Future Parenthood: The Science of Egg Vitrification

We learn more from Dr. Liow Swee Lian, Scientific Director, about how vitrification, an egg-freezing technique, transforms fertility preservation. Learn about the process, benefits, and key considerations, and understand how this advancement empowers women to take control of their reproductive futures.

Oocyte (Egg) freezing, also known as oocyte cryopreservation, is an elective fertility preservation technique that allows women to store their eggs for future use. This procedure can be performed for medical reasons, such as before undergoing treatments that may affect fertility (e.g., chemotherapy or radiation therapy for cancer), certain surgeries that could impact reproductive organs, or when diagnosed with a genetic condition that may decrease fertility.

Elective Egg Freezing (EEF) became available in Singapore on July 1, 2023, with the age limit for the procedure raised from 35 to 37. Previously, egg freezing was permitted only for medical reasons. While the option is now more widely accessible, under the guideline, only legally married couples are allowed to freeze their embryos. For males, sperm freezing is limited solely to medical reasons.

EEF has gained popularity as more women delay starting a family to pursue career goals or other personal aspirations while searching for the right partner. It offers the opportunity to preserve fertility before age-related decline affects the quality and quantity of eggs, making it harder to conceive later in life. By freezing eggs at a younger age, women can increase their chances of a successful pregnancy in the future and potentially help to reduce the need for fertility treatments later.

Egg Freezing Process

One of the most significant advancements in oocyte cryopreservation is the development of vitrification. Vitrification is a rapid freezing technique that has markedly improved the efficacy of oocyte cryopreservation in terms of safety, oocyte survival after thawing, and pregnancy rates. Unlike slow freezing, which can result in damaging ice crystal formation, vitrification avoids this issue entirely, resulting in a significantly higher survival rate after warming.

The process of oocyte cryopreservation using vitrification involves the following key steps:

1. Ovarian Stimulation: Before the eggs can be retrieved, the patient undergoes ovarian stimulation, which involves taking hormonal medications to encourage the ovaries to produce multiple eggs.
2. Egg Retrieval: Once the eggs are mature, they are retrieved from the ovaries through a minor surgical procedure known as transvaginal ultrasound aspiration. This procedure is usually performed under sedation or anesthesia.
3. Oocyte Vitrification: The retrieved eggs are then subjected to the vitrification process. This involves exposing the eggs to a series of cryoprotectants, which protect the cells from damage during the freezing process. The eggs are then rapidly cooled to a very low temperature, effectively "freezing" them in time.
4. Storage: The vitrified eggs are stored in liquid nitrogen at around -196°C. They can remain in storage for many years until the woman is ready to use them.
5. Warming and Fertilization: When the woman uses her frozen eggs, they are carefully warmed and assessed for viability. The surviving eggs are then fertilised with sperm through intracytoplasmic sperm injection (ICSI), where a single sperm is injected directly into the egg.
6. Embryo Transfer: The resulting embryos are cultured for a few days before being transferred to the woman's uterus. If successful, the embryo will implant, and any remaining good embryos will be frozen for future transfers.

From Traditional Slow Freezing to Vitrification?

The shift from traditional slow freezing to vitrification was driven by the need to improve the safety and success rates of egg freezing for reproductive purposes.

The main difference between the two methods lies in the speed of cooling. Slow freezing gradually cools eggs, typically at a rate of 1–2°C per minute, until they reach a storage temperature of about -196°C. Vitrification, on the other hand, cools eggs almost instantly, in less than a minute, to the same temperature.

Due to the slow freezing process, ice crystals form inside and outside the cells. These crystals can damage cell membranes and internal structures, lowering survival rates after thawing.

Vitrification avoids this issue by using higher concentrations of cryoprotective agents and rapid cooling. This causes the cells to enter a glass-like state without forming ice crystals, significantly enhancing the procedure's safety by reducing cellular damage.

With its higher survival and fertilization rates, vitrification is now considered the standard of care for egg freezing in most IVF centres around the world, and a more reliable method for fertility preservation.

Key Considerations for Egg Vitrification and Pregnancy Success

While egg vitrification (oocyte cryopreservation) offers many advantages, there are important factors and limitations to keep in mind:

• Age Factor: The age at which eggs are frozen is one of the most critical determinants of success. Eggs frozen at a younger age are of higher quality and, therefore, have a greater chance of resulting in a successful pregnancy.
• Cost: Egg freezing can be costly, and coverage by insurance plans varies. It's essential to consider both the initial cost of the procedure and the ongoing costs of egg storage.
• Emotional Impact: Although egg freezing provides reassurance and hope for future parenthood, it may also involve emotional stress and uncertainty relating to pregnancy. Hence, women should be prepared for a range of outcomes, including the possibility that pregnancy attempts may not succeed.

Final Note

Egg freezing through vitrification represents a significant advancement in fertility preservation. It empowers women to take charge of their reproductive futures and family planning, whether for medical or social reasons. By understanding the process, benefits, and considerations, they can make informed decisions about their fertility and family planning.

Dr. Liow recently presented and led the discussion of the above topic at the 14th Congress of the Asia Pacific Initiative on Reproduction (ASPIRE 2025), which was held in Singapore.