ionized calcium (i.e., the physiologically important fraction of calcium) and the albumin-corrected serum calcium levels do not change during pregnancy (Seely et al., 1997). Pregnant women consuming “moderate” calcium (800 to 1,000 mg/day) (Gertner et al., 1986; Allen et al., 1991) to “high” calcium (1,950 mg/day) (Cross et al., 1995) are often hypercalciuric as a result of increased intestinal calcium absorption (i.e., absorptive hypercalciuria); as such, pregnancy itself is a risk factor for kidney stones. Urinary calcium excretion increases as early as the 12th week of gestation and averages 300 ± 61 mg/24 hours in the third trimester with hypercalciuric levels not uncommon (Pedersen et al., 1984; Gertner et al., 1986; Allen et al., 1991; Cross et al., 1995; Seely et al., 1997). While urinary calcium excretion goes up in normal pregnancy, it decreases in women who are developing preeclampsia. The risk of preeclampsia can be reduced with supplemental calcium when the dietary calcium intake is very low; however, there appears to be no effect when dietary calcium intake is adequate (Hofmeyr et al., 2006; Villar et al., 2006; Hiller et al., 2007; Kumar et al., 2009).

In the adolescent, whose skeleton is still growing, pregnancy could theoretically reduce peak bone mass and increase the long-term risk of osteoporosis. Most cross–sectional studies that have compared the BMD in teens early postpartum with that in never-pregnant teens have suggested that there is no reason to be concerned about BMD or bone mass after adolescent pregnancy (Kovacs and Kronenberg, 1997). A few smaller observational studies have reported that lower adolescent age at first pregnancy is associated with lower BMD in the adult (Sowers et al., 1985, 1992; Fox et al., 1993). In contrast, an analysis of NHANES III data on BMD by DXA for 819 women ages 20 to 25 years found that women pregnant as adolescents had the same BMD as women pregnant as adults and as nulliparous women (Chantry et al., 2004). This study’s population is diverse and representative of the general U.S. population and thus reassures that teen pregnancy does not reduce BMD in most women. An additional study (O’Brien et al., 2003) found that fractional calcium absorption doubles during adolescent pregnancy (as it does in adults) and during the first 2 months postpartum. Mean BMD of previously pregnant—but not lactating—adolescents was above the expected BMD for age in this study, also suggesting that no loss of BMD had occurred during pregnancy. These data indicate that adolescent women meet the calcium demands of pregnancy by increasing intestinal calcium absorption while preserving maternal bone mass.

Pregnancy: Vitamin D

Maternal outcomes Total calcitriol levels double early in pregnancy and remain at this increased level until delivery (Bikle et al., 1984; Cross et al., 1995; Ardawi et al., 1997; O’Brien et al., 2006; Papapetrou, 2010).