Normal variation in bowel movements: Neonates, especially those who are breastfed, may have a variation in bowel movements, and it's not uncommon for them to go several days without passing stool. This is due to the gut maturation process and the type of feeding.

Constipation: This can be due to various factors such as dehydration, low fiber intake in formula-fed babies, or a change in diet.
Hirschsprung's disease: A condition characterized by the absence of ganglion cells in the distal bowel, leading to a functional obstruction.
Anorectal malformations: Abnormalities in the development of the anus and rectum can cause obstruction and prevent the passage of stool.
Meconium ileus: A condition where the meconium in the intestine of a newborn is thicker and stickier than normal, causing a blockage in the small intestine.

Intestinal obstruction: Conditions such as volvulus, intussusception, or atresias can cause complete obstruction of the intestine and are medical emergencies.
Necrotizing enterocolitis: A serious condition where a portion of the bowel dies, which can lead to perforation and peritonitis.
Spinal cord anomalies: Conditions affecting the spinal cord, such as spina bifida, can affect the nerves controlling bowel movements.

Cystic fibrosis: Although more commonly associated with respiratory symptoms, cystic fibrosis can cause meconium ileus in neonates.
Chronic intestinal pseudo-obstruction: A rare condition characterized by the failure of the intestinal muscles to move food, fluid, and air through the intestines.
Neurogenic bowel: Conditions affecting the nerves that control bowel movements, such as spinal muscular atrophy, can cause difficulties in passing stool.