Normal variation in bowel movement timing: Newborns, especially those who are breastfed, may not have their first bowel movement until 48 hours after birth. This can be a normal variation and not necessarily indicative of an underlying issue.

Hypoganglionosis (Hirschsprung's disease): A condition where nerves are missing from parts of the bowel, leading to severe constipation or intestinal obstruction. It's a common cause of failure to pass meconium in the first 48 hours.
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.
Dehydration: Newborns who are not feeding well or are losing too much fluid may become dehydrated, leading to decreased bowel movements.

Intestinal obstruction: Conditions such as atresias, stenosis, or volvulus can cause complete blockage of the intestine, leading to failure to pass meconium and potentially life-threatening if not promptly diagnosed and treated.
Necrotizing enterocolitis: A serious condition where a portion of the bowel dies. It's more common in premature infants but can occur in term babies, especially if there's a history of perinatal stress or low blood flow.
Cystic fibrosis: Although less common, cystic fibrosis can cause meconium ileus in newborns, which is a medical emergency.

Congenital aganglionic megacolon (Hirschsprung's disease variant): A rare condition where the colon is enlarged due to the absence of ganglion cells, leading to chronic constipation.
Chronic intestinal pseudo-obstruction: A rare condition characterized by impaired intestinal motility, leading to symptoms of intestinal obstruction without any physical blockage.
Abdominal masses or tumors: Rarely, abdominal masses or tumors can cause intestinal obstruction or compression, leading to failure to pass meconium.