Objective To conduct a systematic review of neonatal necrotising enterocolitis (NEC) rates in high-income countries published in peer-reviewed journals.

Methods We searched MEDLINE, Embase and PubMed databases for observational studies published in peer-reviewed journals. We selected studies reporting national, regional or multicentre rates of NEC in 34 Organisation for Economic Co-operation and Development countries. Two investigators independently screened studies against predetermined criteria. For included studies, we extracted country, year of publication in peer-reviewed journal, study time period, study population inclusion and exclusion criteria, case definition, gestation or birth weight-specific NEC and mortality rates.

Results Of the 1888 references identified, 120 full manuscripts were reviewed, 33 studies met inclusion criteria, 14 studies with the most recent data from 12 countries were included in the final analysis. We identified an almost fourfold difference, from 2% to 7%, in the rate of NEC among babies born <32 weeks’ gestation and an almost fivefold difference, from 5% to 22%, among those with a birth weight <1000 g but few studies covered the entire at-risk population. The most commonly applied definition was Bell’s stage ≥2, which was used in seven studies. Other definitions included Bell’s stage 1–3, definitions from the Centers for Disease Control and Prevention, International Classification for Diseases and combinations of clinical and radiological signs as specified by study authors.

Conclusion The reasons for international variation in NEC incidence are an important area for future research. Reliable inferences require clarity in defining population coverage and consistency in the case definition applied.