Research has shown that women how are exposed to particular hormones that are produced by the ovaries for an extended period of time can have a greater chance of developing breast cancer.
These particular hormones are endogenous estrogen and progesterone. It has been shown that women who start their menstruation early, have their menopause start later than normal, are older when they first become pregnant or never give birth at all are all exposed to greater amounts of these hormones.
One of the great things about being pregnant in regards to breast cancer is that the woman is exposed to far less hormones than a woman who is not pregnant. The reason for this is because she is avoiding her monthly menstrual cycle which will expose her to more of these hormones.
Breastfeeding has also been shown to help reduce the exposure to these types of hormones. It is believed that breastfeeding can actually make the cells stronger and less susceptible to cancer cells. This can be very helpful for women who have an increased chance of developing breast cancer.
So, evidence shows that women who are younger when they become pregnant and have multiple births are at a decreased chance of developing breast cancer in the future. However, those on the other side of this spectrum will have an increased chance of developing it instead.