Functional Constipation: This is the most common cause of constipation in infants, often due to changes in diet (e.g., introduction of solid foods), dehydration, or immaturity of the gut. It's characterized by infrequent bowel movements, hard stools, and sometimes pain during defecation.

Dietary Factors: Introduction of cow's milk or certain solid foods can lead to constipation in some infants.
Hirschsprung's Disease (partial): While less common, a partial form of Hirschsprung's disease can present with constipation. It's a congenital condition affecting the nerve cells in the colon, leading to severe constipation or intestinal obstruction.
Hypothyroidism: Congenital hypothyroidism can cause constipation among other symptoms like failure to thrive, dry skin, and cold intolerance.
Anatomical Abnormalities: Conditions like anal stenosis or anteriorly displaced anus can cause constipation.

Intestinal Obstruction: Conditions like intussusception, volvulus, or atresias can cause severe constipation and are medical emergencies.
Spinal Cord Abnormalities: Spina bifida or tethered cord can affect bowel function, leading to constipation.
Botulism: Although rare, infant botulism can cause constipation as an early symptom, followed by weakness and paralysis.
Lead Poisoning: Can cause constipation, abdominal pain, and developmental delays.

Cystic Fibrosis: Can cause constipation due to the thick, sticky nature of the stool, although it's more commonly associated with respiratory symptoms.
Neurological Disorders: Conditions like spinal muscular atrophy can affect bowel movements, leading to constipation.
Chagas Disease: Caused by Trypanosoma cruzi, it can lead to megacolon and constipation, although it's rare in infants and more commonly associated with acute symptoms.
Gastrointestinal Neoplasms: Extremely rare in infants, but tumors can cause intestinal obstruction and constipation.