While term infants who develop NEC often have underlying illnesses predisposing to NEC and are often diagnosed in the first week of life, most premature infants who develop NEC are between 14 and 20 days of age or 30–32 weeks' postmenstrual age. The early clinical presentation may include feeding intolerance, increased gastric residuals, and blood in stools. Specific abdominal signs include abdominal distension, tenderness, abdominal skin discoloration, emesis, and bilious drainage from nasogastric tube. Nonspecific signs include symptoms and signs of neonatal sepsis including increased apnea/bradycardia episodes, temperature instability, hypotension, and circulatory shock.
The clinical course of NEC is variable. While about 30% may have a mild presentation that responds to medical treatment, about 7% may have a fulminant course with rapid progression to NEC totalis, septic shock, severe metabolic acidosis, and death. The modified Bell's staging criteria is often used to classify NEC according to clinical and radiographic presentations.