'Vaccine' Mimicking Childbearing Might Help Avert Breast Cancer

What is it about term pregnancies and lengthy breastfeeding that confers delayed but subsequently lifelong protection against breast cancer? It's not just the elevation in estrogen and progestins. The Collaborative Group and others have shown that oral contraceptives and hormone therapy are associated with increased breast cancer risk during their use and soon after, but a few years later the increased risk is gone.

“It's not just estrogens and progestins that change during pregnancy. We have to be looking for something beyond,” Dr. Beral said.