Cobalamin deficiency in pregnant or lactating women results in a lower supply of the vitamin to the foetus/infant. Low cobalamin levels at birth are associated with elevated cord blood homocysteine and methylmalonic acid (MMA) and lower methylation index. Cobalamin is actively transported from the mother to the child during pregnancy. Maternal or child plasma cobalamin status markers were not considered when defining Adequate Intakes (AI) of cobalamin in infants. In communities with micronutrient insufficiency, infants become strongly depleted the longer lactation lasted or with delayed introduction of/or insufficient complementary foods. The bioavailability of cobalamin from human milk is unknown, particularly when milk haptocorrin content increases a few months after starting breastfeeding. Symptoms of acquired cobalamin deficiency in infancy appear between 3–6 months of age. Diagnosis of cobalamin deficiency should be considered when exploring causes of childhood anaemia, stunting, wasting, low birth weight, underweight and unexplained neurodevelopmental delay in childhood.