Egg freezing also called oocyte cryopreservation, is a relatively new method in fertility preservation, where a mature egg is retrieved from the ovarian follicles after stimulation, frozen, and stored for future pregnancy. Oocyte cryopreservation become the integral part of ART,this method has been used for various legal, social and medical reasons.
There are two types of oocytes cryopreservation. One is slow freezing another one is virtification. In slow freezing technique extracellular ice formation through an equilibrium process cause dehydration therefore,a slow long controlled cooling is required prior to the oocytes are placed in the liquid nitrogen .However in virtification cryoprotactant are present in higher concentration which solidify without forming ice crystal and this allow us to rapidly cool the oocytes by directly plunging them into liquid nitrogen. The primary principle is to protect the oocytes from intracellular ice crystal formation, cellular dehydration and abrupt modification in the solute concentrations at high and low temperature.
Applications of oocytes cryopreservation
-Fertility preservation in malignant and premalignant conditions.
-Egg donor program.
-Accumulation of oocytes in poor responders
-Avoid legal and ethical restrictions of embryo cryopreservation
-Non availability of sperm during oocytes retrieval.
Freezing eggs and banking eggs usually has a good success rate. Around 90 percent of eggs survive after thawing process. Of those, about 75 percent are successfully fertilized, and upto 55 percent will get pregnancy, according to the American Society for Reproductive Medicine. These numbers vary based on the quality of egg and embryo, the health of the patient and other factors.
Safety of oocyte cryopreservation
Some of the study mentioned that pregnancy and infant conceived after oocyte cryopreservation do not present with increased risk of adverse obstetrics outcomes or congenital anomalies. No increase in the number of abnormal or stray chromosomes has been observed in the thawed oocytes. In addition no difference has been found when comparing the incidence of chromosomal abnormalities in the human embryo obtained from fresh or frozen oocytes.
Implantation is rate limiting step and embryo-endometrium asynchrony in controlled Ovarian Hyper Stimulation cycles impairs implantation and it has suggested that the asynchrony problems in fresh cycles can be solved by cryopreservation of all embryo and transferring them subsequently in optimal condition .The first pregnancy and live birth from frozen thawed embryo was reported in Australia by Trounson and Mohr in 1982.In current practice most of the laboratory select embryo for freezing on the basis of morphology.
Relevance of embryo cryopreservations
-It permits to do a single embryo transfer
-Excess cryopreserved embryo can be used for future pregnancy
-Increase cumulative pregnancy rate from frozen cycle
-Can help to achieve an OHSS free clinics.
-Can help to avoid embryo transfer in hostile conditions
-Allow for freeze all and embryo transfer in the next cycle
-In treating recurrent implantation failure due to poor endometrium.