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).

Absorption and Utilization of B₁₂. There is some evidence that the absorption of B₁₂ may increase during pregnancy. An increase in the number of intrinsic factor-B₁₂ receptors was observed in pregnant mice and found to be regulated by placental lactogen (Robertson and Gallagher, 1983). A greater absorption of oral B₁₂ was reported from the single study of pregnant women (Hellegers et al., 1957), but the methods used do not permit quantification of the increase.

Serum total B₁₂ concentrations begin to decline early in the first trimester. In a longitudinal Dutch study of 23 subjects, serum B₁₂ fell significantly by the end of the first trimester, more than could be accounted for by hemodilution (Fernandes-Costa and Metz, 1982). There were further decreases through the sixth month to about half of nonpregnancy concentrations. Some of the later decrease was due to hemodilution. However, transcobalamin I and III increase during the second and third trimesters, and transcobalamin II increases sharply in the third trimester to about one-third more than in nonpregnant, nonlactating control subjects (Fernandes-Costa and Metz, 1982).

Transfer to the Fetus. The serum B₁₂ concentration of the newborn is twice that of the mother, decreasing to adult concentrations at about 6 to 7 months postpartum (Luhby et al., 1958). The placenta