In this age of “I want it NOW”, the last thing patients want to do after their egg retrieval is to wait any longer than necessary to do their embryo transfer. Most patients hear the term “frozen embryo transfer” and think “no way!”, I want to get pregnant “NOW!”. However, evidence is building that holding off on a fresh transfer and instead doing and FET might be in your best interest for getting pregnant.
In a recent study in the medical journal Fertility and Sterility, researchers reviewed outcomes of IVF cycles, between fresh and frozen embryo transfers. Their results suggested that there is reason to believe that IVF outcomes can actually be improved by performing FET compared with fresh embryo transfer. Even though it seems as if fresh would be best, this might not always be the case. Researchers feel that this could be explained by a better embryo-endometrium synchrony achieved with FET, due to a more controlled endometrium preparation.
The success of any IVF cycle relies on three main parameters: embryo quality, endometrial receptivity, and embryo-endometrium interaction. Implantation of the embryo represents one of the most critical components, and is a very time specific window where the endometrium is completely in sync with blastocyst development. Therefore, embryo receptivity is essential for implantation in both FET and IVF cycles. Studies have suggested that controlled ovarian hyperstimulation (the stimulation that grows many follicles to get you ready for your egg retrieval) may adversely affect endometrial receptivity due to the increased estradiol (E2) and progesterone levels produced in stimulated cycles. These altered hormone levels could cause an asynchrony between the endometrium and the transferred embryos, leading to an endometrial environment that is not conducive to embryo implantation. Based on mouse studies, reseachers also have reason to believe that the development of an embryo that has achieved implantation beyond the normal window of endometrial receptivity are more likely to show defects in placental formation and fetal growth.
During an FET cycle, this endogenous production of E2 and progesterone that is present in IVF cycles is absent, and the endometrial preparation is very controlled. This reduces the incidence of missing the “window of opportunity” and is more likely to produce a synchronized endometrial-embryo environment that is more favorable for implantation. Also, with the advance in vitrification techniques, survival rates at thawing are much higher, so we are much more confident that any given embryo will be viable compared with older “slow-freezing” methods of freezing.
Even though we live in a “want it now” world, waiting for your endometrial lining to be at it’s most ideal for embryo implantation, even if that means waiting a few more weeks after egg retrieval to get pregnant, might be the best option. Its one of those times where the wait is definitely worth it!
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