Fetuses require PTHrP and PTH for placental mineral transfer, serum mineral concentrations, endochondral bone development, and skeletal mineralization, whereas calcitriol, calcitonin, and FGF23 are not essential (Fig. 1). It is during the neonatal period that intestinal calcium absorption becomes dependent upon vitamin D/calcitriol, and fetal kidneys become responsive to FGF23 (Fig. 1). This explains why rickets develops postnatally with disorders of vitamin D or FGF23.