Globally, vitamin B12 deficiency is one of the most common micronutrient deficiencies. Poor status is also seen in affluent countries such as in Norway. Vitamin B12 is crucial for normal cell division and differentiation and necessary for the development and myelination of the central nervous system. Deficiency is also associated with impaired fetal and infant growth. In the proposed study we will measure the effect of daily oral vitamin B12 supplementation infants on neurodevelopment. We also aim to measure the impact of B12 supplementation on several other outcomes. Study design: Individually randomized double-blind placebo-controlled trial breastfed infants who will be assigned to a screening group (in which measurements will be obtained immediately) or a control group (in which serum will be stored and measurements done after one year). Pregnant women will be informed about the study during their first antenatal visit at the clinic and that we will re-approach them on their 6-week visit to their public health nurse. Infants who are deficient will be treated with peroral or intramuscular injections with 400 µg cyano-cobalamin. Infants in the control group will not be offered any intervention their blood sample will be stored for one year and then analyzed for the same nutrients as the intervention group. Outcomes: Primary: (i) neurodevelopment in children measured at 12 months of age (ii) growth in children measured by attained weight and length at 12 months. Secondary: (i) neurodevelopment and cognitive functioning in children later in life