Reliable data on causes of death (COD) in preterm infants is needed to assess perinatal care and current clinical guidelines. In this retrospective observational analysis of all deceased preterm infants born <37 weeks’ gestational age (n=278; 2012-2018) at a Swedish tertiary neonatal intensive care unit, we compared preliminary COD from Medical Death Certificates with autopsy defined COD (2002-2018), and assessed changes in COD between two periods (Period 1: 2002-2009 vs Period 2: 2011-2018). Autopsy was performed in 73% of all cases and was more than twice as high compared to national rates of infant autopsies in Sweden (33%). Autopsy defined COD differed from preliminary COD in 34.9% of the cases. Necrotizing enterocolitis (NEC) as COD increased between Period 1 and 2 (5% vs 26%), while intraventricular hemorrhage decreased (13% vs 6%). The autopsy rate did not change between the two study periods (75% vs 71%). We conclude that autopsy changed the preliminary COD in a third of cases, while the incidence of NEC as COD increased markedly during the study period. Since there is a high risk to determine COD incorrectly based solely on clinical findings in preterm infants, autopsy remains a valuable method to obtain reliable COD.