and Skillman, 1971). The increased intestinal calcium absorption causes a net positive calcium balance in the mother early in pregnancy (Heaney and Skillman, 1971). However, in the third trimester, the rapid maternal-fetal calcium transfer results in a maternal calcium balance that is zero or perhaps slightly negative by the end of pregnancy.
There is controversy about the mobilization of calcium from maternal bone during pregnancy and its contribution to fetal calcium needs. A possible loss of BMC has been seen longitudinally using the modern technique of dual-energy X-ray absorptiometry (DXA), but measurements were done 1 to 18 months prior to pregnancy and 1 to 6 weeks postpartum (i.e., not during pregnancy) making it uncertain whether the measured calcium loss had truly occurred during pregnancy (Kovacs and Kronenberg, 1997; Kovacs and Fuleihan Gel, 2006). Further, the effect of pregnancy on bone mineral content may depend on the site examined, with decreases reported for trabecular bone (Black et al., 2000; Naylor et al., 2000; More et al., 2001; Kaur et al., 2003; Ulrich et al., 2003; Akesson et al., 2004; Pearson et al., 2004), but not cortical bone (Naylor et al., 2000; Pearson et al., 2004). Two studies (Kaur et al., 2003; Olausson et al., 2008) used contemporaneous non-pregnant and non-lactating age-matched controls to compare, to the extent feasible, the effects of pregnancy and age on BMD. Kaur et al. (2003) found no significant difference in BMD before and after pregnancy. Olausson et al. (2008) found a significant 1 to 4 percent decrease in whole-body, spine, and total hip BMC before and 2 weeks after pregnancy, whereas controls had an increase in whole-body BMC and a smaller (0.5 to 1 percent) decrease in BMD at the spine and hip. These skeletal changes were unrelated to calcium intake in either group. Collectively, the evidence tends to suggest that mineral mobilization is variable during pregnancy and may contribute, to some extent, to fetal calcium needs.
Relatively few studies have examined the effect of calcium supplementation on either fetal or maternal outcomes. In a placebo-controlled double-blind randomized trial conducted in the United States, Koo et al. (1999) demonstrated that calcium supplementation during pregnancy may benefit the offspring’s bone health, but only in those infants whose mothers had very low calcium intake (600 mg/day), based on a post-hoc subgroup analysis. In contrast to this possible benefit to the offspring, calcium supplementation during pregnancy of Gambian women with low calcium intakes resulted surprisingly in greater decreases in BMC and BMD and related biochemical evidence, consistent with higher bone mineral mobilization during lactation (Jarjou et al., 2010).
Maternal serum calcium levels fall during pregnancy (Pedersen et al., 1984) as a consequence of plasma volume expansion and reduced albumin concentration; lower calcium levels do not imply calcium deficiency. The