Biochemical indices of vitamin B 12 nutrition in pregnant patients with subnormal serum vitamin B 12 levels

To determine the significance of the commonly observed fall in serum vitamin B 12 levels during pregnancy, serum levels of the B 12 metabolites methylmalonic acid (MMA) and homocysteine (Hcy) were measured in a group of 50 pregnant patients with subnormal serum B 12 (range 45‐199 pg/ml) and the results compared with those of 25 pregnant controls (serum B 12 208‐580 pg/ml). Mean values for serum MMA and total Hcy in the subnormal B 12 group were 445.4 nmol/L and 7.03 μmol/L, respectively, which were not significantly different from the mean MMA of 440.5 nmol/L and Hcy of 6.88 nmol/L in the controls. For the total group of patients, neither serum MMA nor serum Hcy levels correlated with serum B 12 . One‐third of pregnant patients showed elevated serum MMA values, independent of B 12 status. Significant elevation of serum Hcy was detected in only two patients, both with subnormal serum B 12 and hematological evidence of B 12 deficiency. We conclude that the usual fall in serum B 12 concentration in pregnancy does not reflect B 12 deficiency at the biochemical level. In establishing true B 12 deficiency in pregnancy, the serum Hcy level (in the absence of folate deficiency) but not serum MMA, is of value. © 1995 Wiley‐Liss, Inc.