Why do newborn babies get vitamin K?

To prevent bleeding. Newborn babies have little vitamin K: little crosses the placenta, and breast milk contains little.^[1] Without it, some babies bleed in the first weeks of life, sometimes inside the head, which can be fatal.^[1]

A Cochrane review found that one injection of 1 mg of vitamin K after birth prevents bleeding in the first week of life.^[2] The American Academy of Pediatrics advises one injection of 0.5 to 1 mg at birth.^[1] Doses by mouth improved blood clotting tests, but the review found no randomized trial that tested them against bleeding.^[2]