Why freeze eggs?
There are two primary reasons why women may wish to consider freezing their eggs. Firstly, women who are planning to undergo medical treatment that may harm reduce their egg reserves (eg chemotherapy), are offered Egg Freezing as a means of Medical Fertility Preservation. Secondly, some women are choosing to freeze their eggs in an effort to safeguard themselves for the future, against the age related decline in fertility that naturally occurs, and these cases we refer to as Social or Elective Fertility Preservation.
Women are born with all the eggs (oocytes) that they need until menopause. Unlike men who produce new sperm every second of their lives, women’s eggs start to decline in number from the time they are born. Only about 400 eggs will normally be ovulated from a woman’s ovaries in her lifetime. Women are most fertile in their 20s and by the time they reach 35y, their fertility has declined by half. This drop in fertility is due to the aging process, when chromosome abnormalities in the eggs also become increasingly prevalent. By the time a women is 37y the proportion of embryos that are produced with normal chromosomes is thought to be only 50%.
What does egg freezing involve?
A woman will undergone controlled ovarian hyper-stimulation to increase the number of follicles growing on the ovaries, this will maximise the number of eggs available for retrieval. Every woman is different and responds differently to treatment so the number and quality of eggs harvested will vary. Once the eggs have been collected the support cells around the eggs are removed. At this time the maturity of the eggs are determined. Only mature eggs are suitable for freezing therefore not all the eggs retrieved may be frozen. Mature eggs are frozen by first exposing them to a cryo-protective solution and then freezing very rapidly using a process called vitrification.
Each egg is individually frozen and stored. Similar to freezing embryos, once the eggs are frozen they are stable for an indefinite period.
Thawing involves the quick warming of the egg to 37oC and the removal of the cryoprotective solution. After a short period of recovery, the eggs are then suitable for insemination by injecting a single sperm into the egg.
How successful is egg freezing?
Recent international research has indicated that, on a per thawed oocyte basis, the use of frozen eggs has the potential to give similar results as using freshly collected eggs. Published results suggest a single pregnancy may be produced on average from about 10 -15 oocytes. The number and quality of eggs collected and the overall success rate for the procedure depends on the woman’s age, the reason for the treatment and other factors. There are instances where no eggs may be collected or suitable for freezing despite everyone’s best efforts.
Predicting your chance of having a baby based on age and number of eggs collected
Each curve in this figure shows the percentage likelihood that a woman of a given age will have at least one baby following a future thawing of her eggs. Based on this table, for a 75% chance of having a baby, you will need an estimated minimum number of eggs at the following age’s:
Like any invasive procedure, egg freezing does come with associated risks. Vitrification, a process of snap freezing has been used for embryos for relatively longer period of time than for eggs. Egg freezing is a relatively new procedure and there is currently not a large number of studies to address long term implication of freezing eggs.
Eggs themselves are naturally at a very fragile phase when they are vitrified in comparison with their embryos counterparts, and hence the survival rate of eggs is lower than what is expected for embryos. The survival of the eggs depends on their quality and not the quantity.
Pregnancy rates and miscarriage rates are influenced by the age of the egg, but there are many other pregnancy complications such as pre-eclampsia and age of the women at the time of pregnancy that can also impact the outcome and freezing eggs do not mitigate these risks.