If estrogen exposure increases the risk of breast cancer, then why is pregnancy (early/more number of pregnancies) considered a protective mechanism, while the estrogen levels are actually increased in pregnancy ?
During pregnancy, the breast lobules not only proliferate but they also become terminally differentiated- a feature that is protective against the development of cancer. Estrogen isn't the only hormone that is elevated during pregnancy, progesterone is also increased which serves to protect against the cancerous effects of estrogen. This is because progesterone promotes the differentiation and secretory activity of breast tissue.
One of the risk factors for developing breast cancers includes early menarche or late menopause because of two reasons: 1) Number of cycles decreases. 2) High estrogen levels in pregnancy help in mammary gland proliferation as well as rapid epithelial proliferation, so it actually does increase malignancy growth rate in the start but it also increases the immune system which is protective against new cancers. 3) This association is a weak one and it only affects cancers with positive receptors and has no effect in negative receptor cancers.