relatively few studies have examined the effect of calcium supplementation on either fetal or maternal outcomes.
Maternal serum calcium falls during pregnancy (Pedersen et al., 1984), but this is likely not important from a physiological perspective in that it reflects the fall in serum albumin caused by plasma volume expansion and therefore does not imply calcium deficiency. Reports indicate that the concentration of ionized calcium remains normal during pregnancy (Frolich et al., 1992; Seely et al., 1997).
Pregnant women consuming moderate (800 to 1,000 mg/day [Gertner et al., 1986; Allen et al., 1991]) to high (1,950 mg/day [Cross et al., 1995]) levels of calcium are often hypercalciuric due to increased intestinal calcium absorption (i.e., absorptive hypercalciuria), and as such pregnancy itself can be a risk factor for kidney stones.
Within the developing human fetus, calcium metabolism is regulated differently from that of its mother. Serum calcium, ionized calcium, and phosphorus are raised above the maternal values, while PTH and calcitriol are low. The high calcium and phosphorus as well as the low levels of PTH all contribute to suppression of the renal 1α-hydroxylase and maintenance of low levels of calcitriol.
In adolescents, whose skeleton is still growing, pregnancy could theoretically reduce peak bone mass and increase the long-term risk of osteoporosis. Although most cross-sectional studies comparing BMD in teens early post-partum to never-pregnant teens (reviewed by Kovacs and Kronenberg, 1997) suggest that BMD or bone mass after adolescent pregnancy is not adversely affected, a few smaller associational studies report that adolescent age at first pregnancy is associated with lower BMD in the adult (Sowers et al., 1985, 1992; Fox et al., 1993). Chantry et al. (2004) analyzed data from the Third National Health and Nutrition Examination Survey (NHANES III) on BMD as measured by dual-energy X-ray absorptiometry (DXA) for 819 women ages 20 to 25 years and found that women pregnant as adolescents had the same BMD as nulliparous women and women pregnant as adults.
Breast milk calcium content is homeostatically regulated, and maternal calcium intake does not appear to alter the breast milk calcium content (Kalkwarf et al., 1997; Jarjou et al., 2006). Generally, human breast milk will provide two to three times the amount of calcium to the infant during 6 months of lactation as the pregnant woman will have provided to the fetus during the preceding 9 months of pregnancy. To meet the calcium demands of pregnancy, key physiological changes in the female will also occur, but the adaptations differ from those that take place during pregnancy