All newborn babies have a natural degree of vitamin K deficiency at birth, usually about 40 to 70% less than adult levels. These levels are adequate to protect babies from bleeding, but can be quickly depleted if the baby is unable to replace and build up their vitamin K stores in the weeks and months following the birth (to keep up with what their body is using). In rare cases the levels can lower even further than those at birth, creating an unsafe deficiency.

During the pregnancy vitamin K1, (obtained through food eaten by the mother), does not cross the placenta to the baby in large quantities (only about 3 to 4 %). The Vitamin K2 that is produced by the mother's bowel bacteria is unable to be transmitted to the baby.

Once babies are born, they start to obtain a little Vitamin K1 through feeding. Breast milk contains very small amounts of vitamin K1, (about 1 to 2 micrograms per litre). Infant formula milks are regulated by law to contain a minimum of 30 micrograms per litre. This means that babies who are exclusively breastfed are at an increased risk of having a vitamin K deficiency in the early months.

Most of the Vitamin K needed by the baby is obtained through a slow build up of their gut bacteria in the weeks, or months, after the birth, producing their vitamin K2. A baby's bowel is 'sterile' when inside the uterus (meaning no bacteria) and does not start producing vitamin K2 until after they are born. Once in the 'outside world' the baby's bowel starts colonising the normal 'body-friendly' bacteria. This is done by sucking on their hands, latching on the breast etc. starting the production of vitamin K2. The vast majority of babies will increase their vitamin K to adult levels by the time they are 6 weeks old.

While most newborn babies are able to create adequate Vitamin K2 to maintain and increase their levels, occasionally a baby does not achieve this and may experience Vitamin K deficiency.