The whole not-having-babies thing adds risk of breast cancer. Having babies and feeding them naturally are both very large risk-reducers for breast cancer, so it follows that NOT having babies and NOT feeding them naturally amounts to increased risk of getting breast cancer.
Thinking about this rationally: things that disrupt the natural use of the human body tend to lead to increased risk of cancer. Cigarette smoking "sedates" the mechanisms in the lungs that clear out smoke and tar; birth control pills hijack the natural cycle of a woman; non-food stuff we eat (chemical preservatives, HFCS, dough "conditioners" in "bread," almost all the ingredients in Twinkies...) confuses the intestine and disrupts digestion and insulin response; etc. Proper and frequent use of body parts diminishes risk of cancer and other diseases: lactating uses the mammary glands and ducts, produces oxytocin (responsible for returning the uterus to it's non-pregnant size and location and aids in human bonding as well as having a calming effect on both mother and child), and speeds, through use, replacement of old tissue with new, reducing the risk for breast cancer; pregnancy uses all sorts of systems and produces all sorts of good-for-mother-and-baby hormones, reducing the risk of breast and cervical cancer; hard exercise uses the cardiovascular system and the musculoskeletal system reducing the chance of heart and lung diseases; weight-bearing exercises compact both muscle and bone, reducing the incidence of arthritis, osteoporosis, and other bone- & muscle-loss-related diseases and conditions; proper use of the digestive system including insulin response and bile production by the regular intake of a balanced diet along with that hard exercise eliminates the occurrence of type 2 diabetes and gall bladder diseases...
It stands to reason that disruptions to the normal operation of the human body will increase risks of all sorts of diseases and injuries. Abortion is a major disruption, I'm thinking.
