Egg freezing is the preservation of mature eggs following hormone stimulation to achieve pregnancy at a later time. It is currently offered to women who are interested in preserving their fertility or delaying pregnancy electively, or for those women with a diagnosis of cancer at risk of losing their egg reserve and the potential to have a child due to surgery, chemotherapy or radiation. Egg reserve decreases significantly with the increasing reproductive age and any woman in their reproductive years can be considered a candidate for egg freezing, regardless of age.
Egg freezing has been offered very selectively since the initial cases in mid-late 1980s, but its true application to clinical practice has become viable recently. This is due to advanced laboratory techniques with better freezing methods that result in high survival and fertilization rates. Additionally, increased awareness in fertility preservation and access to medical information are also important variables.
Egg freezing can be utilized by any woman who would like to delay childbearing. Hormone treatment for 8-10 days is followed by aspiration of eggs under anesthesia in the office setting and eggs are then frozen for fertility preservation purposes. When the pregnancy is desired at a later time in life, eggs are then thawed and fertilized with sperm and embryos are transferred into the uterus. This process is called in vitro fertilization (IVF), a common type of treatment used for infertility patients.
Eggs can be frozen by two methods today, which are the slow-freeze method and vitrification. Vitrification method is the ultra-rapid freezing technique which seems to be better than slow freezing method based on medical literature. Survival of eggs following freeze-thaw -- and overall pregnancy rates -- are much higher with the vitrification method. Additionally, eggs can be tested for chromosomal abnormalites at the time of the freezing process and only chromosomally normal eggs are then kept for the long term storage.
An advanced genetic test called comparative genomic hybridization (CGH) can be utilized to identify chromosomally normal eggs during egg freezing. Once the eggs are aspirated from the ovaries, mature eggs are identified under the microscope and the polar body from each egg is removed from the area adjacent to the egg before the freezing process is initiated.
When ovulation occurs, the egg splits into two cells and one of them is called “The Egg” and the second one is the polar body. Polar body is the extrusion of excessive genetic material that allows the sperm to complete the genetic make up of the embryo. The egg provides 23 chromosomes and the sperm brings another 23 making the total number 46. If the egg is chromosomally normal (23 chromosomes) following ovulation, the polar body should also have 23 chromosomes. When the polar body is tested and is found to be normal, the egg is also considered to be normal as they should be the mirror image of each other genetically. For this reason polar body testing is helpful in identifying chromosomally normal eggs during the egg freezing procedure.
Typically, once the eggs are aspirated and mature ones identified, polar bodies are removed adjacent to each egg and then the eggs are frozen immediately. Polar bodies are then tested with the CGH test for chromosomal abnormalities. Once the normal eggs are identified they are kept frozen until pregnancy is desired. Eggs can be kept frozen for a few days to years without any compromise depending on when they are needed to achieve a pregnancy.
Based on our published data in the literature, survival rates following vitrification of eggs along with CGH testing is very high at 96%, and the pregnancy rates are approximately 75% per attempt when frozen eggs are used to achieve a pregnancy. Egg freezing should be considered as an option with high success rates for fertility preservation.