Why does bone mass start to decline in postmenopausal women?

Once a women reaches middle age, bone mass in the hip starts to decline, because there is an imbalance between the volume of the various bone components which are removed during the normal metabolism and buildup of the bone, and those reincorporated during the bone remodeling process. When menopause occurs, the rate of bone loss accelerates. This is particularly fast in the first ten years after menopause.