Methionine and folate are the key components of one carbon metabolism, providing the methyl groups for numerous methyl transferase reactions via the ubiquitous methyl donor, s-adenosyl methionine. Methionine metabolism is responsive to nutrient intake, is regulated by several hormones and requires a number of vitamins (B12, pyridoxine, riboflavin) as co-factors. The critical relationship between perturbations in the mother’s methionine metabolism and its impact on fetal growth and development is now becoming evident. The relation of folate intake to fetal teratogenesis has been known for some time. Studies in human pregnancy show a continuous decrease in plasma homocysteine, and an increase in plasma choline concentrations with advancing gestation. A higher rate of transsulfuration of methionine in early gestation and of transmethylation in the 3rd trimester was seen in healthy pregnant women. How these processes are impacted by nutritional, hormonal and other influences in human pregnancy and their effect on fetal growth has not been examined. Isocaloric protein restriction in pregnant rats, resulted in fetal growth restriction and metabolic reprogramming. Isocaloric protein restriction in the non-pregnant rat, resulted in differential expression of a number of genes in the liver, a 50% increase in whole body serine biosynthesis and high rate of transmethylation, suggesting high methylation demands. These responses were associated with a significant decrease in intracellular taurine levels in the liver suggesting a role of cellular osmolarity in the observed metabolic responses. These unique changes in methionine and one carbon metabolism in response to physiological, nutritional and hormonal influences make these processes critical for cellular and organ function and growth.