A significant proportion of couples at reproductive age rely on assisted reproductive technology to overcome infertility. In vitro fertilisation (IVF) involves typically the use of exogenous gonadotropins to stimulate the ovary to produce oocytes, which are collected surgically.
After fertilization by conventional IVF or intracytoplasmic sperm injection (ICSI), embryos are cultured in the embryology laboratory for a few days before being replaced into the uterus (fresh embryo transfer). Spare embryos can be vitrified and stored in liquid nitrogen to be transferred in a subsequent cycle.
Over the years, concerns have arisen about possible adverse outcomes of transferring embryos back to the uterus immediately after controlled ovarian stimulation (COS) as regards to obstetrical and perinatal outcomes. It has been suggested that high levels of hormone during COS could create a relatively hostile environment for embryo implantation whilst increasing the risk of ovarian hyperstimulation syndrome (OHSS).
With the remarkedly improvement of vitrification as an alternative to the slow-freezing technique for human embryos, a new strategy the so-called “freeze-all” (FA) or “elective frozen embryo transfer” (eFET) was introduced. This approach involves COS, followed by the elective cryopreservation of the entire cohort of viable embryos to be transferred to the uterus in subsequent cycles in a possibly more physiological environment, thus avoiding the supra-physiologic hormonal levels observed during COS.
The initial reports suggested that this policy could lead to improved pregnancy rates and reduced perinatal complications, which resulted in a steady increase and widespread use of FA globally. However, as data accumulated, it became clear that the use of FA to unselected couples undergoing ART offered no additional benefits over the conventional approach.
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Nonetheless, current evidence based on randomized controlled trials and observational studies indicates that FA might be justified in selected clinical scenarios, such as those involving the risk of OHSS. By contrast, there is a lack of evidence to support the FA policy for other indications, such as implantation failure or high progesterone levels on the trigger day. This review summarizes the clinical effectiveness of FA with the main focus on the health of offspring.