Never heard of vitamin K? It helps the blood to clot — which is why all babies need to get a shot of it before they leave the hospital.

What is a vitamin K shot? A single injection of vitamin K containing .5 to 1 milligram of the nutrient is given in one of your baby’s thighs.

When is it done? Newborns will get a vitamin K shot usually within the first six hours of birth — and certainly by the time they leave the hospital.

Why do babies need vitamin K? Vitamin K is necessary for the blood to clot. All babies have low levels of K when they’re born, so the shot reduces the chances of vitamin K deficiency bleeding (VKDB), which happens in a small percentage of babies. Newborns who have VKDB may have blood in their stool or urine, or they may ooze blood from the skin around the umbilical cord. The bleeding can be fatal. Babies are at higher risk of VKDB if they don’t get the shot, if they’re exclusively breastfed (because cow’s milk formula contains more vitamin K than human milk), or if their mothers take antiseizure medications. The condition is most common in the first days of life, but it can also appear after two weeks.

Is it standard? Yes, receiving the vitamin K shot is common practice for all babies. The American Academy of Pediatrics (AAP) first started recommending it in 1961.

Should you ask for it? You won’t have to.

Are vitamin K shots safe? The vitamin K shot is safe, even though in the early 1990s, British researchers published data that found an association between vitamin K injections in newborns and childhood leukemia. But since then, many scientists have looked at the same data as well as additional research and found no link. The AAP has also analyzed the British data, plus research on children in the US, and found no association between the shot and childhood leukemia or other cancers. The AAP has also stated that recent studies on how childhood leukemia develops show that it’s unlikely vitamin K injections have anything to do with it.

Can you ask for vitamin K as an oral dose instead of as a shot for your newborn? There are oral doses of vitamin K, but they aren’t as effective at preventing the VKDB that can develop after the first few days of your baby’s life. Because an oral dose of K can't protect babies against later-stage VKDB (which can happen anytime between the age of two weeks and six months) as well as the shot can, it's typically not an option for newborns in the United States.