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Objective Fetomaternal transfusion (FMT) is associated with increased perinatal mortality and morbidity, but data on postnatal outcomes are scarce. Our aim was to determine the incidence of adverse short-termand long-term sequelae of severe FMT.Design Retrospective cohort study.Setting Dutch tertiary neonatal intensive care unit.Patients Liveborn neonates with FMT admitted in 2017–2022.Main outcome measures Severe FMT was defined as ≥30 mL of fetal red blood cells in the maternal circulation diagnosed with positive Kleihauer-Betke/flow cytometry test. Adverse outcomes were compared between severe and mild FMT (10–30 mL blood loss) to highlight the impact of FMT severity. Primary outcome was an adverse composite outcome consisting of neonatal mortality or severe neurological morbidity (ie, severe cerebral injury and/or neurodevelopmental impairment (NDI) at 2 years). Secondary outcome was perinatal asphyxia.Results 109 neonates with FMT were included, 16 with severe FMT and 93 with mild FMT. Neonatal mortality occurred in 19% (3/16) of neonates with severe FMT and in 4% (4/93) with mild FMT (p=0.063). Perinatal asphyxia was diagnosed in 25% (4/16) of neonates with severe FMT compared with 6% (6/93) with mild FMT (p=0.038). Long-term outcome was assessed in 60 neonates. NDI occurred in 22% (2/9) of children with severe FMT compared with 16% (8/51) with mild FMT (p=0.637). Adverse outcome occurred in 43% (95% CI 38 to 50%) of neonates with severe FMT compared with 18% (95% CI 17% to 24%) with mild FMT (p=0.074).Conclusion Neonatal mortality or long-term neurological morbidity occurred in 38%–50% of children with fetal blood loss and anaemia due to severe FMT.