Necrotizing enterocolitis (NEC) is a serious intestinal disease that primarily strikes premature infants, and it develops when the lining of the intestine becomes damaged, allowing bacteria to invade and destroy bowel tissue. In the United States, roughly one infant dies from NEC every day, with 356 deaths recorded in 2022 alone. The condition doesn’t have a single cause. Instead, it results from a collision of factors: an immature gut, an imbalanced mix of intestinal bacteria, and triggers like formula feeding that together overwhelm a fragile digestive system.
How NEC Develops in a Premature Gut
A full-term baby’s intestinal lining acts as a strong barrier, keeping bacteria on the inside of the gut where they belong and preventing them from crossing into surrounding tissue. In premature infants, this barrier is underdeveloped. The cells lining the intestine are loosely connected, and the immune system that patrols the gut is both immature and prone to overreacting.
When bacteria do cross this weakened barrier, the immune response can spiral out of control. Rather than containing the threat locally, the immature immune system triggers widespread inflammation that damages healthy intestinal tissue. Blood flow to the affected area drops, starving the tissue of oxygen. Without adequate blood supply, patches of the intestinal wall begin to die. This is the “necrosis” in necrotizing enterocolitis. In severe cases, the bowel wall develops holes (perforations), allowing intestinal contents to leak into the abdominal cavity and cause life-threatening infection.
Prematurity and Low Birth Weight
Gestational age is the single strongest predictor of NEC. The disease is one of the leading causes of illness and death in infants born before 28 weeks. The earlier a baby is born, the less developed the intestinal lining, immune defenses, and blood flow regulation to the gut. Premature infants remain at risk for NEC for several weeks after birth, and the age when symptoms appear is inversely related to how early they were born. A baby born at 25 weeks might not develop NEC until several weeks of life, while a term infant with NEC typically shows signs within the first one to three days.
Birth weight compounds this risk. Babies classified as small for gestational age, meaning their birth weight falls below the 10th percentile for how far along the pregnancy was, face more than double the NEC risk compared to appropriately sized babies of the same gestational age. In one study of nearly 3,000 small-for-gestational-age infants, just over 5% developed NEC.
The Role of Gut Bacteria
The bacteria colonizing a newborn’s intestines play a central role in whether NEC develops. Healthy preterm infants gradually build up a diverse mix of gut bacteria, including beneficial species that help maintain the intestinal barrier. Infants who go on to develop NEC show a distinctly different pattern well before symptoms appear.
A large prospective study that analyzed over 3,500 stool samples from 166 very low birth weight infants found that babies who later developed NEC had higher proportions of a class of bacteria called Gammaproteobacteria, which are gram-negative bacteria that can trigger intense immune responses. At the same time, these infants had significantly lower levels of strictly anaerobic bacteria, particularly a group called Negativicutes. This imbalance, an overgrowth of potentially harmful bacteria and a shortage of protective ones, precedes NEC and likely contributes to the inflammatory cascade that damages the gut wall.
Several factors common in the NICU can disrupt this bacterial balance: antibiotic use, delayed feeding, and limited exposure to the mother’s skin and breast milk. Each of these reduces the diversity of beneficial bacteria and creates an opening for more harmful species to dominate.
Formula Feeding as a Risk Factor
What a premature infant is fed matters enormously. Breast milk, whether from the baby’s own mother or a donor, contains antibodies, immune cells, and growth factors that strengthen the intestinal lining and promote the growth of healthy gut bacteria. Formula made from cow’s milk protein lacks these protective components.
In a randomized trial comparing extremely premature infants fed exclusively human milk to those receiving cow’s milk-based formula, the formula group had an NEC rate of 21% compared to just 3% in the human milk group. Surgical NEC, the most severe form requiring an operation, occurred in four infants in the formula group and zero in the human milk group. Separately, infants who received their own mother’s milk had 50% less NEC and late-onset bloodstream infection compared to those fed either donor milk or formula.
This is one of the clearest and most actionable findings in NEC research: breast milk is powerfully protective. The benefit comes from both its immune properties and its effect on shaping the infant’s gut bacteria toward a healthier composition.
Other Contributing Factors
Beyond prematurity, bacterial imbalance, and feeding type, several other conditions increase the risk of NEC. Reduced blood flow to the intestines is a recurring theme. Infants who experienced oxygen deprivation during birth, those with congenital heart defects that alter blood flow, and babies who needed blood transfusions all face elevated risk. Each of these situations can compromise the already fragile blood supply to the premature gut.
Prolonged antibiotic courses in the first weeks of life also raise NEC risk, likely by wiping out beneficial bacteria and allowing harmful species to flourish unchecked. Interestingly, NEC occasionally affects full-term infants, though this is much rarer. When it does, it tends to appear earlier (within the first few days of life) and is more often linked to specific conditions like heart defects or birth complications rather than the prematurity-driven pathway seen in most cases.
Early Warning Signs
NEC often begins with subtle signs that can easily be mistaken for the routine instability of a premature infant. The earliest symptoms are typically feeding intolerance: the baby’s belly becomes slightly distended, milk isn’t moving through the stomach as quickly as expected, and the infant may vomit. Nurses sometimes describe the baby as simply “acting different.”
Over hours to a day, these mild signs can progress. The abdomen becomes visibly swollen and tender. Bowel sounds decrease or disappear. Blood may appear in the stool. Systemically, the baby may develop more frequent pauses in breathing, a slower heart rate, temperature instability, and increasing lethargy. In the most severe cases, the disease progresses rapidly to circulatory collapse. The speed of this progression, sometimes within hours, is part of what makes NEC so dangerous.
Prevention in the NICU
The most effective prevention strategy is prioritizing human milk feeding, starting as early as possible and continuing exclusively when feasible. For mothers who cannot produce enough milk, pasteurized donor milk is the next best option and is increasingly available in NICUs.
Probiotics are gaining traction as a preventive measure. Multiple NICUs now routinely give premature infants supplements containing combinations of beneficial bacteria, typically species of Lactobacillus and Bifidobacterium. The goal is to shift the gut bacterial environment toward the healthier pattern seen in infants who don’t develop NEC. While clinical guidelines note there isn’t yet enough evidence to recommend one specific product over another, the overall body of research supports their use in high-risk preterm infants.
Other protective strategies include careful, gradual introduction of feedings rather than aggressive advancement, skin-to-skin contact with parents to promote healthy bacterial colonization, and judicious use of antibiotics to avoid unnecessarily disrupting the developing gut microbiome.