Swallowed maternal blood: This is the most common cause of hematemesis in newborns, occurring when the baby swallows blood during delivery, often from a vaginal tear or episiotomy.

Gastroesophageal reflux: Newborns can experience reflux, which may lead to hematemesis due to irritation of the esophagus.
Stress-induced gastric mucosal damage: Stress from birth, hypoxia, or other perinatal stressors can cause gastric mucosal damage leading to bleeding.
Infection: Certain infections, such as esophageal or gastric infections, can cause inflammation and bleeding.

Esophageal or gastric perforation: Although rare, perforation can occur due to various reasons like instrumentation or congenital anomalies, leading to severe consequences if not promptly diagnosed and treated.
Coagulopathy: Conditions affecting blood clotting, such as vitamin K deficiency bleeding (VKDB), can lead to significant bleeding, including hematemesis.
Intestinal obstruction: Conditions like pyloric stenosis or intestinal atresias can cause vomiting, which may be bloody if there is associated mucosal damage.

Congenital anomalies of the gastrointestinal tract: Anomalies like esophageal duplication cysts or gastric diverticula can rarely cause hematemesis.
Allergic protcolitis: An allergic reaction to cow's milk protein in breast milk or formula can cause rectal bleeding, but in rare cases, it might present with hematemesis.
Arteriovenous malformations: Rare vascular malformations in the gastrointestinal tract can cause bleeding, including hematemesis.