Premature birth and low birth weight implies insufficient intrauterine growth and fetal malnutrition. During the first months of life where the infant is breastfed, the low stores aquired during fetal life, may cause specific B vitamin deficiencies. In this study infants with a birth weight below 3000 g will be studied at 6 weeks, 4 and 6 months. At 6 months infants with biochemical signs of impaired cobalamin status (i.e.: tHcy > 97.5 percentile for cobalamin treated infants, i.e.: tHcy>6.5 µM/L) will be randomised to cobalamin treatment or placebo. At 7 months the investigators will evaluate the effect of cobalamin or placebo treatment according to infant biochemical status and neurodevelopment.