What Are the Risk Factors for Necrotizing Enterocolitis?

Prematurity and formula feeding are the two most consistent independent risk factors for necrotizing enterocolitis (NEC), a sudden and devastating intestinal disease that primarily strikes the smallest, most vulnerable newborns in neonatal intensive care units.¹ But the full picture involves a constellation of additional factors, from the medications a baby receives to the health of its mother’s placenta, and researchers are still untangling how these risks combine to trigger the disease in one infant but not another.

Why Prematurity Is the Dominant Risk Factor

NEC affects roughly 7% of preterm infants born weighing less than 1,500 grams, making it the most common gastrointestinal surgical emergency in this population. The more premature the infant, the higher the risk. This is not simply about being small. The premature gut is structurally and immunologically immature in ways that set the stage for the disease. The intestinal lining has weaker barrier integrity, reduced blood flow regulation, and an immune system that overreacts to normal gut bacteria in ways a full-term infant’s gut does not.

At the molecular level, the premature intestine has heightened activity of a receptor called Toll-like receptor 4 (TLR4), which detects bacterial products. In a mature gut, TLR4 helps manage the relationship between the body and its resident bacteria. In a premature gut, TLR4 overactivation triggers an uncontrolled immune response that damages the mucosal lining, reduces blood flow, and can ultimately lead to tissue death.² Researchers have shown that this TLR4 signaling is essentially required for the development of the barrier failure that precedes NEC.³

Birth weight matters independently of gestational age. In one study of very preterm infants, each kilogram increase in birth weight dramatically reduced the odds of developing NEC, even after accounting for other factors.⁴ This makes intuitive sense: a larger preterm infant generally has a more developed gut and better overall physiological resilience.

Formula Feeding Versus Breast Milk

The protective effect of breast milk against NEC is one of the strongest and most consistent findings in neonatal research. A landmark multicenter study found that confirmed NEC was six to ten times more common in exclusively formula-fed babies than in those receiving only breast milk. Among babies born after 30 weeks of gestation, confirmed NEC was 20 times more common in those fed formula exclusively compared to those whose diet included breast milk.⁵ Even mixed feeding (some formula plus some breast milk) offered partial protection compared to formula alone.

When a mother’s own milk is unavailable, donor human milk still provides meaningful protection. A systematic review found that infants receiving donor milk were about three times less likely to develop NEC than those receiving formula, and four times less likely to have confirmed NEC.⁶ This is why most neonatal units now prioritize human milk, whether from the mother or a milk bank, for their highest-risk infants.

Part of what makes breast milk so protective appears to be specific sugar molecules called human milk oligosaccharides (HMOs). These are indigestible to the infant but serve critical biological functions. One particular HMO, 2′-fucosyllactose (2′-FL), has been shown in animal models to protect against NEC by maintaining blood flow to the intestinal lining and reducing the inflammatory signaling that damages it.⁷ More recent preclinical work has found that multiple HMOs, including 2′-FL and 6′-sialyllactose, can protect the enteric nervous system within the gut wall, preventing the loss of specialized cells called enteric glia and reversing the gut motility problems seen in NEC.⁸ These findings help explain why breast milk’s protection goes beyond simply being a gentler food: it contains active biological compounds that directly counter the pathways driving NEC.

The Role of Gut Bacteria

A healthy newborn gut is rapidly colonized by bacteria after birth, and this colonization normally proceeds in an orderly pattern. In preterm infants who develop NEC, this process goes wrong. Instead of a diverse, balanced microbial community, the gut becomes dominated by potentially harmful species while beneficial bacteria are scarce. This microbial imbalance, or dysbiosis, appears to be central to how NEC develops: abnormal bacterial populations interact with the immature gut lining, triggering the loss of barrier integrity, inflammation, and tissue death that define the disease.⁹

Several of the other risk factors for NEC likely exert their influence through their effects on the microbiome. Formula feeding shapes the gut flora differently than breast milk does. Antibiotics wipe out bacteria indiscriminately. Antacids change stomach acidity and let different organisms survive the trip to the intestines. Understanding the microbiome as a common pathway helps explain why so many seemingly unrelated exposures can all increase NEC risk.

Medications That Raise the Risk

Two classes of medications stand out as contributors to NEC risk: antibiotics and gastric acid suppressants. Both are commonly used in the NICU, often for good reason, which makes these associations particularly important for clinicians to weigh carefully.

Prolonged early antibiotic therapy is a well-documented risk factor. In extremely low birth weight infants, each additional day of empirical antibiotics (antibiotics given before culture results confirm an infection) was associated with increased odds of NEC, death, or both.¹⁰ A multicenter study reinforced this by showing that infants whose early empirical antibiotics were limited to 72 hours or less had significantly lower NEC rates than infants who received prolonged courses.¹¹ The likely mechanism is disruption of normal bacterial colonization at a critical window, leaving the gut vulnerable to the kind of dysbiosis that triggers NEC.

Gastric acid suppressants, particularly H2 blockers (a type of antacid), have also been linked to higher NEC rates. A meta-analysis found that exposure to these drugs was associated with roughly 80% higher odds of NEC in preterm, very low birth weight infants.¹² The reasoning is straightforward: stomach acid is one of the body’s first defenses against swallowed pathogens. By suppressing acid, these medications allow more bacteria to survive into the intestines, potentially fueling the dysbiosis that precedes NEC.¹³ This has led many neonatal units to become far more conservative about prescribing acid suppressants to preterm infants.

Red Blood Cell Transfusions

The relationship between blood transfusions and NEC has been debated for years. Preterm infants frequently need red blood cell transfusions because of anemia, and clinicians have long noticed that some cases of NEC seem to cluster shortly after a transfusion. A large multicenter observational study found a striking pattern: among post-transfusion NEC cases, about a third occurred within two days of the transfusion, and roughly half within three days, with risk dropping off as more time passed.¹⁴

However, the picture is not straightforward. A secondary analysis of the TOP trial, a randomized clinical trial comparing different transfusion thresholds, found that among extremely low birth weight infants within the hemoglobin ranges studied, red blood cell transfusions were not temporally associated with a higher risk of NEC during the 72 hours after transfusion.¹⁵ This apparent contradiction may reflect the difference between observational studies (which can be confounded by the fact that sicker infants both need more transfusions and are more prone to NEC) and controlled trials. The question remains open, and many units still exercise caution around feeding practices during and shortly after transfusions.

Maternal and Placental Factors

Risk for NEC does not begin at birth. Conditions during pregnancy can set the stage for the disease. Chorioamnionitis, an infection or inflammation of the fetal membranes, has been identified as a predictor of NEC in multiple studies. One study found that histological chorioamnionitis, along with abnormal umbilical artery flow and chronic placental changes, were significant predictors of NEC when examined through multivariate analysis.¹⁶ Mothers of NEC infants in this study also had higher rates of preeclampsia.

Interestingly, while chorioamnionitis appeared as a risk factor in some analyses, other studies found that it dropped out as a significant independent predictor when birth weight and other variables were accounted for, with maternal blood markers like the neutrophil-to-lymphocyte ratio and multiparity (having had multiple previous pregnancies) emerging instead.¹⁷ The inconsistency across studies reflects one of the challenges of NEC research: many risk factors are tightly intertwined with prematurity itself, making it difficult to tease apart which factors are truly independent contributors and which are proxies for how premature or how sick the infant is.

Congenital Heart Disease

Infants born with congenital heart disease face a heightened risk of NEC even when they are born at full term. The mechanism appears to be different from the classic prematurity-driven pathway. In these babies, abnormal blood circulation reduces blood flow to the intestines, creating the kind of oxygen deprivation that can damage the gut lining and set the stage for NEC. Babies with certain types of heart defects that cause low cardiac output or steal blood away from the gut are at particular risk.¹⁸ This has implications for management: the NEC that develops in cardiac patients may need to be approached differently from classical NEC in preterm infants, because the underlying cause is fundamentally different.

How NEC Differs in Full-Term Infants

Although NEC is overwhelmingly a disease of prematurity, it can occur in full-term babies, and when it does, it looks different. Full-term infants with NEC tend to develop symptoms earlier in life, perhaps because they start feeding sooner. There is a correlation between the age at which feeding begins and the onset of NEC in these babies.¹⁹ The disease also tends to affect different parts of the intestine and has different underlying causes: most full-term infants who develop NEC have a predisposing condition such as heart disease, birth asphyxia, or sepsis.²⁰

The recognition that term NEC has a distinct cause and presentation matters because it changes how clinicians think about prevention. The strategies that protect preterm infants, such as prioritizing breast milk and limiting antibiotic exposure, may still help, but addressing the underlying predisposing condition (correcting a cardiac defect, managing infection) is often the more critical intervention in this group.

Genetic Susceptibility

Not every premature, formula-fed infant develops NEC, which has long suggested that individual biology plays a role. Researchers have begun to identify genetic variants that influence susceptibility. Two genes have drawn particular attention: ATG16L1, involved in a cellular cleanup process called autophagy, and SIGIRR, which helps regulate immune signaling in the gut. In animal models, loss of SIGIRR function made mice much more susceptible to NEC, while a specific variant of ATG16L1 was associated with decreased risk.²¹

Genetic research in NEC is still in relatively early stages. No genetic test can yet predict which infant will develop the disease, and the variants identified so far likely explain only a fraction of the variation in susceptibility. But this work confirms what clinicians have long suspected: some babies are biologically primed to develop NEC while others with identical environmental exposures are not.

Probiotics as Prevention

Given the central role of microbial dysbiosis in NEC, the idea of giving preterm infants beneficial bacteria as prevention has attracted enormous research interest. The evidence is now substantial. A Cochrane systematic review pooling data from 57 trials and nearly 11,000 infants found that probiotics may reduce the risk of NEC by roughly half, with about 33 infants needing to be treated to prevent one case.²² An earlier version of this analysis was even more emphatic, stating that the evidence “strongly supports a change in practice.”²³ Additional meta-analyses have reached similar conclusions, with one estimating a roughly 64% reduction in relative risk.²⁴

Despite this evidence, probiotic use in NICUs remains inconsistent around the world. Some units have adopted routine probiotic supplementation for at-risk infants; others have not, citing uncertainty about the optimal strain, dose, and duration, and occasional safety concerns about administering live bacteria to immunologically fragile infants. The more recent Cochrane review rated the certainty of the NEC evidence as “low,” reflecting heterogeneity across trials rather than a lack of effect. Still, the overall direction of the evidence is consistent enough that professional guidelines in several countries now support probiotic use in very preterm infants.

Early Detection and the Challenge of Prediction

One of the cruelest features of NEC is that it strikes suddenly. An infant can appear stable one hour and show signs of catastrophic intestinal damage the next. This has driven research into methods that might predict the disease before symptoms emerge, including specialized ultrasound techniques to assess blood flow to the gut, near-infrared spectroscopy to measure oxygen levels in intestinal tissue, blood-based biomarkers of inflammation, and analysis of stool bacteria to detect dangerous shifts in the microbiome before they cause disease. Researchers have reviewed these emerging approaches, but none has yet achieved the reliability needed for routine clinical use. The difficulty is that NEC’s risk factors are common while the disease itself is relatively rare even in high-risk populations, making prediction challenging.

Long-Term Consequences Beyond the Gut

NEC is not just an acute intestinal crisis. Surviving infants carry increased risk for long-term complications, particularly in brain development. NEC strikes during a period of rapid neurological growth, and the combination of severe illness, inflammation, surgical stress, nutritional deprivation, and prolonged hospitalization can leave lasting marks on the developing brain.²⁵

The consequences are worst for infants who develop short bowel syndrome after surgical removal of damaged intestine. In one study, moderate-to-severe neurodevelopmental impairment was present in 77% of extremely preterm children with short bowel syndrome, compared to 44% of preterm peers who had never had NEC. These children also showed persistently impaired growth, with length and head circumference remaining well below average at follow-up.²⁶ Even among NEC survivors who retain enough intestine to avoid short bowel syndrome, cognitive, language, and motor development can be affected, though to a lesser degree. These long-term outcomes underscore why preventing NEC matters so much: the stakes extend far beyond the neonatal period.

Why Risk Factors Are So Hard to Untangle

If NEC had a single cause, it would have been solved decades ago. The reality is that the disease likely requires multiple hits occurring in sequence or in combination: an immature gut, abnormal bacterial colonization, some kind of inflammatory or ischemic trigger, and possibly a genetic predisposition. Most of the individual risk factors discussed above are common among preterm infants, yet only a minority develop NEC. A baby might be premature, receive antibiotics, and be formula-fed, yet never develop the disease. Another baby with fewer apparent risk factors might be hit suddenly and severely.

This multi-hit model is why research has increasingly focused on how risk factors interact rather than treating each one in isolation. The microbiome sits at the center of many of these interactions, serving as both a target of external exposures (antibiotics, feeding type, acid suppressants) and an upstream driver of the immune and inflammatory processes that cause tissue damage. Researchers are building animal models, primarily in neonatal mice, rats, and piglets, that try to replicate these overlapping insults, and translating those findings into clinical strategies remains the field’s central challenge.