Freezing embryos involves preserving them at an ultra-low temperature of -196°C using special liquid nitrogen tanks. This technique is commonly used in various scenarios, including:

Situations Requiring Embryo Freezing:

  • Surplus embryos from an IVF egg retrieval or embryo transfer cycle.
  • For patients at risk of ovarian hyperstimulation syndrome (OHSS), embryos can be frozen and transferred at a later time.
  • In cases where embryo transfer is difficult, such as complications with catheter insertion into the uterine cavity. It is also used during PGD/PGS when waiting for genetic testing results.
  • For patients undergoing fresh egg donation cycles, the embryos are typically frozen for a minimum of 6 months.
  • If the patient experiences infections, fever, or severe diarrhea during the egg retrieval cycle, freezing the embryos is advised.
  • Fertility preservation, such as for cancer patients to preserve reproductive function, or freezing embryos at a younger age for potential transfer at an older age.

Suitable Embryos for Freezing:

  1. Cleavage Stage Embryos:
    • High-quality embryos with 2-8 cells (Grade 1 or Grade 2) and cytoplasmic fragmentation of less than 20% are suitable for freezing. Embryos with inconsistent blastomere sizes or high fragmentation rates may have reduced survival rates.
  2. Pronuclear Stage:
    • The fertilized egg must have an intact zona pellucida, healthy cytoplasm, and two clearly visible pronuclei. The timing of freezing is critical since the microtubule system, which forms during DNA synthesis and spindle assembly, is highly sensitive to temperature fluctuations. Improper freezing at this stage can lead to chromosomal dispersion. Therefore, pronuclear freezing must be done while the pronuclei are still clearly visible, typically within 20-22 hours post-fertilization.
  3. Blastocyst Stage:
    • High-quality blastocysts are selected for freezing on Day 5 or Day 6 based on the size of the blastocoel, the rate of development, and the quality of the inner cell mass (ICM) and trophectoderm (TE). High-quality blastocysts have an expanded blastocoel, a clearly visible ICM, and trophectoderm cells forming a continuous layer along the inner wall of the zona pellucida, which has thinned by this stage.

Ethical Considerations and Usage:

The decision on how to use frozen embryos is entirely up to the couple, and it can vary based on personal values and beliefs. Some couples may view embryos as just cells, while others see them as the beginning of life. Additionally, some couples may have emotional considerations about the relationship between the frozen embryos and their future children.

Regardless of the perspective, medical professionals require informed consent from both partners before any action is taken. The most common approach for using frozen embryos is thawing and transferring them for implantation.

Preservation Duration and Success Rates:

Frozen embryos are typically stored for around 5 years, although they can be preserved for longer periods. In fact, there was a case reported in the U.S. where a 20-year-old frozen embryo was successfully thawed, transferred, and resulted in the birth of a healthy child.

It is important to note that not all embryos survive the thawing process. However, with recent advancements in vitrification (a rapid freezing method), the survival rates have improved significantly:

  • Cleavage stage embryos have a thaw survival rate of 90%-95%.
  • Blastocyst stage embryos have a thaw survival rate of 95%-99%.