To retrieve eggs for freezing, a patient undergoes mandatory screening in the form of Ovarian reserve testing & infectious disease testing. Post which the procedure is the same as in-vitro fertilization (see IVF Cycle Details). The only difference is that following egg retrieval, they are frozen for some time before they are thawed, fertilized, and transferred to the uterus as embryos.
It takes approximately 3 weeks to complete the egg-freezing cycle and is consistent with the initial stages of the IVF process including:
- 1-2 weeks of birth control pills to temporarily turn off natural hormones (this step can be skipped if there is urgency, such as before cancer therapy).
- 9-10 days of hormone injections to stimulate the ovaries and ripen multiple eggs.
Once the eggs have adequately matured, they are removed with a needle placed through the vagina under ultrasound guidance. The eggs are then immediately frozen to sub-zero temperature (-196 degrees). This is done to stop all the biological activity in the egg. Various special methods are employed for egg freezing as mentioned below:
- Usage of cryoprotectants – These cryoprotectants help the egg from forming an intracellular harmful ice crystal when stored in deep freezing.
- Slow-freezing technique – This technique involves the use of programmable freezers that freeze the eggs very slowly so that the intracellular ice crystals cannot be formed. This technique protects the eggs from exposure to the toxic substances present in the cryoprotectants. In this slow-freezing method, initially, the cryoprotectants are used in a very low concentration. The high concentration of cryoprotectants is only used when there is a gradual decline in the temperature and a decrease in the metabolic rates of the eggs.
- Vitrification – In this technique, initially, the higher concentration of the cryoprotectants is used with rapid cooling so that there is no time for the intracellular ice crystals to form (currently used & popular technique).
Around 75 to 80% of the eggs stay damage-free even after freezing and thawing. When the patient is ready to attempt pregnancy (this can be several years later) the eggs are thawed, injected with a single sperm to achieve fertilization, and transferred to the uterus as embryos.