What is the rationale for injecting vitamin k to newborn`s thigh?

Stop bleeding: Newborns do not have adequate vitamin k at birth which is necessary for proper clotting of the blood. This injection serves as a prophylactic against bleeding episodes of the newborn.

2 things: The thigh is the best muscle mass to accept the shot. The newborn has just gone through the trauma of a delivery and will not clot its blood adequately until 8 days or later without the shot. Early in the use of CT scans they found as many as 3% of infants had small intracranial bleeds thought to represent birth trauma. The shot is a an insurance against extending a bleed with significant damage.