Lancet paper found that pregnancies that end as a spontaneous or induced abortion do not increase a woman's risk of developing breast cancer.
The Lancet 2004
Beral V., Bull D., Doll R., Reeves G., Breast cancer and abortion: collaborative reanalysis of data from 53 epidemiological studies, including 83?000 women with breast cancer from 16 countries. Lancet, 2004 Mar 27;363(9414):1007-16.