Elective oocyte cryopreservation, commonly known as egg freezing, is a medical process used for fertility preservation. This process involves stimulating the ovaries to produce multiple eggs, retrieving them, and then rapidly freezing them for future use in in vitro fertilization (IVF) to achieve a pregnancy later in life. The central challenge lies in determining the precise number of eggs that offer a reasonable chance of live birth, as this number is entirely dependent on the individual’s age at the time of the retrieval. Understanding the statistical targets and biological realities is the first step in the process.
How Age Affects Egg Quality and Quantity
A woman’s age is the most significant factor influencing the required number of frozen eggs because it directly impacts the quality of the oocytes. Unlike the body’s other cells, an egg’s quality declines with chronological age, primarily due to an increasing rate of chromosomal errors called aneuploidy. This means that a 30-year-old’s eggs are genetically healthier than a 40-year-old’s eggs. The decline in quality is not linear but accelerates significantly after age 35. For women under 35, a high proportion of retrieved eggs are chromosomally normal, or euploid, making them capable of developing into a healthy embryo. Conversely, by the early 40s, the majority of eggs retrieved are aneuploid and will not result in a viable pregnancy.
Recommended Target Numbers for Live Birth Success
The target number of eggs to freeze is determined by balancing the woman’s age at retrieval with her desired probability of a live birth. Fertility societies have developed statistical models to guide this decision, emphasizing that a higher number provides a greater assurance of success. These models focus on the number of mature (MII) oocytes, which are the only ones capable of being fertilized.
Age 35 and Under
For women under 35, the target number of mature eggs to achieve a 70% chance of at least one live birth is approximately 14 to 15. This lower number reflects the higher proportion of chromosomally normal eggs available in this age group. A woman in this category seeking a 50% chance of one live birth may need to freeze as few as six eggs.
Age 35 to 40
The required number increases substantially for women in their later reproductive years due to the sharp decline in egg quality. For a woman aged 35 to 37, achieving a 70% chance of one live birth typically requires banking around 15 mature oocytes. This number nearly doubles for women aged 38 to 40, where an estimated 25 to 26 mature eggs are needed for the same 70% probability.
Age 40 and Over
For women over 40, the required quantity continues to rise dramatically, sometimes exceeding 30 to 40 eggs for a reasonable chance of success. This often necessitates multiple stimulation and retrieval cycles. These numbers are statistical probabilities, not guarantees, and individual outcomes can vary widely depending on the patient’s unique ovarian response.
Biological Factors Influencing Retrieval Yield
The total number of eggs retrieved during a single cycle, known as the retrieval yield, is highly individualized and is not directly related to the statistical target number. This yield is governed by a woman’s ovarian reserve, which is the number of eggs remaining in her ovaries. Specialized testing helps estimate this reserve before a cycle begins.
Two primary biomarkers are used to assess ovarian reserve: Anti-Müllerian Hormone (AMH) levels and the Antral Follicle Count (AFC). AMH is a hormone secreted by the small follicles in the ovaries, and its level in the blood correlates positively with the pool of remaining eggs. AFC is determined via transvaginal ultrasound, where a fertility specialist counts the small, fluid-filled sacs, or antral follicles, visible on the ovaries. Both AMH and AFC are strong predictors of how the ovaries will respond to the stimulation medication and how many eggs are likely to be retrieved.
Understanding Attrition: Survival Rates After Thawing
The high number of eggs recommended for freezing is necessary because of a predictable and sequential loss of viability, or attrition, that occurs at every step after the eggs are thawed. This biological filtering process means that not every frozen egg will result in a viable embryo. The first stage of attrition occurs immediately after the eggs are warmed, with modern flash-freezing techniques, known as vitrification, yielding high survival rates.
The loss of viability continues through fertilization and development:
- Typically, 90% to 97% of mature eggs successfully survive the thawing process and remain biologically intact.
- The next loss occurs during fertilization, where approximately 70% of the surviving eggs successfully fertilize using Intracytoplasmic Sperm Injection (ICSI).
- The most significant attrition happens as these fertilized eggs are cultured in the lab to the blastocyst stage (day five or six). Only 30% to 50% of fertilized eggs are expected to progress to this stage.
Considering all these steps, a woman may need to start with 10 frozen eggs to yield just one to three high-quality blastocysts. This illustrates why a high starting number is recommended to ensure the eventual transfer of a viable embryo.