The Breast Cancer Prevention Institute has on their webpage on studies oninduced abortion causing breast cancer [versus miscarriage spontaneous abortion]:
http://bcpinstitute.org/FactSheets/BCPI-FactSheet-Epidemiol-studies.pdf
There are 70 total studies done: 55 showed that induced abortion caused breast cancer.
They also state that in their breast cancer prevention pamphlet:
On page 10 of their booklet Breast Cancer Risks and Prevention, they talk about how induced abortion causes breast cancer. They state that spontaneous abortion (miscarriage) is not a factor.
Webpage to this booklet is:
I have not been doing any reading recently on this matter of an abortion/breast cancer connection. When last I was paying attention, I read it that an induced abortion of a first pregnancy was shown to increase susceptibility to breast cancer but a direct cause/effect relationship was not proven. A tenuous distinction, admittedly, probably based on readings of statistical tea leaves..