Vitamin K in pregnancy: evidence and safety

How does vitamin K work?

Vitamin K functions as a coenzyme for vitamin K-dependent carboxylase, the enzyme required to build proteins involved in hemostasis and bone metabolism. On the clotting side that runs through prothrombin, clotting factor II, which NIH ODS describes as a vitamin K-dependent plasma protein directly involved in blood clotting. On the bone and vascular side it runs through osteocalcin, a vitamin K-dependent protein present in bone that may be involved in bone mineralization or turnover, and through matrix Gla protein, found in vascular smooth muscle, bone, and cartilage and studied for whether it helps reduce abnormal calcification. Those are the mechanisms in an adult body. The newborn question on this page is a separate matter of placental transfer, not of what the vitamin does once it is present.

What does the evidence say about vitamin K in pregnancy?

• The adequate intake for vitamin K is 90 mcg a day during pregnancy and lactation, for both the 14 to 18 and 19-and-older age groups. 1
• No tolerable upper intake level exists for vitamin K, because the Food and Nutrition Board found its potential for toxicity low and no adverse effects from food or supplements have been reported in humans or animals. 1
• Newborns start life vitamin K deficient for three reasons: poor placental transfer, liver clotting factor synthesis at only 30 to 50 percent of adult levels, and gut bacteria that have not colonized yet. Without prophylaxis, late vitamin K deficiency bleeding affects roughly 4.4 to 10.5 per 100,000 live births. 2
• Pooling six trials covering 21,493 pregnant women and newborns, antenatal vitamin K had no statistically significant effect on neonatal bleeding (relative risk 1.16, 95 percent CI 0.59 to 2.29). Maternal plasma vitamin K1 did rise. 3
• Maternal supplementation at 5 mg daily raises breastmilk vitamin K1 from about 1 mcg/L at baseline to 27 and 59 mcg/L at two weeks, yet LactMed states plainly that this is not an adequate or safe substitute for vitamin K given directly to the newborn after birth. 4

What's on the label?

ODS describes phylloquinone (K1) as the main dietary form, present primarily in green leafy vegetables, and menaquinones (K2) as predominantly bacterial in origin, present in modest amounts in various animal-based and fermented foods. Supplements use vitamin K1 as phylloquinone or phytonadione and vitamin K2 as MK-4 or MK-7. ODS cites one study finding both phytonadione and MK-7 well absorbed, with MK-7 having the longer half-life, which is the honest extent of the form comparison available.

What should I know before taking vitamin K?

Please consult your healthcare provider before starting, stopping, or stacking supplements in pregnancy, especially if you take prescription medication, live with a chronic condition, or have a procedure or delivery scheduled. Herbal supplements are not required to demonstrate purity; brands that publish third-party test results give the most confidence in what is actually in the capsule.