concentrates B₁₂, which is then transferred to the fetus down a concentration gradient. Fetal and maternal B₁₂ serum concentrations are quite strongly correlated (Fréry et al., 1992). It appears that only newly absorbed B₁₂ is readily transported across the placenta and that maternal liver stores are a less important source of the vitamin for the fetus (Luhby et al., 1958). This implies that current maternal intake and absorption of the vitamin during pregnancy have a more important influence on the B₁₂ status of the infant than do maternal B₁₂ stores. The importance of adequate maternal intake during pregnancy is supported by the appearance of B₁₂ deficiency in infants at 4 to 6 months when their mothers have been strict vegetarians for only 3 years (Specker et al., 1990).

Fetal Accumulation. The human fetus accumulates an average of 0.07 to 0.14 nmol/day (0.1 to 0.2 µg/day) of B₁₂, a range based on three studies of the liver content of infants born to women who were adequate in B₁₂ (Baker et al., 1962; Loria et al., 1977; Vaz Pinto et al., 1975) and an assumption that the liver contains half the total body B₁₂ content. Placental B₁₂ is negligible (0.01 nmol/L [14 ng/L]) (Muir and Landon, 1985). The low body content of B₁₂ in the newborn implies that pregnancy is unlikely to deplete maternal stores.

On the basis of a fetal deposition of 0.1 to 0.2 µg/day throughout pregnancy and evidence that maternal absorption of the vitamin becomes more efficient during pregnancy, the EAR is increased by 0.2 µg/day during pregnancy. No distinction is made for the age of the mother.

The RDA for B₁₂ 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 B₁₂; 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 B₁₂ the RDA is 120 percent of the EAR).