Vitamin K is formed in the stomach when we eat, thats why babies dont have any as they dont really eat, It only takes a couple of days to be made once any form of feeding has been started. also there has been a huge increase in childhood cancers since it has been introduced as a regular drug.
I just have to correct this. Vit K is formed by BACTERIA that are found in the stomach, not by eating. It takes approx. 8-10 days for the bacteria to populate the stomach in significant quantities to produce enough vit K for baby to utilise. As the amniotic Sac is (unless ruptured) a sterile environment, they do not have this bacteria when they are born. They get it from the Vagina and from the breasts (NO, NIPPLES ARE NOT STERILE!).
However this lack of K is only a concern when baby has a low blood count and/or has had a traumatic birth. This is why it has been proven that when you allow the placenta to stop pulsating on its own and utilise gravity to get as much bubba blood as possible out of the placenta and into the baby, that the supplement is not really needed. And FYI, the childhood cancer increase only happened in the UK, because there was a carcinogenic contaminant in their stores of VitK.
To this day, it has not been proven "safe or necessary", just "not always necessary, and not unsafe". The mods have placed some great articles earlier in the thread.