Why is having fewer pregnancies or fewer kids a risk factor for cancer of the breast?

Later pregnancies : Pregnancy is protective against cancer because there is a natural change in hormone fluctuations. The estrogen produced in pregnancy, estriol is very protective against breast cancer. However this occurs only if the pregnancy occurs before 30, not after. Breast feeding stops the hormones which can also be protective; so its age, how often pregnant and breast feeding that are the factors to see.

Estrogen exposure: Prolonged exposure to and higher concentrations of estrogen increases the risk of breast cancer. The more pregnancies the fewer menses. Breast feeding accomplishes the same goal. Also age at 1st birth is associated with bc risk too. Check this out from acs: http://bit.Ly/oysxjs.

Unknown: "risk factor" refers to a statistical association between two items. We can postulate why , but those at best are informed guesses. With out a controlled scientific experiment , we do not know the true answer. Statistics can mislead us.