Hormones play an important role in many breast cancers which is why if a woman has or doesn't have children and if she does or doesn't take hormonal birth control impacts her chances of cancer.
Hormone positive cancers (ER+, PR+) feed off of the hormones estrogen and progesterone. Estrogen is especially important at growing breast cancers. Women who are pregnant and breastfeeding have low estrogen and are not 'feeding' cancer cells. This is why we see such a high rate of breast cancer in nuns. They never have the estrogen suppressing experiences of pregnancy and breastfeeding. The more babies you have and the more you breastfeed the lower your chances of breast cancer.
Hormonal birth control is exactly that, hormones. Artificial estrogen and progesterone which are dosed and added to a woman's natural cycle in a way to suppress ovulation and change the quality of the uterine lining. They feed hormone receptor positive cancer....literally.
The drugs Tamoxifen and Femara that many women with breast cancer take after treatment are not chemo as many people think but simply estrogen suppressors.
Hormone positive cancers are more common and occur in older women and tend to be less aggressive cancers. Hormone negative cancers tend to occur in younger women and are often very aggressive and are linked to BRCA1/2. My sil had a prophylactic double mastectomy. The pathologists found a very small tumor that was previously undetectable. Her mother and her 6 aunts all had breast cancer. Yes, she is BRCA 1/2 +
There is a great book out there called Emperor of All Maladies which is about the history of cancer and it's treatment. Fascinating. Can get a little heavy when it hits the biochem and RNA/DNA but worth powering through.
One little breast cancer factoid: Nabby Adams (the daughter of John and Abigail Adams) had breast cancer. She underwent a mastectomy (while conscious) but sadly died of metastatic breast cancer a few years later. She was only in her mid 30's.