WOMEN'S HEALTH:Consider one of the most potent forces now marketed to rapidly aging baby-boomers: Replacement hormone therapy. Estrogen is touted to alleviate symptoms from hot flashes, prevent osteoporosis (the so-called "brittle bone" disease of aging) and reduce the risk of cardiovascular disease.But estrogen is not without risk. Because "pure" estrogen increases the risk of uterine cancer, some women take a combination pill which combines estrogen and progestin. Although this lowers the risk.