Vitamin K is given as an injection to newborns to prevent something called "hemorrhagic disease of the newborn" (HDN). This consists of bleeding from multiple sites in otherwise healthy infants in the first week of life. It occurs because newborn blood takes longer to clot than adult blood, due to a natural deficiency of vitamin K during this time. Vitamin K is a required precursor for many of the clotting factors. Levels are low in the newborn because of several factors, the most important being low placental transfer of vitamin K, low concentration in breastmilk, and low intake of milk during the first few days of life. The incidence of HDN has been reported to be between 0.5 percent and 1.7 percent of newborns.

Interestingly, the practice in some religious and ethnic groups of scheduling circumcision on day eight of life may have it's historic roots in the increased incidence in bleeding during the first week of life.