How does having a baby lower the risk of breast cancer in women?

Less estrogen: The placenta puts out progesterone, which acts as an estrogen antagonist (off switch) in the body, to prevent ovulation while you're pregnant. The cyclic ramp up in estrogen is also avoided. This period of lower estrogen exposure lowers overall cancer risk.

Less cycles.: Early menopause, late menarche and first full term pregnancy by age 18 lower the risk of breast cancer. This is apparently done because the number of menstrual cycles is lowered over a woman's life by these factors. This lowers a woman's exposure to endogenous hormones, like estrogen.