Fresh vs. Frozen Embryo transfer: Which is More Successful?

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The last decade has witnessed a dramatic improvement in frozen embryo implantation and pregnancy rates. This has resulted in a growing number of IVF practitioners recommending to their patients that they consider frozen  over fresh transfers, preferentially.

The dramatic switch was largely brought about by two developments:

  1. The first was the introduction, almost a decade ago, of ultra-rapid embryo cryopreservation (vitrification), which freezes the embryo rapidly (about 60,000 times faster than was the case using the older “conventional” slow freezing approach). The slow, “conventional” freezing approach resulted in the formation of intracellular ice that damaged the embryos, resulting in a low survival rate. Those that did survive had significantly diminished implantation potential. In contrast, the ultrarapid freezing process (vitrification) avoided intracellular ice formation with the result being that >90% of pre-vitrified embryos survived, essentially unaffected by the freeze-thaw process and with an implantation and pregnancy generating potential at least as good as (and possibly even better than) fresh embryos.
  2. The second was the move to preferentially vitrify and cryopreserve blastocysts rather than early (day 2-3) cleaved embryos. Embryos that fail to develop to the expanded blastocyst stage (day 5-6) will in the vast majority of cases, be chromosomally compromised (aneuploid) and thus unable to propagate viable and healthy offspring. Moreover, implantation and pregnancy rates could be markedly improved by selectively transferring expanded blastocysts. Thus fewer embryos need be transferred at a time, thereby reducing the incidence of high order multiple pregnancy (triplets or greater) with its incumbent risks to both mother and babies.

We are definitely entering an era where “staggered” embryo transfers (i.e. embryos being transferred in a different cycle than where ovarian stimulation and egg harvest was conducted), are fast becoming commonplace and for the following reasons, I believe this to be a move in the right direction:

By reducing the imperative to transfer available embryos fresh in order to give them the best chance of propagating a pregnancy, it will reduce the incentive to transfer “multiple embryos” at a time, and so herald a significant reduction in high order multiple pregnancies.
For the many women who are particularly sensitive to fertility drugs and “overstimulate” with fertility drugs, it will (by avoiding pregnancy in that cycle), remove the risk of fulminant severe ovarian hyperstimulation syndrome (OHSS) caused by ever increasing production of hCG.
It provides an opportunity to control and optimize the endometrial environment for implantation.
It provides an opportunity for older women and those with DOR who are running out of time on the “biological clock” to bank euploid embryos for subsequent dispensation to the uterus, and in many cases, provide a tangible alternative to donor egg-IVF.
With pre-implantation Genetic Technology rapidly advancing, there is little doubt that we will in time be able to diagnose and perhaps (through genetic engineering) even reverse many chromosomal/genetic abnormalities affecting the embryo. Frozen Embryo Transfers, by providing the time/opportunity to implement such strategies, could ultimately provide the most important advantage of all, namely to optimize the quality of life after birth.


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