Background <p>Nonketotic hyperglycinemia is a rare metabolic disorder caused by glycine accumulation due to defects in the glycine cleavage system. While severe metabolic disorders can theoretically affect fetal growth, nonketotic hyperglycinemia is not recognized as a common or typical cause of symmetric intrauterine growth retardation.</p> Case presentation <p>This case report describes a Iranian 36-week male neonate with symmetrical intrauterine growth restriction, born to consanguineous parents with a history of preeclampsia and prior fetal demise. Despite initial stabilization, the infant developed metabolic acidosis, recurrent apnea, and seizures. Laboratory findings revealed elevated glycine levels (2560&#xa0;µmol/L), confirming nonketotic hyperglycinemia. Despite seizure management and antibiotic therapy, the infant deteriorated and died on day 11.</p> Narrative review and conclusion <p>Nonketotic hyperglycinemia typically presents with lethargy, hypotonia, seizures, and apnea, often fatal in the neonatal period. Glycine’s dual role as an excitatory and inhibitory neurotransmitter explains the seizures and hypotonia seen in nonketotic hyperglycinemia. While acute symptoms may resolve, survivors face severe neurological impairment. Early recognition of nonketotic hyperglycinemia is critical for genetic counseling and palliative care. This case highlights the diagnostic challenge of nonketotic hyperglycinemia, particularly when presenting with intrauterine growth restriction, which can mimic other neonatal conditions such as sepsis or hypoxic-ischemic encephalopathy. Consanguinity and prior fetal loss should raise suspicion for metabolic disorders, and according to this case presentation, symmetric intrauterine growth restriction in high-risk pregnancies should prompt metabolic screening. Despite advances, nonketotic hyperglycinemia remains a devastating condition with high mortality and neurodevelopmental morbidity, emphasizing the importance of further research and early intervention strategies.</p>