What is Embryo Cryopreservation?
Embryo cryopreservation, also known as embryo freezing, is an advanced laboratory technique that preserves embryos at extremely low temperatures for future use.
After eggs are retrieved and fertilized in the laboratory during an in vitro fertilization (IVF) cycle, the resulting embryos are cultured for several days—typically until they reach the blastocyst stage (about 5–7 days after fertilization).
At this point, healthy embryos can be selected for freezing. The most common method used today is vitrification, a rapid-freezing process that prevents the formation of ice crystals, which can damage delicate cellular structures.
Frozen embryos are stored in liquid nitrogen tanks at -196°C (-321°F), where biological activity is essentially halted, allowing them to remain viable for many years without degradation.
When a patient is ready for pregnancy, the frozen embryos can be thawed and transferred into the uterus during a carefully timed cycle. The success rate after thawing is very high with modern vitrification techniques, and in many cases, pregnancy rates from frozen embryo transfers are comparable to, or even better than, fresh transfers due to optimal preparation of the uterine environment.
Embryo cryopreservation is a cornerstone of modern reproductive medicine, offering flexibility, safety, and hope for individuals and couples who wish to preserve their fertility and family-building options.
When is Embryo Cryopreservation Needed?
You may consider embryo cryopreservation in situations such as:
- Surplus embryos from an IVF cycle, kept for future pregnancy attempts.
- Waiting for preimplantation genetic testing (PGT) results before transfer.
- Fertility preservation before medical treatments like chemotherapy or radiation.
- Endometrial lining not optimal for embryo transfer during the IVF cycle.
- Personal or scheduling reasons delaying immediate embryo transfer.
Embryo Cryopreservation – Step-by-Step Overview
1. Egg Retrieval (Oocyte Retrieval)
After controlled ovarian stimulation, mature eggs are collected from the ovaries through a short, minimally invasive procedure under light anesthesia.
- Purpose: Obtain high-quality eggs for fertilization.
- Typical timing: About 34–36 hours after the trigger injection (hCG or GnRH agonist).
2. Fertilization
The retrieved eggs are fertilized with sperm in the laboratory. Two main methods are used:
- Conventional IVF: Sperm and eggs are placed together in a culture dish, allowing natural fertilization.
- ICSI (Intracytoplasmic Sperm Injection): A single healthy sperm is directly injected into each mature egg, often used when sperm quality or quantity is low.
- Purpose: Create viable embryos for future transfer.
3. Embryo Screening (Optional)
Some embryos undergo Preimplantation Genetic Testing (PGT) before freezing:
- PGT-A: Detects abnormal chromosome numbers.
- PGT-M: Screens for specific inherited genetic disorders.
- PGT-SR: Detects structural chromosomal rearrangements.
- Purpose: Increase the chances of a successful pregnancy and reduce the risk of genetic diseases.
4. Freezing (Vitrification)
Healthy embryos are cryopreserved using vitrification, a rapid-freezing method that prevents ice crystal formation and preserves cellular integrity.
- Stored in liquid nitrogen tanks at -196°C (-321°F).
- Purpose: Maintain embryo viability for years without biological degradation.
5. Thawing (Warming)
When ready for pregnancy, embryos are gently warmed in a controlled process to restore them to their pre-freeze state.
- Purpose: Ensure embryos remain healthy and ready for transfer.
- Survival rate: Over 90% with modern vitrification methods.
6. Embryo Transfer
The thawed embryo(s) are placed directly into the uterus using a thin catheter.
- Purpose: Achieve implantation and pregnancy.
- Timing: Usually done in a medicated or natural cycle to optimize uterine receptivity.
Why is Embryo Cryopreservation Important?
- Preserves fertility for individuals undergoing cancer treatment or other medical procedures that may harm reproductive potential.
- Allows flexibility in family planning by storing surplus embryos for future pregnancies.
- Supports genetic testing and safe timing of embryo transfer after PGT results are available.
- Optimizes pregnancy rates by enabling transfers when the uterus is most receptive.
- Provides security for those who may not be ready to start a family immediately but want to maintain their biological options.
Key Advantages
- Preserves reproductive potential for future use.
- Flexibility in planning pregnancy timing.
- High survival rate with modern vitrification methods.
- Allows for PGT results to guide embryo selection.
Important Considerations
- Requires an IVF cycle to create embryos before freezing.
- Long-term storage is generally safe but may involve ongoing storage fees.
- Success rates depend on embryo quality and maternal age at the time of retrieval.