Choosing the Embryo Stage for Freezing
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Embryo freezing can be performed at two different developmental stages, depending on the medical strategy and embryo quality. At the cleavage stage, embryos have already divided into four to eight cells, about 72 hours post-fertilization. In contrast, at the blastocyst stage, embryos have developed into a more complex structure of 200–300 cells, usually five to seven days post-fertilization. The choice of stage depends on embryo viability, implantation likelihood, and the experience of the medical team. Freezing at the blastocyst stage is considered particularly effective, offering greater certainty about embryo quality and future survival.
Vitrification Process
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The most widely used freezing technique is vitrification, which involves ultra-rapid cooling to prevent ice crystal formation and preserve cellular integrity. Initially, a special cryoprotective agent (CPA) is added to the embryos, acting as an antifreeze to protect cell structure. The embryos are then placed directly into liquid nitrogen tanks at -196°C, where they remain in complete dormancy and stability for long-term storage. Each embryo is stored in a separate container (similar to a small straw), which is uniquely labeled to ensure full identification and traceability. This method boasts exceptionally high survival rates upon thawing.
Alternative Slow Freezing Method
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Although rarely used today, slow freezing is another cryopreservation technique. In this process, a smaller amount of CPA is added, and the embryos are placed in a specialized machine that gradually lowers the temperature over about two hours. They are then stored in liquid nitrogen for long-term preservation. While this method was widely used in the past, its application has significantly declined due to lower survival rates compared to vitrification.
What Happens After Freezing
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When the time comes to use the frozen embryos, the thawing process is carried out with great precision. The embryos are removed from liquid nitrogen and gradually returned to normal temperature. The cryoprotective agent is then carefully removed to prevent cellular stress. Finally, the embryos are assessed by the scientific team, and if they meet the required quality criteria, they are transferred into the woman’s uterus during a controlled embryo transfer cycle. A major advantage of this method is that embryos retain the biological age they had at the time of freezing, preserving their quality regardless of how much time has passed.
Embryo Freezing: Who It’s For
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Embryo freezing is a targeted solution for couples who wish to preserve their ability to start a family in the future. In cases where multiple embryos are produced during an IVF cycle, the surplus can be cryopreserved for future use, either to achieve a second pregnancy or as a backup in case of initial failure. This practice allows couples to plan their family development more comfortably and with less stress, while maintaining consistent chances of success.
Application in Preimplantation Genetic Testing (PGT)
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Embryo freezing is also essential in cases involving preimplantation genetic testing (PGT), as embryos are kept frozen until genetic analysis is completed. This period is used to extract valuable information that allows for the selection of embryos free from genetic abnormalities or inherited diseases. In this way, freezing actively supports the future child’s genetic safety, ensuring embryo transfer only involves healthy genetic material.
Fertility Preservation for Patients Undergoing Medical Treatments
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Patients about to undergo treatments such as chemotherapy, radiation, or surgeries that may impact fertility can turn to embryo freezing to preserve their reproductive potential. This preventive strategy offers hope and a solution to those facing serious health challenges, allowing them to maintain their family plans regardless of the illness’s progression.
Prevention of Ovarian Hyperstimulation Syndrome (OHSS)
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Embryo freezing can also be used to prevent ovarian hyperstimulation syndrome (OHSS), a serious IVF complication. By opting for a “freeze-all” strategy, immediate embryo transfer is avoided, allowing the body to return to normal and reducing the risk of complications. This enhances both the safety and overall success of the treatment.
Freezing in Cases of Elevated Hormonal Levels
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Freezing is also applied in women with elevated progesterone levels during the IVF cycle, especially when levels exceed 1.5 ng/ml on ovulation trigger day. This hormonal imbalance may reduce implantation chances by up to 20%, making embryo freezing the safest and most effective choice. Delaying embryo transfer and performing it in a hormonally stable cycle increases the chances of a successful pregnancy.
