The development of OHSS is always accompanied by elevated E2 levels during the process of ART, and E2 has been implicated as a potential etiologic factor, one of the possible reasons is that elevated E2 increases capillary permeability so VEGF and other chemical mediators or precursors which augment fluid extravasation increase and thus OHSS developed. This evidence concerns the gene VEGFA and ovarian hyperstimulation syndrome.