Eli5: How come nulliparity vs having 1-2 pregnancies(most women have) create a difference in risk of reproductive cancers?

Question

Women spend most of their lives without pregnancy, and when they’re pregnant, it’s usually 1-2 years of their whole life.

Shouldn’t it be not that important when it comes to elevated risk of reproductive cancers (breast, uterine, ovarian etc) if the woman is nullipar?

What’s the explanation?

In: Biology

Answer 1

Because high estrogen levels increase the risk for reproductive cancers. The more menstrual cycles a woman has, the higher her risk of breast cancer. So if a woman has an early menarche and a late menopause, she is at higher risk. If she practices extended breastfeeding and does not menstruate for a year and a half after giving birth, that reduces her risk of breast cancer. Estrogen levels are higher when a woman is cycling, and estrogen has a stimulating and proliferative effect on reproductive tissue. Women who are overweight generally have higher levels of circulating estrogen. This makes them at higher risk for endometrial hyperplasia, endometrial cancer and breast cancer. Female children who are overweight are more likely to have an early menarche due to their higher levels of circulating estrogen.