But an early abortion not only does not confer that protection but goes the other way and confers in creased risk. Thus, the extra estrogen ultimately causes abnormal cells to grow into full blown cancer. Many early studies did not distinguish between spontaneous and induced abortion. Thus, the body of knowledge in this area is inconsistent. Why a miscarriage wouldn't carry an increased risk as an abortion does is not entirely clear.
Miscarriage is harder to measure epidemiologically and has many causes.
Simply put, a miscarriage is a natural termination of an abnormal pregnancy, whereas abortion is almost always the abnormal termination of normal pregnancy. Thus, they are very different events and the difference in consequences should not be too surprising. Of the 800,000 women every year who get abortions and who have never had a full-term pregnancy, we are increasing the average lifetime risk among them by at least 50 percent. By the year 2020 we are going to see 40,000 - 50,000 breast cancer cases minimum, every single year, that are due to induced abortions. A 1994 National Cancer Society Institute study of over 1,800 women in Washington state found a significant 50 percent increase risk among women who had any abortions.