Background: Low Birth Weight (LBW) remains a major determinant of neonatal survival and long-term health. This systematic review updates the evidence on risk factors for LBW, focusing on studies published between 2020 and 2026, with particular attention to findings from Iran and comparable international data.

Methods: A systematic search of observational studies was conducted in PubMed, Scopus, Web of Science, Google Scholar, Embase, Cochrane Library, Magiran, Scientific Information Database (SID), and IranDoc. Studies published from January 2020 to June 2026 were screened. Quality was assessed using STrengthening the Reporting of OBservational studies in Epidemiology (STROBE) and Newcastle-Ottawa checklists.

Results: From 1,847 screened articles, 35 met inclusion criteria (28 Iranian, 7 international). Pooled prevalence of LBW in Iran is approximately 8.0% (95% CI: 7–9%). Major risk factors with significant associations include maternal smoking, hypertension, underweight BMI, depression, anemia, periodontitis, inadequate prenatal care, food insecurity, and environmental exposures. Long-term consequences include increased risks of metabolic syndrome, glucose disorders, asthma, and cerebral palsy.

Conclusions: LBW is influenced by multiple modifiable factors. Effective prevention requires integrated interventions addressing nutrition, prenatal care, mental health, smoking cessation, and chronic disease management.