Why does having an abortion so greatly increase the chance of getting breast cancer?

It doesn't.: Early retrospective studies suggested a link that has not been supported by more rigorous prospective studies. This is likely due to a retrospective bias, where women with cancer were probably more likely to be truthful with their medical histories than unaffected people, especially about as sensitive of a topic as abortion.

It doesn't! : This is a myth that has been perpetuated for political reasons. It is not true. World experts have studied this question and they all agree: there is no increased risk of breast cancer in women who have had an induced or spontaneous abortion. To read more, check out this link: http://www.Cancer.Gov/cancertopics/causes/ere/workshop-report.

It does not: I give myself out to be pro-life and am embarrassed that this is still being put forward despite its being discredited scientifically. In some old studies, women with breast cancer were told that the cause was often having had an abortion and they were urged to confess having undergone the procedure. You can see why this study got an association. The best recent work shows little or no effect.