Calcium is required for skeletal growth in all vertebrate offspring. In eutherian mammals, calcium is provided by the mother via the placenta during fetal growth and via milk until weaning. Transferring calcium to offspring during pregnancy and lactation significantly stresses maternal calcium homeostasis. During human pregnancy, the extra calcium requirements are met primarily by an increase in absorption of calcium from the diet and by a modest increase in rates of bone resorption. In nursing mothers, the calcium required for milk production is generated by a dramatic increase in rates of bone resorption and a decrease in the rate of renal calcium excretion. To consider the evolution of these maternal adaptations in bone and calcium metabolism, comparisons are made across different species of mammals, and the fundamental problem of maternal transfer of calcium to young is explored in lower vertebrates. These comparisons suggest that maternal adaptations in calcium and bone metabolism during pregnancy and lactation in mammals originate from adaptations in bone and mineral metabolism that supply calcium for egg production in lower vertebrates.