Controlled ovarian stimulation (COS) is crucial for optimizing in vitro fertilization (IVF) / intracytoplasmic sperm injection (ICSI) success. Multiple factors influence the ovarian response to COS, making predictions about oocyte yields not so straightforward. As a result, ovarian response may be poor or suboptimal, or even excessive, all of which has negative consequences to the affected patient. The study of gene polymorphisms regulating the female reproductive function may help to clarify the mechanisms responsible for gonadal function and fertility, and also the inter-individual variability in ovarian response to COS.