Newborn babies only have 30 to 60% the amount of blood clotting factors that adults have. Babies this young do not yet possess the bacteria in their colon that are required to produce vitamin K, nor do they synthesize enough of the precursor proteins required.

Furthermore, vitamin K does not cross the placenta during pregnancy to reach the fetus inside the womb. It can take several weeks before a newborn baby is able to produce adequate levels of vitamin K and for this reason, babies are administered a vitamin K injection after birth as part of routine practice.
 

In the USA, 0.5 to 1.0 mg of vitamin K1 is administered to all babies shortly after birth, as recommended by the Committee on Nutrition of the American Academy of Pediatrics. In the UK, a single intramuscular injection of 1mg is usually given soon after birth and a second option of three oral doses over the course of a month is also offered. However, intramuscular administration is more effective than administering vitamin K orally.

Another reason why newborns have low vitamin K levels is that breast milk is low in this vitamin (1 to 4 μg per litre). Infant formula is manufactured to contain vitamin K and these formulas typically contain up to 100μg per litre. However, even babies who are fed this formula often have a low vitamin K level for several days.

If left untreated, a low vitamin K level can lead to a condition called hemorrhagic disease of the newborn (HDN), which can lead to severe bleeding. Premature babies have an even lower level of vitamin K than babies born at full term, meaning they are at an even greater risk of HDN. The bleeding that occurs in HDN can be very severe and bleeding in the brain can lead to brain damage. Hospitalization and blood transfusion are sometimes required to treat babies who develop this condition.

In the UK, HDN is very rare and the majority of cases occur when parents refuse the preventative treatment after birth. The condition is probably a common cause of death and handicap in developing countries where prophylactic treatment is not routinely administered to newborns, although this is likely to be poorly documented.