Introduction

Oocyte cryopreservation sometimes known as egg freezing is a strategy to preserve a person assigned female at birth’s fertility. This strategy involves freezing oocytes (immature eggs) when they are younger and saving them for when the individual wishes to have a child. It is often used when the individual is at risk of becoming prematurely infertile e.g. by undergoing chemotherapy for cancer treatment or at risk of premature menopause.

However, it is becoming an increasingly common method due to shifting societal trends such as an increase in older mothers for career, education or other personal reasons. There is no guarantee that oocyte cryopreservation will be successful for all, however in 50% of oocyte retrievals it will result in a successful birth.

Understanding oocyte cryopreservation

Oocyte cryopreservation procedure 

The method involves several key steps beginning with controlled ovarian stimulation. Controlled ovarian stimulation involves daily injections of gonadotropin gonadotropin-releasing easing hormones to promote and extend levels of FSH.¹

FSH is a hormone which stimulates the development of follicles (fluid-filled sacs with immature eggs) into oocytes, this process is called folliculogenesis. Increasing FSH levels stimulate the development and growth of oocytes. Sometimes with these injections, multiple follicles are selected meaning multiple eggs. Then drugs are administered to mimic the LH surge which will induce oocyte release from the ovaries.

Oocytes can then be retrieved around 36 hours later. The procedure for oocyte retrieval involves the insertion of a needle through the vaginal wall followed by suction to remove the oocyte. The oocytes of the best quality are then selected by an embryologist and then frozen.¹

There are two techniques to freeze oocytes, traditional slow freezing or vitrification, a rapid freezing technique. Slow freezing reduces the temperature by 2-3°C until the egg reaches -196°C. The vitrification process is newer and more experimental but has the best outcomes for freezing eggs because, unlike slow freezing methods, it stops ice crystals from forming and instead water becomes a glass-like solid state.²

This is important as oocytes tend to be more fragile than embryos and sperm, which are less prone to damage in the freezing process. To further prevent damage to oocytes cryoprotectants are used in both techniques to prevent damage to tissues.^2,3

Using oocytes after cryopreservation 

Whilst oocyte cryopreservation guarantees autonomy and preserves fertility for women it is often an insurance choice, with only around 1 in 10 women returning to use their eggs.^4,5

If a woman decides to use her frozen oocyte the oocytes first need to be thawed; this is done by transferring oocytes from ultra-low temperatures to a series of warming solutions, and removing the cryoprotectants. Once finished thawing oocytes are examined to check for oocyte quality and check if there are no malignant (cancerous) cells. Suitable oocytes are then used for fertilisation by IVF (in vitro fertilisation) or ICSI (intracytoplasmic sperm injection) if there is a problem with the sperm.  

IVF involves fertilising the egg with the sperm in a laboratory to create an embryo. The embryo is then implanted into the uterus using a catheter. This is a simple procedure similar to a Pap smear as it requires no sedation. Two weeks following implantation a pregnancy test can be taken to see if IVF has worked. However, IVF is only available on the NHS for those under the age of 43 who have been trying to conceive for two years via unprotected sex and IUI (intrauterine insemination) has failed several times. 

Evaluating the efficacy of egg freezing

Success and outcomes

The mean age for oocyte cryopreservation is approximately 37 years old, with around 10 oocytes frozen per retrieval. Every 3 in 4 oocytes survive thawing and 1 in 2 result in a live birth.⁴ The more oocytes that are retrieved and frozen the greater the success rate for live births from embryo transfer.⁴

The successful outcomes of oocyte cryopreservation depend on the age of the patient, as there is a sharp decline in fertility rates at 35. For example, there is no longer a 3 in 4 chance of oocytes surviving when a person is older than 38.⁶ It is important to remember other factors such as sperm quality and uterine health determine the likelihood of a successful pregnancy.

Risks and complications

Although oocyte cryopreservation is a safe and well-established strategy there are still minimal risks associated. In less than 5% of cases, ovarian hyperstimulation syndrome occurs which is a complication resulting from the excess of hormones caused by injections.⁷

The ovaries swell and fluid leaks into the abdomen causing pain. The symptoms should usually resolve with time, however pain medication and anti-sickness drugs can be given if required. In addition, those who decide to use their oocytes will also experience the risks of IVF. These risks include:⁷

• Multiple pregnancies 
• Pregnancy-related high blood pressure
• Premature delivery
• Low birth rate  

Psychosocial factors  

Historically oocyte cryopreservation was used for patients who are about to undergo chemotherapy or radiotherapy to treat cancer or other conditions associated with infertility like severe endometriosis, and autoimmune diseases. However, a changing societal landscape means women might choose to freeze their eggs for other reasons. Therefore this choice is a complex and personal one which depends on a variety of social and psychological factors. Moreover, after freezing eggs, there might be concerns if and when to use oocytes for pregnancy.

It is important to consider that freezing your oocytes does not guarantee IVF will be successful afterwards. This can sometimes lead to further worries regarding the time to use oocytes and whether they will have a successful pregnancy. Thus it is critical to have a support system of friends, family or your partner for emotional support. If available counselling or therapy is useful to help decide if oocyte cryopreservation is right for you.⁸

Alternative fertility preservation strategies 

Embryo cryopreservation

Embryo cryopreservation freezes and stores embryos. This usually occurs after trying IVF or ICSI to get pregnant. The embryo cryopreservation procedure also involves freezing and oocyte extraction like oocyte cryopreservation. The major difference is the egg is fertilised with sperm to form the embryo prior to cryopreservation.

This technique is used as embryo cryopreservation has a higher success rate than oocyte cryopreservation. Embryos are more resistant to thawing with 90-95% of frozen embryos surviving resulting in a 50% chance of pregnancy per embryo.⁹ Whereas one frozen oocyte has only a 2-5% chance of leading to a live birth.⁹ Hence why multiple egg retrievals are sometimes required for oocyte cryopreservation. 

The major disadvantage to women is loss of autonomy since embryo cryopreservation requires sperm. Egg freezing allows them to preserve fertility independent of whether they have a partner. This gives more future options regarding whether they choose to use a partner or an anonymous sperm donor.

Furthermore, if a couple decides to freeze embryos and then splits there can be a complicated aftermath as to the fate of embryos. Therefore, embryo cryopreservation is more likely to lead to a successful pregnancy but comprises the person’s autonomy. 

Ovarian tissue cryopreservation

Ovarian tissue cryopreservation is another strategy to preserve fertility, however is only recommended where IVF is not an option. This is usually in pre-pubertal girls who have undergone chemotherapy or radiotherapy.

Alternatively, young people undergoing gender affirmation surgery may also want to preserve their ovarian tissue. The procedure uses laparoscopic oophorectomy to remove the ovarian cortex (where folliculogenesis occurs) from one ovary and is then frozen and checked for malignant cells. If a person decides to use ovarian tissue it will then be reimplanted back onto her ovaries. 

This strategy has been used for over 20 years, although only around 100 babies have been born via this route worldwide. It has a success rate of approximately 50%. Another advantage of this method is that with a sufficient number of follicles, they have the ability to restore endocrine function in 93-95% of cases,¹⁰ and return to the normal menstrual cycle giving multiple opportunities for pregnancy.

Unfortunately, often there are not sufficient follicles in the ovarian cortex graft which can limit longevity and tissue survival.¹¹ This is due to thawing reduces ovarian stroma and granulosa cells that are essential for ovulation.¹²

Summary

Oocyte cryopreservation is an important strategy to independently preserve fertility. It is used when people are facing infertility due to a medical condition or other personal reasons. The procedure involves ovarian stimulation, egg retrieval and freezing by slow freezing or vitrification.

While the success rate is influenced by a variety of factors, it generally has a 50% chance of live birth from 10 oocytes. There are some risks associated such as ovarian hyperstimulation syndrome or those associated with IVF.

However alternative strategies like embryo cryopreservation offer a greater likelihood of pregnancy, though it requires sperm from either a partner or donor. Ovarian tissue cryopreservation is suited specifically to prepubertal girls. Therefore to conclude there are a variety of different options for preserving fertility which strategy is the best is specific to an individual?