Treatment Tips

Why Don't All Eggs Become Pregnancies?

5 mins read

Many women are often told that 20 is the number to aim for when going through egg freezing. Since egg retrieval is such a physically, emotionally, and mentally taxing and demanding process, most people don't want to go through it more than once unless absolutely necessary. Some women may be disappointed to find out that only a small number of eggs were successfully retrieved during one cycle and viable embryos made it through, even though they were told a much higher number during the ultrasound checkup of their ovaries. This can be due to many different reasons.

The process from egg retrieval to embryo transfer is laden with challenges, and various factors can influence the final count of viable eggs and embryos. In this article, we explore some of the major influences that could affect final egg and embryo count.

First things first: Egg, or oocyte, quality

The quality of eggs retrieved during IVF cycles significantly impacts fertilization rates and embryo development. As women age, the proportion of chromosomally abnormal eggs increases, reducing the likelihood of successful fertilization and embryo formation. For more, read our blog post on [What’s the Connection Between Age and Women’s Fertility? Discover With Sow Fertility.]. Moreover, factors such as ovarian response to stimulation medications, duration of ovarian stimulation, and the presence of underlying reproductive conditions can influence egg quality. Advanced techniques such as preimplantation genetic testing can help identify chromosomally normal embryos, and improve the chances of a successful pregnancy in older individuals or those with a history of recurrent miscarriages.

Next comes the freezing (a.k.a cryopreservation) process

Cryopreservation, the process of freezing and storing eggs and embryos for future use, plays a pivotal role in fertility treatment outcomes. The survival of eggs following thawing depends largely on the cryopreservation technique employed. Traditional slow freezing methods, while widely used in the past, are associated with potential damage to cellular structures due to ice crystal formation. In contrast, vitrification, a rapid freezing technique, has emerged as the preferred method for egg and embryo cryopreservation due to its ability to minimize ice crystal formation and preserve cellular integrity. For egg freezing, expect that a number of eggs don't survive the thawing process. High survival rates post-thawing are essential for maximizing the available pool of eggs for fertilization and subsequent embryo development.

Understanding fertilization rates

Following egg retrieval, the fertilization process is another process where the final count can be affected. In conventional IVF, where eggs are mixed with sperm in a culture dish, fertilization rates may vary depending on factors such as sperm quality, egg quality, and laboratory conditions. Intracytoplasmic sperm injection (ICSI), a laboratory technique where a single sperm is injected directly into the egg, helps in cases of male factor infertility or previous fertilization failure to bypass natural barriers to fertilization. While ICSI can improve fertilization rates, it is essential to balance its use judiciously to avoid potential risks such as an increased incidence of chromosomal abnormalities in embryos.

After fertilization, embryos are cultured

Once the embryo is fertilized, the next step is making sure the embryo has optimal culture conditions to support the development from fertilization to the blastocyst stage. The blastocyst stage refers to the final stage of embryo growth before they are frozen or transferred. Only 30-50% of embryos make it to this stage. Factors such as culture media composition, pH levels, temperature, and gas concentrations can influence embryo viability and morphology. Furthermore, advancements in embryo culture techniques, such as sequential media formulations and single-step culture systems, aim to mimic the physiological conditions of the female reproductive tract and enhance embryo quality. All of these things may affect the final count.

Last but not least: implantation and endometrial receptivity

Now that you have the final count of viable embryos, it's time to transfer. In the context of embryo transfer, the receptivity of the uterine lining, or endometrium, plays a crucial role in implantation and subsequent pregnancy. Factors such as endometrial thickness, structure, and receptivity to hormones affect the likelihood of successful implantation following embryo transfer. For women in their 40s or 50s, endometrial preparation protocols, including hormone replacement therapy (HRT), may help to optimize endometrial receptivity and synchronize embryo transfer with the window of implantation.

Conclusion

As you can see, fertility treatment processes represent a complex interplay of biological, technical, and environmental factors, all of which influence the final count of viable eggs and embryos. From freezing techniques to embryo culture conditions and endometrial receptivity, it is normal for the count to decrease at each stage of the treatment journey. Knowing the entire process and what to expect, you may feel less disappointment and more prepared to face each step. By understanding the multifaceted nature of these factors and leveraging advances in reproductive science and technology, you can work better with your medical provider to maximize the chances of achieving a successful pregnancy.

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