The RDA for vitamin C is set by assuming a coefficient of variation (CV) of 10 percent (see Chapter 1) because information is not available on the standard deviation of the requirement for vitamin C; the RDA is defined as equal to the EAR plus twice the CV to cover the needs of 97 to 98 percent of the individuals in the group (therefore, for vitamin C the RDA is 120 percent of the EAR). As with the RDA for younger women, the calculated RDA of 72 mg has been rounded up to 75 mg/day.

Plasma vitamin C concentration decreases with the progression of pregnancy, probably secondary to hemodilution (Morse et al., 1975) as well as active transfer to the fetus (Choi and Rose, 1989). This decrease in plasma concentration has not been shown to be associated with poor pregnancy outcomes. The placenta apparently clears oxidized ascorbic acid from the maternal circulation and delivers it in the reduced form to the fetus (Choi and Rose, 1989). Ascorbic acid deficiency during pregnancy is associated with increased risk of infections, premature rupture of the membranes (Casanueva et al., 1993; Pfeffer et al., 1996), premature birth (Casanueva et al., 1993; Tlaskal and Novakova, 1990), and eclampsia (Jendryczko and Tomala, 1995). In addition, both serum and amniotic fluid concentrations of ascorbic acid are decreased in pregnant smokers compared to nonsmokers (Barrett et al., 1991).

Although the amount of vitamin C required by the growing fetus is unknown, it is known that maternal plasma vitamin C concentra-