Constipation: This is a common issue in newborns, especially if they are formula-fed or have started solids. The justification for this being the most likely diagnosis is that constipation is a frequent and relatively benign condition in infants, often due to dietary factors or normal variations in bowel habits.

Breastfeeding issues: If the infant is breastfed, issues such as inadequate latch, poor milk supply, or the mother's diet affecting the baby's stool consistency could be contributing factors.
Formula issues: Similar to breastfeeding, if the infant is formula-fed, the type of formula or an intolerance to a component of the formula could lead to constipation or changes in bowel habits.
Normal variation in bowel habits: Newborns can have a wide range of normal bowel habits, and some may naturally go longer between bowel movements without any underlying pathology.

Hirschsprung's disease: A congenital condition characterized by the absence of ganglion cells in the distal bowel, leading to severe constipation or intestinal obstruction. Early diagnosis is crucial for effective management.
Intestinal obstruction: Conditions such as intussusception, volvulus, or atresias can cause bowel obstruction, leading to failure to pass stool. These conditions are emergencies requiring immediate medical attention.
Hypothyroidism: Congenital hypothyroidism can cause constipation among other symptoms. Early detection and treatment are vital for preventing long-term developmental issues.

Cystic fibrosis: Although more commonly associated with respiratory symptoms, cystic fibrosis can also cause gastrointestinal symptoms, including constipation or intestinal obstruction due to thick, sticky stool.
Spinal cord anomalies: Conditions affecting the spinal cord, such as spina bifida, can impact bowel function and lead to constipation or incontinence.
Neurological disorders: Certain neurological conditions can affect the nerves controlling bowel movements, leading to constipation or other bowel habit abnormalities.