BackgroundTrisomy 18 syndrome (T18) is the second most common autosomal trisomy and has a high risk of fetal loss and stillbirth. Aggressive surgical treatments for the respiratory, cardiac, or digestive systems of patients with T18 were previously futile, while the results of recent studies are controversial. Over the past decade, there have been approximately 300,000 to 400,000 births annually in the Republic of Korea; however, there have been no nationwide studies on T18. This nationwide retrospective cohort study aimed to determine the prevalence of T18 in Korea and its prognosis according to the presence of congenital heart disease and relevant interventions.MethodsThis study utilized NHIS-registered data between 2008 and 2017. A child was defined as having T18 if the ICD-10 revision code Q91.0–3 was reported. Subgroup analysis was performed for children with congenital heart diseases, and survival rates were compared based on the history of cardiac surgical or catheter interventions. The primary outcomes in this study were the survival rate during the first hospitalization period and the 1-year survival rate.ResultsOf the children born between 2008 and 2017, 193 were diagnosed with T18. Of these, 86 died, with a median survival of 127 days. The 1-year survival rate for children with T18 was 63.2%. The survival rate in the first admission of children with T18 who did and did not have congenital heart disease was 58.3% and 94.1%, respectively. Children with heart disease who underwent surgical or catheter intervention had a longer survival time than those who did not.ConclusionsWe suggest these data could be used in ante- and postnatal counseling. Ethical concerns about the prolonged survival of children with T18 remain; however, the potential benefits of interventions for congenital heart disease in this population need further study.Supplementary InformationThe online version contains supplementary material available at 10.1186/s12887-023-04056-4.