes transforming the breast during the first pregnancy, which increases the risk of breast cancer. Progestogens stimulate considerable growth of undifferentiated cells in the breast during the first pregnancy, which, if pregnancy continues, results in the budding and branching of the network of milk ducts. This period of rapid growth during the first trimester is when these breast cells are most vulnerable to carcinogens. Interruption of pregnancy during this period prevents these breast cells from becoming differentiated during the second and third trimesters. No less than 26 published studies indicate that abortions prior to the first live birth raises the risk of breast cancer (1.9-3.9 higher risk). Abortions after a first full-term pregnancy have little effect, since the number of undifferentiated cells is considerably smaller due to the earlier pronounced breast development during each full-term pregnancy. Accurate epidemiologic research is not easy because of confounding factors (e.g., oral contraceptive use) and political pressures. Worldwide, one finds high breast cancer rates correlated with high induced abortion rates. In the past, rich women, who tended to have more access to abortion, had 2 times higher breast cancer rates than poor women, but breast cancer rates have increased 53% among poor women while they fell 1% among rich women with the advent of public funding of abortions. Women of religions that strongly object to abortion have lower breast cancer rates than those whose religion allows abortion. Older black women are at a lower risk of breast cancer than older white women, but younger black women are a significantly higher risk than their white peers.