['Fasting plasma total homocysteine (tHcy) decreases during pregnancy. Previous reports suggested that this is due to folic acid supplementation, hemodilution, or a decrease in albumin. However, these hypotheses have not been tested in a longitudinal study.', 'We investigated the relation between pregnancy-related physiologic changes and tHcy in a group of healthy women who were either unsupplemented or supplemented with folic acid.', 'In a longitudinal study from preconception throughout pregnancy, we studied 54 unsupplemented women and 39 women who were supplemented with folic acid during the second or third trimester of pregnancy. tHcy, hematocrit, and serum albumin were determined preconceptionally and at 8, 20, and 32 wk of pregnancy.', 'For the entire group, geometric mean tHcy concentrations at preconception (8.2 micro mol/L) were significantly greater (P < 0.001) than those at 8 wk of pregnancy (6.4 micro mol/L). When the unsupplemented and supplemented groups were regarded separately, geometric mean tHcy concentrations at preconception were significantly greater than those at 20 (5.22 and 4.18 micro mol/L, respectively) and 32 (5.16 and 4.42 micro mol/L, respectively) wk of pregnancy (P < 0.001 for both). Mean reductions from preconception concentrations at 8, 20, and 32 wk of pregnancy were significantly greater (P < 0.001) for tHcy (-11.5%, -25.5%, and -24.5%, respectively) than for hematocrit (-1.9%, -4.2%, and -4.3%, respectively) or serum albumin (-1.1%, -9.8%, and -13.4%, respectively). There was no correlation between changes in either hematocrit or serum albumin and changes in tHcy.'] Is the pregnancy-related decrease in fasting plasma homocysteine explained by folic acid supplementation , hemodilution , or a decrease in albumin in a longitudinal study?