Calcium and nutrients are transferred from mothers to fetuses or infants during pregnancy or lactation, respectively, promoting metabolic changes in the mother, many uncharacterized. To evaluate these changes, we undertook two parallel studies. In one we analyzed fourteen clinical cases of vertebral fragility fractures, at or before three months after partum, in mothers who breastfed their infants. In the other, we enrolled 79 additional pregnant subjects, some who chose to breastfeed and others who did not, and analyzed changes in bone metabolic status starting between 34 and 36 weeks of gestation and ending one month after partum. In the larger group, bone-resorbing and bone-forming parameters such as serum TRACP5b and osteocalcin, respectively, significantly increased after partum. Among parameters that changed after partum, serum PTH and the bone-resorbing markers serum TRACP5b and urine NTX were significantly higher in mothers who only breastfed infants compared to mothers who fed infants formula or a mix of both. However, bone-forming parameters were comparable between breastfeeding and non-breast-feeding groups after partum, suggesting that elevated bone-resorption occurs only in the breastfeeding group. Radiographic analysis after partum demonstrated that no subject among the 79 analyzed showed vertebral fractures, even those who breastfed exclusively. Among fracture cases analyzed, subjects exhibited significantly lower bone mineral density than did non-fracture cases in breastfeeding-only subjects. We conclude that bone metabolic status significantly changes over the period between pregnancy and post-partum lactation, and that low bone mineral density seen in a small subset of breastfeeding-only cases likely causes post-partum vertebral fragility fractures.