Our bodies require vitamin K to make our blood clot. Vitamin K comes from the diet (mostly in green leafy vegetables) and from bacteria in the intestines that make vitamin K as well; most healthy children and adults need both sources, and vitamin K deficiency is rare. However, that is not the case in newborn infants.
Newborns are at risk of a disease now called vitamin K deficient bleeding (VKDB). Premature and breast-fed babies are at a higher risk. Babies are born with low vitamin K for several reasons. The placenta does not transfer much vitamin K from the mother to the baby. Breastmilk is relatively low in vitamin K, and babies are born with sterile intestines with no bacteria to make vitamin K. The liver in newborns is also not very efficient in using vitamin K. All of these factors make the baby prone to severe or fatal bleeding due to low vitamin K. Most babies have normal levels by one month of age.
There are two forms of VKDB. One is an early form, seen in the first few days or weeks of life. The other is a late form, usually seen from two to twelve weeks of life, usually in breastfed infants. The early form can be prevented by giving a vitamin K supplement by mouth or as an injection. However, oral vitamin K has not been shown to be effective at preventing the late form of the disease.
The injection of Vitamin K prevents the disease in full-term infants and in most premature infants. This became standard practice in 1961. The disease would actually occur in up to 2% of infants if Vitamin K were not given. Infant formulas are also supplemented with vitamin K.
There have been concerns raised about a connection between the Vitamin K shot and childhood leukemia and cancer; research continues to show that there is no association. Although this practice is not without controversy (even in the pediatric community), currently the benefits seem to outweigh the risks (or the risks of not getting it are higher than the risks of the injection).
