Ovarian hyperstimulation syndrome (OHSS) is a serious complication of controlled ovarian hyperstimulation (COH). The pathophysiology of the syndrome is poorly understood and there is no reliable test to predict patients who will subsequently develop severe OHSS. The syndrome almost always presents either after hCG administration in susceptible patients or during early pregnancy, especially multifetal gestation. Despite many years of clinical experience, there are no precise methods to completely prevent severe OHSS, except by withholding the ovulation-inducing trigger of hCG. We offer a triage aimed at eliminating the occurrence of severe OHSS based on several clinical observations, including the role of GnRH-antagonist in COH protocols and the ability to transfer embryos in the blastocyst stage.