Estrogens regulate the bone metabolism, adversely affecting peak bone mass and prohibiting the bone loss. The high gestational estrogens has made us suppose that the maternal bone would be protected. However, the maternal bone mineral density (BMD) is found to decrease. In addition, postpartum lactational low estrogens and PTHrP induce to lower the BMD. These two gestational phases cause complicated bone metabolisms, resulting to bone loss with different pathophysiologies., Breastfeeding places significant stress on calcium metabolism and, as a consequence, directly influences bone metabolism. In short time after the end of lactation, the bone recovers to prepregnant state.