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In-vitro fertilization and embryo freeze-all (IVF-freeze-all) involves creating embryos in a standard IVF process but freezing of all embryos so that a fresh embryo transfer into the uterus, which is normally performed within five days of the egg retrieval, is avoided.  Multiple indications for embryo freeze-all exist, including a suboptimal (non-receptive) uterine lining, the finding of a uterine lesion which may hinder implantation of embryos (i.e. uterine polyp, fibroid, or scarring), concerns for ovarian hyperstimulation syndrome (OHSS) which may be exacerbated by the transfer of fresh embryos, the employment of preimplantation genetic testing (PGT) for embryo chromosome screening, and diminished uterine receptivity/increased risk for embryo implantation failure which may occur under certain IVF clinical conditions more common with fresh embryo transfers. 

With exceptionally reliable embryo freezing techniques used in modern IVF laboratories, embryo freeze-all has revolutionized IVF care and success rates, allowing great flexibility and optimization of IVF treatment, which no longer relies on fresh embryo transfers to achieve pregnancy under suboptimal conditions.  In fact, under certain clinical conditions, assisted reproductive techniques (ART) using frozen embryos yield superior pregnancy and live birth rates than those achieved with fresh embryos. 

When an indication exists for embryo freeze-all, the IVF treatment is divided into two separate but equally important treatment cycles:  the IVF-freeze-all cycle, and the frozen embryo transfer (FET) cycle.  The freezing of all embryos resulting from the IVF cycle allows time for the correction of uterine factors associated with implantation failure, before an embryo transfer is finally done.  For instance, if the uterine lining during the ovarian stimulation phase of the IVF treatment is inadequately thick for ideal receptivity of an embryo, this can be corrected during an FET cycle before a frozen-thawed embryo is implanted, thereby optimizing implantation rates.  In the case of a diagnosed uterine polyp or fibroid or intrauterine scarring (intrauterine adhesions leading to Asherman’s syndrome), known to hinder implantation of an embryo, the IVF cycle can still proceed without delay and an embryo freeze-all is performed; after which surgical correction of the lesion is performed to normalize the uterine cavity, followed by an FET cycle and the transfer of a frozen-thawed embryo into a more receptive uterine environment.  

Embryo freeze-all is particularly important for patients of advanced maternal age and/or those with diminished ovarian reserve where a delay in initiating IVF (and creating embryos), due to the need for surgery, for instance, may be detrimental to IVF success.  Since the time span between the IVF-embryo freeze-all cycle and the subsequent FET cycle does not negatively impact ART success rates, patients can be rest assured that clinically indicated embryo freezing followed by a delay in initiating an FET cycle will improve rather than decrease their ART success rates. 

A common indication for embryo freeze-all following IVF is elevated hormone levels often reached during the IVF stimulation treatment.  In particular, estrogen and/or progesterone levels, which exceed certain threshold levels, are associated with an increased risk for embryo implantation failure when fresh embryos are transferred into the uterus within five days of the egg retrieval procedure.  In contrast, since ovarian stimulation is uncommon during the majority of FET cycles, estrogen and progesterone levels typically remain low during FETs, simulating a natural cycle, and therefore allowing frozen-thawed embryos to be transferred into a more receptive uterine environment, resulting in superior outcomes.  In addition to improved implantation and live birth rates resulting from frozen embryo transfers, FET cycles are associated with lower miscarriage rates, a lower risk of preterm labor, and a possible lower risk for ectopic pregnancies as compared with fresh embryo transfers.