Background/AimLate vitamin K deficiency bleeding (VKDB) during early infancy is a serious problem worldwide. Vitamin K (VK) deficiency commonly occurs in newborns who are exclusively breastfed. Protein Induced by VK Absence (PIVKA‐II) has been identified as an early indicator of subclinical VK deficiency in neonates, surpassing prothrombin time. To assess PIVKA‐II levels at 48 h, 1 and 3 months of age in full‐term newborns who were exclusively breastfed and received varying VKDB prophylaxis regimens.MethodsA prospective observational study was conducted in four hospitals, enrolling 105 newborns. PIVKA‐II levels were measured using a sandwich‐type enzyme‐linked immunosorbent assay.ResultsAt 48 h of age, there was no significant difference in PIVKA‐II concentrations between newborns who received intramuscular administration of 1 mg of phylloquinone (VK1) and those who received oral administration of 2 mg of VK1 at birth. At 1 and 3 months of life, infants who received any supplementation regimen between 2 and 14 weeks exhibited significantly lower PIVKA‐II concentrations compared to infants who received only 1 mg of intramuscular VK1 at birth. The prophylaxis involving a dose of 1 mg of intramuscular VK1 at birth followed by oral administration of 150 μg/day of VK1 from the 2nd to the 14th week of life showed the lowest PIVKA‐II blood concentrations.ConclusionsOral supplementation of VK1 after discharge significantly reduced PIVKA‐II concentrations in exclusively breastfed term infants. These findings suggest the importance of oral VK1 supplementation in exclusively breastfed infants during their first 3 months of life to avoid the risk of VK insufficiency.