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Frozen Embryo Transfer is a procedure designed to cryopreserve (freeze) surplus embryos or cryopreserve embryos in the fresh stimulation cycle to increase the overall success rate (freeze all) in couples undergoing the IVF process.
Slow freezing is known as equilibrium freezing in which the gametes are frozen at a slow temperature fluctuation of 0.3 to 2 degree Celsius per minute until it reaches the final temperature of -196 degrees. The exchange of intra and extra cellular spaces because of the equilibrium freezing and low concentration of the cryoprotectants allows safe freezing without serious osmotic and deformation affect to the cells. This requires more time consumption and expensive programmable freezer. Vitrification is called as fast freezing of flash freezing in which non-equilibrium freezing is done with high concentration of the cryoprotectants thereby completely eliminating the ice crystal formation. It is less time consuming and is a fast process without any expensive equipment. Most advanced centres including ours follow Vitrification.
As per the Human Fertilization and Embryology act , the embryos or gametes can be frozen upto 10years. In India, we can freeze embryos upto 5 years at a stretch after which we have to renew the freezing if needed.
In many cases FET gives higher pregnancy rates especially in the indications stated above, like hormonal imbalances, fluid in endometrium or thin endometrium. However, in other cases, especially in women with regular cycles. fresh embryos are transferred. (https://www.bmj.com/content/370/bmj.m2519).
Initially, the embryos are removed from liquid nitrogen to room temperature. At this stage the embryo is completely dehydrated. The embryo is then rehydrated by moving it into samples of low osmolality. It then regains its original shape. It is then placed in culture medium for its continued growth.
The endometrium is usually prepared from the first or second day of the menstrual cycle by starting the lady on estradiol valerate. When the endometrial thickness reacges 8 mm , progesterones are started and the embryos are transferred a few days later. Following transfer , the lady is put on some medications ( luteal phase support) for 2 weeks till the pregnancy test is done.
The success rate of FET depends on multiple factors like age of the patient, quality of eggs, sperms, embryos ,endometrium, freezing thawing technique and transfer technique. It cannot be the same for every couple. Usually, the results vary between 30-60%/ET.