From the article above:
Summarizing Brind's argument, Francis explained that upon conception, the level of estrogen in a woman's body increases dramatically. This results in the development of undifferentiated cells in the breast, which pose an additional cancer risk. Late in the pregnancy, these cells become milk-producing cells, cease posing a greater cancer risk, and in fact provide added protection against cancer. If a woman has an abortion before that stage - and the vast majority of abortions would occur before then - her body is left with a high number of undifferentiated cells which increase the risk of her contracting breast cancer, it is argued. Francis said a woman who suffers a miscarriage well into a pregnancy - in a motor accident, for example - would face the same risk. However, in cases where a spontaneous, early miscarriage occurs, the woman would not have had the surge in estrogen in the first place, and therefore would not face the additional cancer risk.
The only way to "prove" the link in a scientific sense would be to do studies where one cohort has an abortion, and one does not, controlling for other factors such as diet, smoking, drup use, exposure to environmental toxins, etc (which is indeed done in studies all the time)then 10 to 20 years later dissect the breast tissue to see the ratio of differentiated and metastatic cells from non differentiated in each cohort and compare them.
Since this is not feasible, retrospective studies and prospective studies comparing reported cancers and history of abortions are done.
Using the latter method, the only method available, 28 out of 35 studies, or 80%, found a positive link between abortion. Given this fact combined with the theoretical explanation given above,
HOW MUCH MORE "PROOF" DO WE NEED BEFORE WOMEN INSIST ON PROPERLY INFORMED CONSENT???