Vitamin B12 (cobalamin) is necessary for development of the fetus and child. Pregnant women who are vegetarian or vegan, have Crohn's or celiac disease, or have undergone gastric bypass surgery are at increased risk of B12 deficiency. Low serum levels of B12 have been linked to negative impacts in cognitive, motor, and growth outcomes. Low cobalamin levels also may be related to depression in adults. Some studies indicate that B12 supplementation may improve outcomes in children, although more research is needed in this area. Overall, the mechanisms of B12 action in development remain unclear. Further studies in this area to elucidate the pathways of cobalamin influence on development, as well as to prevent B12 deficiency in pregnant women and children are indicated.