Gestational Age and NEC Incidence
Necrotizing enterocolitis (NEC) is most common in extremely premature infants born at less than 28-29 weeks' gestation, with the highest incidence occurring in those born at 23-24 weeks' gestation (10-20% incidence). 1, 2
Gestational Age-Specific Risk Stratification
The incidence of NEC follows a clear inverse relationship with gestational age at birth:
- 23-24 weeks' gestation: 10-20% incidence 1
- 25-27 weeks' gestation: 5-10% incidence 1
- ≥28 weeks' gestation: <5% incidence 1
More than 85% of all NEC cases occur in very low birth weight infants or very premature infants (those born at less than 32 weeks' gestation). 3, 4
Birth Weight Correlation
The gestational age pattern is mirrored by birth weight, reinforcing that the most immature infants face the highest risk:
- **<750 g birth weight**: >10% incidence 1
- 750-1000 g birth weight: 5-10% incidence 1
- >1000 g birth weight: <5% incidence 1
Timing of Disease Onset
The age of onset of NEC is inversely related to the postmenstrual age at birth - meaning the more premature the infant, the later NEC typically develops after birth. 3 The syndrome typically strikes premature infants during the first 2 weeks of life, usually occurring in the second week after initiation of enteral feeds. 5, 6
Clinical Context
This gestational age pattern exists because extremely premature infants have multiple vulnerabilities that predispose them to NEC: their immune system is not yet adapted to ex-utero life, they have immature gastrointestinal tract development (shorter crypts and villi, reduced mucus production, increased intestinal permeability), and they require prolonged intensive care with central vascular catheters and delayed enteral feeding. 1, 4
Important caveat: While NEC predominantly affects extremely premature infants, it can occur in term and late preterm infants, though this is less common and typically associated with congenital heart disease or other anomalies. 3