Some factors associated with pregnancy are known to increase a woman's chances of developing breast cancer:

• After a woman gives birth, her risk of breast cancer is temporarily increased. This temporary increase lasts only for a few years.
• A woman who during pregnancy took DES (diethylstilbesterol), a synthetic form of estrogen that was used between the early 1940s and 1971, has a slightly higher risk of developing breast cancer. (So far, research does not show an increased breast cancer risk for their female offspring who were exposed to DES before birth. Those women are sometimes referred to as DES daughters.