Why does my baby need a vitamin K shot?

Vitamin K is essential to help blood clot.  Babies born with a vitamin K deficiency may have unexpected bleeding (0.25% to 1.7% incidence) during the first week of life in previously healthy-appearing neonates (classic hemorrhagic disease of the newborn [HDN).  Late HDN, a syndrome defined as unexpected bleeding due to severe vitamin K deficiency in infants aged 2 to 12 weeks, occurs primarily in exclusively breast-fed infants who have received no or inadequate neonatal vitamin K prophylaxis. The efficacy of neonatal vitamin K prophylaxis (either oral or parenteral) in the prevention of classic HDN is firmly established.

Since time is of the essence, the American Academy of Pediatrics recommends all babies receive vitamin K rather than perform laboratory tests to find out which baby is deficient and waste precious time.