Maternal vitamin B12 deficiency during pregnancy and lactation, particularly in the context of a strict vegan diet, results in severely low infant serum B12 levels (<125 pg/mL) and megaloblastic anemia, regardless of reported maternal supplementation.

When a mother has very low vitamin B12, her breast milk has almost no B12. The baby uses up the small amount stored during pregnancy and quickly runs out. Without B12, the baby’s bone marrow can’t make normal red blood cells, so it produces large, broken cells that don’t work. At the same time, the baby’s brain can’t build the fatty insulation around nerve fibers, so nerves don’t send signals properly. This causes anemia and neurological damage.