IntroductionOvarian hyperstimulation syndrome (OHSS) is a common but serious complication of in vitro fertilisation. Despite available strategies to reduce OHSS incidence, a small proportion of patients will develop the clinically significant disease with substantial morbidity. Efforts toward better understanding and the prevention of severe disease are required to improve patient outcomes.AimsThe aims are to: (1) formulate clinically relevant OHSS classification for inpatient settings and data collection/reporting; (2) estimate OHSS prevalence requiring hospital admission in Victoria; and (3) determine the extent of OHSS preventability with clinical strategies.Materials and MethodsThis retrospective cohort study included all OHSS admissions in a tertiary referral centre, January 2016–December 2021, which included approximately 40% of all cases of hospitalisation for OHSS in the State of Victoria. Patient characteristics, treatment regimes, fertility treatment outcomes, timing classification, and clinical markers of disease severity were studied. Patients were classified as having mild, moderate, or severe OHSS with a novel inpatient classification system.ResultsOf 199 OHSS cases presenting to the tertiary institution, 107 were classified as moderate/severe, with no significant difference between age, body mass index, length of stimulation and follicle number between mild/moderate and severe groups. There were more cases of early hyperstimulation (137) compared to late (62) presentation, of which 53% were severe. The average length of stay overall was 3.1 days, and 5.2 days for severe presentations. In 15% of severe cases, an agonist trigger was used.ConclusionsThe overall prevalence of OHSS requiring hospital admission appears to be low (approximately 0.6% of all stimulated cycles). Established risk factors may not accurately predict clinically relevant OHSS risk. Further monitoring, clinician and patient education are required to minimise the risk of significant OHSS that results in hospital admissions.