The practice of freezing embryos for deferred frozen embryo transfer (FET) has given rise to the so-called freeze-all strategy. The freeze-all policy was first introduced by clinicians in an attempt to prevent ovarian hyperstimulation syndrome (OHSS) ( ) and has increasingly been integrated into in vitro fertilization (IVF) for several indications, including among others progesterone rise at the end of the follicular phase, pre-implantation genetic testing for aneuploidy and embryo pooling in patients with low ovarian reserve ( ). Freeze-all may be planned and then is referred to as “elective” freeze-all, while in some cases unplanned (or “non-elective”) freezing is performed. Given the popularity of the freeze-all protocol and the dramatic increase of FET cycles over the last decade, several RCTs and meta-analyses aimed to evaluate the efficacy of FET versus fresh ET in different IVF populations.
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