Let’s talk about vitamin K. What is it?

Vitamin K is a group of vitamins mostly found in leafy green foods (think kale, spinach, broccoli, Brussels sprouts, asparagus, even kiwi), and it plays a big role in helping our blood clot. If it’s green, chances are it’s got vitamin K.

Let’s break down Vitamin K!

Vitamin K is a fat-soluble vitamin, which means it's stored in the liver and transported throughout your body via the bloodstream. While we get it from food, our gut bacteria also produce it, but that doesn’t happen overnight. Our bodies use it quickly and excrete it just as fast, which is why we have to keep topping up through diet.

But here’s the issue: Babies are born vitamin K deficient. Newborns don’t have enough gut bacteria to produce vitamin K yet, and the vitamin doesn’t pass easily through the placenta during pregnancy. Even if you're planning to breastfeed, there still won’t be enough vitamin K transferred through your milk.

Now Here’s Why It Matters

Babies do have the clotting factors they need at birth, but they can’t actually use them without vitamin K. It’s like buying your dream house but not getting the keys—you’ve got everything you need except the one thing that lets you in. And that “key” is vitamin K.

Without it, babies are at risk for something called Vitamin K Deficiency Bleeding (VKDB), which can happen spontaneously, not just from trauma or procedures like circumcision. Even in smooth, non-traumatic births, VKDB can still occur. In fact, a 2013 report documented six spontaneous cases following uncomplicated vaginal deliveries.

Worried your amazing placenta is handling it? I get it. But vitamin K just doesn’t cross the placenta effectively, because it’s not hanging out in your bloodstream freely. It’s locked away in your liver. What about breast milk? Still nope. Not enough vitamin K makes it through.

So, How Do We Help These Little Humans?

With a one-time vitamin K injection within the first 6 hours of life.

And I know—you’ve got questions. You love your baby more than anything, and every decision feels monumental. Concerns usually come down to three big themes:

belief systems
infant welfare
outside influences

Let’s talk about them.

The Risk Without the Shot

Babies who don’t receive a vitamin K shot are 80 times more likely to experience severe bleeding. The most common site? The brain. And 1 in 5 babies with VKDB dies.

There are three types of VKDB:

Early VKDB: Happens in the first 24 hours, often in babies whose mothers took medications that interfere with vitamin K.
Classical VKDB: Occurs between days 2–7, when vitamin K levels are at their lowest. Common bleeding sites include the umbilical cord, GI tract, skin, nose, and circumcision site.
Late VKDB: The rarest, affecting about 1 in 14,000–25,000 babies. It shows up between 1 week and 6 months (peaking at 2–8 weeks), mostly in exclusively breastfed infants who didn’t receive the shot. Up to 60% of these cases involve brain bleeding.

Babies start making their vitamin K around day 7, but they don’t reach sufficient levels until 6 months old. That’s a long time to be vulnerable.

Why is the Shot Recommended?

The intramuscular shot contains 0.5-1 mg of vitamin K, depending on the baby’s weight. It's stored in the liver and slowly released over months. Oral vitamin K, by comparison, requires three carefully timed doses—and even then, it’s not as effective, especially for late VKDB. It’s easy to miss a dose in the chaos of newborn life, and studies show 1-6 out of 100,000 babies who get oral vitamin K still experience bleeding.

That's why both the Canadian Pediatric Society and the American Academy of Pediatrics recommend the shot. It's the most effective, low-risk option.

But What About Side Effects?

Let’s talk about the black box warning. This warning exists because adults with liver disease or on blood thinners may receive large IV doses of vitamin K, which in rare cases can cause a severe allergic reaction (anaphylaxis). But this risk isn’t the same with the small intramuscular dose given to newborns.

Since the 1960s, millions of babies in the U.S. alone have received the vitamin K shot. There has been one documented allergic reaction in a baby in 2014 in Turkey. That baby recovered. There have been zero deaths from vitamin K injections in babies.

For context? Ibuprofen has a black box warning. So do some antibiotics. This label isn’t a sign that something is inherently dangerous—it’s about context and use.

Let’s Talk Aluminum

Yes, the shot may contain a tiny amount of aluminum: up to 0.05 micrograms. Adults consume 2–10 milligrams of aluminum daily (that’s 2000–10,000 micrograms). The amount in the shot is minuscule, and babies are capable of eliminating it efficiently.

There’s more aluminum in your placenta than there is in the shot.

What about Preservatives?

There may be a small amount of benzyl alcohol (a preservative), but no mercury. If you’re concerned, you can ask your provider if a preservative-free version is available because many are!

What About the Leukemia Rumour?

This concern came from a small 1992 study that involved fewer than 800 children. Since then, multiple large-scale studies (including one with 1.4 million children) have found no connection between vitamin K and leukemia. No biological mechanism supports it either.

My Final Thoughts?

As a mom of four, I know how hard it is to watch your baby get poked. But I also know the peace of mind that comes from protecting them, especially when it’s with one dose that can prevent a life-threatening condition.

If you've never heard of vitamin K deficiency bleeding, that’s a good thing. It means we’re doing our jobs in preventing it. One little poke helps keep it that way.

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The Lowdown on Vitamin K and Why It Matters for Your Newborn