Later in the pregnancy, the cells will specialize. Some will produce milk, others will line the milk ducts, and so on. Each cell will have its own job to perform. Hormones are naturally produced chemicals which tell cells what to do. Estrogen causes the cells in the breast to rapidly multiply, but none of the cells know what to do.
Abortion interrupts the series of changes within the body.
With an abortion, millions of unspecialized cells grow rapidly without any direction. Unspecialized cells are the only ones that are naturally vulnerable to cancer. Rapidly growing unspecialized cells are the most vulnerable of all, because the cancer occurs when cells grow and there is less chance to correct the genetic errors which occur during cell growth. A great deal of research has been done in this area, and repeatedly, reputable studies have found unusually high abortion rates among women with breast cancer. Interrupted pregnancies were identified as a possible risk factor for breast cancer in 1979.