EGG FREEZING: AN IN-DEPTH LOOK AT SURVIVAL RATES AND EMBRYO GRADING

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Egg freezing, or oocyte cryopreservation, has become an increasingly popular option for individuals seeking to preserve their fertility.

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This process involves extracting eggs from the ovaries, freezing them for future use, and then fertilizing them later through in vitro fertilization (IVF). With advancements in freezing techniques and IVF technology, egg freezing has become a viable solution for those wanting to delay childbearing due to medical, personal, or career reasons.

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In this article, we’ll explore the process of egg freezing, the survival rates of frozen eggs, and how embryos are graded post-fertilization.

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WHAT IS EGG FREEZING?

Egg freezing is a medical procedure where a woman’s eggs (oocytes) are retrieved, frozen, and stored for future use. It’s typically done when a woman is in her early 30s to early 40s, allowing her to have the option of having biological children later in life when her fertility may have declined. The procedure is generally done in the following steps:

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1. Ovarian Stimulation : The individual receives hormone injections to stimulate the ovaries to produce multiple eggs during one menstrual cycle.

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2. Egg Retrieval: A doctor retrieves the eggs through a minor surgical procedure performed under sedation.

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3. Freezing: The eggs are frozen using a rapid freezing technique called vitrification, which ensures the eggs remain viable.

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4. Storage: The eggs are stored in liquid nitrogen until they are ready to be used.

SURVIVAL RATE OF FROZEN EGGS

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The survival rate of frozen eggs refers to the percentage of eggs that remain viable after the freezing and thawing process. With older freezing techniques, survival rates were low, and many eggs were damaged during freezing or thawing. However, vitrification, a rapid freezing technique introduced in the early 2000s, has significantly improved survival rates.

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Vitrification: This method involves cooling eggs very quickly to prevent the formation of ice crystals inside the egg, which can damage cell structures.

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Vitrification has revolutionized egg freezing by improving the survival rate to 90–97% after thawing, depending on the clinic and the procedure’s specifics.

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While survival rates have dramatically improved, some loss is still inevitable. Typically, around 5–10% of eggs may not survive the thawing process, but the majority remain viable.

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FERTILIZATION AND EMBRYO DEVELOPMENT FROM FROZEN EGGS

After eggs are thawed, they are fertilized using Sperm Injection (ICSI)a technique where a single sperm is directly injected into each egg.

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This method is necessary because freezing and thawing can sometimes harden the outer shell of the egg (the zona pellucida), making it difficult for sperm to penetrate naturally.

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Once fertilized, the eggs develop into embryos. Not all embryos will progress equally, and it’s essential to assess embryo quality to determine the best candidates for implantation.

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The embryo development process and grading system play a crucial role in predicting the chances of successful pregnancy.

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EMBRYO GRADING

Embryo grading is the process by which embryos are evaluated based on their appearance and development stages. Grading helps embryologists select the most viable embryos for transfer.

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It is a subjective process, though most grading systems follow a similar pattern. Here’s how embryo grading typically works:

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1. EMBRYO DEVELOPMENT STAGES

Embryos are assessed at several stages of development:

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Day 1 (Fertilization Check): This is when the eggs are fertilized, and embryologists check for signs of successful fertilization, such as the presence of two pronuclei (one from the egg and one from the sperm).

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Day 3 (Cleavage Stage): By Day 3, the embryo will have divided into 6–10 cells. Embryologists assess whether the cells are dividing evenly and whether there is any fragmentation (cellular breakdown).

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Day 5–6 (Blastocyst Stage): The blastocyst is a more advanced stage of development, where the embryo has formed a hollow ball of cells. Embryos at this stage are typically the best candidates for implantation.

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2. GRADING SYSTEM

Most embryologists use a grading system that evaluates embryos based on two primary factors: morphology (the shape and structure of the embryo) and cellular division.

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Grade A (Excellent): Embryos are evenly divided, with little or no fragmentation, and show signs of healthy development.

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Grade B (Good): These embryos may have minor fragmentation or uneven division but are still considered viable for implantation.

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Grade C (Fair): Embryos with significant fragmentation or uneven development. They have lower chances of implantation and pregnancy.

Blastocyst grading** also includes assessing the inner cell mass (ICM) and the (TE):

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ICM: The cluster of cells that will develop into the fetus.

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TE: The cells that will form the placenta.
Grade 4AA is considered excellent, while lower grades (such as 3BB) indicate embryos that may have lower implantation potential.

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SUCCESS RATES: FROZEN EGGS VS. FRESH EGGS

The good news is that research shows no significant difference between embryos created from frozen eggs and those created from fresh eggs. Studies have demonstrated that:

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Frozen embryosfrom eggs that were stored and later thawed have similar implantation and live birth rates as fresh embryos, provided the eggs were frozen at a young age (ideally before 35).

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Blastocyst formation from frozen eggs are also comparable to fresh ones, although some variability remains depending on egg quality and freezing conditions.

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However, success rates decline when older eggs (typically frozen after age 38) are used. Frozen eggs from older women may have a higher likelihood of chromosomal abnormalities, which can affect embryo development.

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Genetic screening(such as PGT-A) can be performed on embryos to assess chromosomal health before implantation, improving success rates and reducing the chances of miscarriage.

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FINAL THOUGHTS: IS EGG FREEZING A RELIABLE OPTION?

Egg freezing is a reliable and increasingly common method for preserving fertility. While the survival rates of frozen eggs have significantly improved due to advancements in vitrification, the key to success lies in freezing eggs at a younger age, ideally in the mid-20s to early 30s. This increases the likelihood of high-quality embryos and better overall outcomes.

While egg freezing is not a guarantee of future pregnancy, it offers individuals the ability to delay childbearing and maintain the option of having children later in life. By understanding the survival rates of frozen eggs and the grading process for embryos, individuals can make informed decisions about their reproductive health and fertility preservation.