<2mo
Swallowed maternal blood (from chapped nipples)
Stress ulcer
Vascular malformation
Hemorrhagic disease of newborn (vitamin K deficiency)
Coagulopathy/bleeding diathesis
Swallowed maternal blood (from chapped nipples)
Stress ulcer
Vascular malformation
Hemorrhagic disease of newborn (vitamin K deficiency)
Coagulopathy/bleeding diathesis
2mo–2yr
Gastroenteritis
Toxic ingestion
Mallory-Weiss tear
Vascular malformation
Esophagitis
Stress ulcer
Coagulopathy/bleeding diathesis
GI duplication
Ingested foreign body
Gastroenteritis
Toxic ingestion
Mallory-Weiss tear
Vascular malformation
Esophagitis
Stress ulcer
Coagulopathy/bleeding diathesis
GI duplication
Ingested foreign body
>2yr
Gastroenteritis
Mallory-Weiss tear
Peptic ulcer disease
Toxic ingestion
Vascular malformation
Gastritis
Varices
Hemobilia
Ingested foreign body
Gastroenteritis
Mallory-Weiss tear
Peptic ulcer disease
Toxic ingestion
Vascular malformation
Gastritis
Varices
Hemobilia
Ingested foreign body
<2mo
Swallowed maternal blood
Milk allergy
Infectious colitis
Intussusception
Volvulus
Meckel diverticulum
Necrotizing enterocolitis
Vascular malformation
Hemorrhagic disease of newborn
Hirschsprung disease
Congenital duplications
Swallowed maternal blood
Milk allergy
Infectious colitis
Intussusception
Volvulus
Meckel diverticulum
Necrotizing enterocolitis
Vascular malformation
Hemorrhagic disease of newborn
Hirschsprung disease
Congenital duplications
2mo–2yr
Anal fissure
Anal fissure
Gastroenteritis
Milk allergy
Intussusception
Volvulus
Meckel diverticulum
Hemolytic uremic syndrome
Henoch-Schönlein purpura
Polyps; benign, familial
Inflammatory bowel disease
GI duplication
Dieulafoy lesion
>2yr
Anal fissure
Anal fissure
Gastroenteritis
Hemorrhoids
Polyps
Colitis (infectious, ischemic)
Meckel diverticulum
Intussusception
Hemolytic uremic syndrome
Henoch-Schönlein purpura
Inflammatory bowel disease
Angiodysplasia
Celiac disease
Dieulafoy lesion
Rectal ulcer syndrome
Peptic ulcer disease
Anal fissure
Anal fissure
Gastroenteritis
Milk allergy
Intussusception
Volvulus
Meckel diverticulum
Hemolytic uremic syndrome
Henoch-Schönlein purpura
Polyps; benign, familial
Inflammatory bowel disease
GI duplication
Dieulafoy lesion
Anal fissure
Anal fissure
Gastroenteritis
Hemorrhoids
Polyps
Colitis (infectious, ischemic)
Meckel diverticulum
Intussusception
Hemolytic uremic syndrome
Henoch-Schönlein purpura
Inflammatory bowel disease
Angiodysplasia
Celiac disease
Dieulafoy lesion
Rectal ulcer syndrome
Peptic ulcer disease
Gastrointestinal bleeding 
<2mo
Lower gastrointestinal bleeding
Swallowed maternal blood 
Burns
Stress ulcer
Dark raised skin lesions
Vascular malformation