Oestrogen is not currently known to have a direct bone building action. Oestrogen is unlikely to deal with this osteoporosis, but does increase risks for developing certain forms of cancer. In addition, women are naturally exposed to higher levels of oestrogen from the age of 35 onwards due to regularly not ovulating. Progesterone, which has a bone building effect, is only produced in significant amounts after ovulation. Oestrogen is produced in varying large amounts throughout the menstrual cycle, and only significantly declines just before a period, and still osteoporosis cripples thousands of people each year.
Osteoporosis is a slow progressive disease.
Bone loss starts to occur in most women from the mid thirties. It does not happen overnight with the last menstrual period, but develops alongside high oestrogen levels. Specific bone cells, known as osteoblasts, have receptor sites for progesterone. This is the hornmone that ensures that new bone is built. Those women taking combined HRT do not experience much benefit from the synthetic form of progesterone added to it.