Assisted reproductive techniques have improved significantly since 1978 when the first IVF baby was born. Both sperm and embryos have been successfully cryopreserved and resulted in many pregnancies. Data from the 2003 CDC ART report alone documented 17.8% of cycles were preformed using frozen embryos in those clinics reporting.

Unlike sperm and embryos, oocyte freezing has not enjoyed the same success and is still considered by some to be experimental. It is thought that worldwide less than 150 babies have been born using eggs that have been frozen; however, recent advances in cryobiology and a better understanding of oocyte physiology have improved the chances of achieving pregnancy using frozen eggs.

There are many patients who would benefit from this option. They include the following:

• The young cancer patient who is facing chemotherapy and radiation that will likely render her sterile once treatment is completed
• Women who need to have their ovaries removed
• As a way to salvage an interrupted IVF cycle
• Couples for whom embryo cryopreservation presents a moral or ethical dilemma
• Career oriented women wanting to delay childbearing until a later date
• Women with a family history of premature ovarian failure wishing to preserve their fertility

The human oocyte is the largest cell in the body and has a high water content. Problems occur during the freezing process because ice crystals form that damage DNA, intracellular organelles and membranes. There are two techniques commonly used. The first, a Slow-Freeze method, is familiar to embryologists who use it to freeze embryos. Using this method the eggs are frozen slowly using cryoprotectants (anti-freeze), sucrose and solvents to reduce the amount of water that forms in the cell, placed in a vial or straw and cooled gradually. At -32° C the straw is put into liquid nitrogen (-196° C). Vitrification, an ultra-rapid freezing technique, is the alternative method used. The eggs are loaded onto a thin film of medium and plunged directly into liquid nitrogen that leaves no time for ice crystal formation.

Regardless of the freezing method used, the zona pellucida or outer coating of the egg, undergoes changes due to the premature release of substances that are normally released to prevent multiple sperm from entering the egg. This causes the zona to harden making it less likely to fertilize when sperm are placed around the egg in conventional insemination. To overcome this deficit, ICSI is used.

Based on the data reported egg freezing appears to be safe however it will be many years until there are enough births to determine the true safety of this technique. No major abnormalities have been reported in the 150 babies born so far and to date no pregnancies have occurred in women over the age of 38. Finally, although egg freezing is a viable option with promising results it should still be considered a research protocol and done with ethics board informed consent.

References:
Paulson R. Oocyte Freezing: The New Frontier. Paper presented at: The ART of Donor Oocytes; June 9-11, 2006: Charleston, SC.

Pool T. Egg Freezing: Application of Oocyte Cryopreservation. Paper presented at: Smart Art VII From Physiology to Clinical Success; August 3-5, 2006: Las Vegas, NV.

Porcu E et al. Obstetric, Perinatal Outcome and Follow Up of Children Conceived from Cryopreserved Oocytes. Fertil Steril 2000; 74: (Supplement 1): S48.