An increasing drain of maternal reserves mineralises the foetal skeleton. In total, the developing foetal skeleton gains up to 33 g of calcium, and about 80% of this is deposited during the third trimester when the foetal skeleton grows rapidly. This high demand for calcium is largely met by a doubling of maternal intestinal calcium absorption, mediated by calcitriol and other hormones, which is usually enough to meet the daily calcium needs of the foetus without long-term consequences to the maternal skeleton. The increased calcium absorption early in pregnancy may stimulate the maternal skeleton to prepare for the peak foetal demands that occur later in pregnancy. Subsequently, during lactation, sufficient calcium must be supplied in the breast milk to enable skeletal growth of the infant. Calcium homeostasis in human pregnancy and lactation is illustrated in Fig. 37.1.