IntroductionOvarian Hyperstimulation Syndrome (OHSS) is a life-threatening iatrogenic complication of In vitro fertilisation (IVF). This study aimed to quantify rates of Ovarian Hyperstimulation Syndrome (OHSS) requiring intensive care unit (ICU) admission and assess whether trends have changed between 1996-2020 commensurate with the introduction of safer IVF practices.MethodsData regarding Intensive Care Unit (ICU) admission across England, Wales and Northern Ireland was gathered retrospectively from the Intensive Care National Audit and Research Centre (ICNARC) database. 38,957 female patients aged between 18-55 years were admitted to ICU for OHSS or related conditions between 1996-2020. The primary outcome was the rate of OHSS requiring ICU admission expressed as a proportion of the number of fresh IVF cycles conducted in that year according to Human Fertility and Embryology Authority (HFEA) records. Baseline characteristics (for example, age, ethnicity, BMI), biochemical parameters (such as renal function, serum electrolytes), length of ICU stay and duration and need for organ support, were also compared between ICU patients with ‘confirmed OHSS’ and those ‘without OHSS’.ResultsThere were 238 cases of ‘confirmed OHSS’ requiring ICU admission recorded between 1996-2020. Rates of OHSS requiring ICU admission declined over the study period (P=0.006); the annual rate of severe OHSS requiring intensive care admission halved when comparing those occurring between 1996-2007 and 2008-2020 (OR=0.37, 95% CI 0.37-0.45; P<0.0001). Patients spent a mean of 3.5 days in the ICU, with 86.3% of patients with ‘confirmed OHSS’ requiring at least 2 days of higher level (i.e., level 2 or 3) care. Patients with ‘confirmed OHSS’ required a shorter duration of renal, advanced cardiovascular, and advanced respiratory support than patients ‘without OHSS’ (P<0.0001 for all comparisons). There was no significant difference in BMI or ethnicity between those with ‘confirmed OHSS’ and those ‘without OHSS’, however women with ‘confirmed OHSS’ were younger (34 versus 41 years old, p<0.0001).DiscussionAlthough absolute rates of OHSS requiring ICU admission recorded in this study are likely to represent a significant underestimate of all clinically significant OHSS, rates of OHSS requiring ICU admission have decreased since 1996 in concordance with the introduction of modern IVF practices.