The adult skeleton contains approximately 99% of calcium in the body, and that content is perceived to be stable in the young to middle-aged adult female, unless ovarian failure and certain medications or illnesses have interfered. In fact, calcium and bone metabolism is substantially altered during the normal reproductive periods of pregnancy and lactation, and bone density can drop and regain 3–10% in the span of a few months in normal, healthy women. The primary cause for this disruption is, of course, the calcium requirements of the rapidly mineralizing skeleton of the fetus and neonate. The fetus and placenta actively pump calcium from the maternal circulation, while hormonal changes in the mother ensure a sufficient supply of calcium to the breast milk and, thereby, the nursing infant. Although the daily maternal calcium losses in the third trimester are similar to the daily calcium losses in breast milk of an exclusively lactating woman, it seems that the adjustments made in each of these reproductive periods differ significantly. This article summarizes what is currently known about altered calcium homeostasis in pregnancy and lactation; the interested reader is referred to a recent comprehensive review for more information and detailed references (1).