Ovarian hyperstimulation syndrome is a disorder associated with exogenous gonadotropin and is an iatrogenic complication of ovulation induction therapy in ART during the luteal phase and or early stages of pregnancy. Severe OHSS is considered a life-threatening condition and occurs in 0.2-1.2% of stimulated cycles. In literature, sporadic cases of spontaneous OHSS have been reported amongst naturallyconceived pregnancies and non-pregnant women with primary hypothyroidism, polycystic ovarian disease, molar pregnancies, pituitary adenomas and in women with Follicle Stimulating Hormone (FSH) receptor mutations. The characteristic pathophysiology of OHSS is due to release of number of vasoactive amines including Vascular Endothelial Growth Factor (VEGF) and other pro-inflammatory factors resulting in increased capillary permeability, leakage of fluid from the vasculature, third space fluid accumulation and intravascular dehydration. Fatal cases are associated with cerebral infarction, pulmonary thromboembolism and massive pulmonary edema. Even though the condition is extremely rare, spontaneous OHSS can be lethal if not timely diagnosed or tend to be mismanaged due to misdiagnosis. We report a case of spontaneous ovarian hyperstimulation syndrome in a 39-year-old woman, pregnant after receiving medicated thawed embryo transfer cycle. Although the onset of OHSS was not until 12 weeks of gestation, the corresponding diagnosis was considered in presence of twin pregnancy characterized by elevated β-hCG levels, bilateral enlarged ovaries, ascites with other physical and biochemical findings suggestive of intravascular depletion.