Ovarian stimulation protocols in fresh cycles and in elective freeze-all cycles.

Ovarian stimulation is applied to women undergoing IVF treatment with the aim of selecting more than one follicle. Until a few years ago, fresh embryo transfer was the norm, but not without the risk of ovarian hyperstimulation syndrome (OHSS). In recent years, vitrification has allowed the successful freezing of all oocytes or embryos, thus favoring multiple embryo transfer in subsequent thawing cycles and eliminating the risk of OHSS. Certainly, excess embryos from fresh embryo transfer cycles can be also frozen and thawed in future transfers. In terms of the number of oocytes needed, research in recent years has shown differences between fresh embryo transfer cycles and elective freezing of all embryos, termed elective freeze-all cycles, with the fresh cycle requiring an optimal number to avoid OHSS, while in elective freeze-all cycles there appear to be no specific upper limit. Consequently, the approach to ovarian stimulation may also differ between the two types of cycles. Although GnRH antagonists are used in both cycles, in elective freezing cycles progestins tend to replace antagonists in preventing the endogenous LH surge. However, it is unclear whether prevention of the LH surge is required in such cycles, since luteinization affecting the endometrium is not clinically relevant, as no embryo transfer occurs. This narrative review describes the current experience in ovarian stimulation for IVF, highlighting the differences between fresh and elective freeze-all cycles and the potentially different approach.