Can pregnancy induce breast cancer?

Less likely: Pregnancy causes many hormone changes in the body. For one thing, pregnancy stops monthly menstrual cycles and shifts the hormone balance toward progesterone rather than estrogen. This is why women who become pregnant while they are young and have many pregnancies may have a slightly lower risk of breast cancer later on. They are exposed to less estrogen. Women who have had no children or who had their first pregnancy after age 30, on the other hand, have a slightly higher breast cancer risk.

Not likely,: pregnancies are associated with decreased lifetime breast cancer risk.