How does the embryo freezing procedure work?
La freezing of embryos, technically known as embryo cryopreservation, is a process by which embryos created during an in vitro fertilization (IVF) cycle are preserved at extremely low temperatures. This procedure begins after the successful fertilization of the egg with sperm in the laboratory, when the embryos have reached the appropriate stage of development, usually between day 3 (cell stage) and day 5 or 6 (blastocyst stage).
The most commonly used method today is vitrification, an advanced technique that converts embryos to a glass-like state without forming ice crystals that could damage cells. Embryos are exposed to high concentrations of cryoprotectors, substances that protect cells during the freezing process, and are then rapidly immersed in liquid nitrogen at a temperature of approximately -196°C. According to data from the American Society for Reproductive Medicine (ASRM), this modern method offers embryo survival rates greater than 90%, a significant advance over previous techniques (Trusted Source).
Once vitrified, embryos can be safely stored for years without significant deterioration in their viability. The entire process is carried out under strictly controlled conditions in specialized IVF laboratories, where each step is supervised by expert embryologists.
What happens during the embryo thawing process?
When it's time to use the frozen embryos, a carefully controlled thawing process takes place. The embryos are removed from storage in liquid nitrogen and undergo a gradual heating protocol. The cryoprotectors are gradually eliminated and replaced with suitable solutions that allow the embryo to resume its normal metabolic activity.
After thawing, embryologists evaluate the viability of embryos by looking at the integrity of their cells and their ability to resume development. Embryos that successfully survive this process are considered suitable for embryo transfer. This procedure is performed using a thin catheter inserted through the cervix to place the embryo in the uterus, a generally quick and painless process that does not require anesthesia.
Accuracy and optimal timing are crucial to maximizing the chances of success. Specialists carefully determine when to perform the transfer based on the woman's natural menstrual cycle or through a hormonally controlled cycle to ensure that the endometrium (the lining of the uterus) is in optimal condition for implantation.
What preparation is required before embryo freezing?
Before starting a cycle that involves freezing embryos, a full medical evaluation is required. This includes blood tests to check hormone levels, detection of infectious diseases, evaluation of ovarian reserve through the AMH test and ultrasound to examine the reproductive organs.
The patient will receive medications for ovarian stimulation, administered primarily by injections for approximately 8-12 days. These medications are designed to stimulate the ovaries and promote the development of multiple follicles, each containing an egg. During this period, regular monitoring is performed using ultrasound and blood tests to adjust doses as needed and determine the optimal time for egg retrieval.
Egg retrieval is performed using an outpatient procedure under light sedation. Using an ultrasound guided needle, the doctor removes mature eggs from the ovarian follicles. These eggs are then fertilized in the laboratory, either by Conventional IVF or ICSI (intracytoplasmic sperm injection), depending on the particular circumstances of each case. The resulting embryos are cultured for several days before selecting the most viable ones for cryopreservation.
How long can embryos stay frozen?
Correctly cryopreserved embryos can remain viable for extremely long periods. Although there are documented cases of successful births from embryos that were frozen for more than 25 years, there is no definitive biological limit to how long they can remain in storage without losing their potential.
The longevity of frozen embryos depends primarily on the stability of storage conditions in liquid nitrogen. The World Health Organization indicates that, with modern methods of vitrification and adequate storage, there is no scientific evidence to suggest a significant deterioration in embryonic viability over time (Trusted Source).
However, it is important to note that many countries set legal limits on the length of storage of frozen embryos. In Mexico, these regulations may vary, so it's critical to discuss the legal and administrative implications of long-term storage with the medical team at your fertility clinic.
What are the main benefits of freezing embryos?
Embryo freezing offers multiple significant benefits for patients undergoing fertility treatments. One of the most prominent is the preservation of fertility for the future, allowing individuals or couples to postpone parenthood without biological clock pressure. This option is especially valuable for those facing medical treatments that could affect their reproductive capacity, such as chemotherapy or radiation therapy.
From a medical perspective, cryopreservation reduces the need to repeat the process of ovarian stimulation and egg retrieval, reducing the risks associated with these procedures. This is particularly relevant for patients with conditions such as ovarian hyperstimulation syndrome (OHS), where immediate transfer could compromise the patient's health. A study published in the journal Fertility and Sterility demonstrated that success rates with transfer of frozen embryos may be even higher than those of fresh embryos in certain cases, because they allow the body to recover completely after hormonal stimulation (Cobo et al., 2022).
Cryopreservation also facilitates long-term family planning, providing the possibility of completing the family in the future using embryos from the same genetic cohort. This means that siblings can be genetically complete even if they are born years apart, an important consideration for many families.
How does freezing embryos improve fertility success rates?
Embryo freezing technology has revolutionized fertility treatments by allowing better synchronization between embryonic development and uterine receptivity. In conventional IVF cycles, the uterus may not be in optimal condition for implantation due to the effects of ovarian stimulation medications. Cryopreservation allows the transfer to be performed in a later cycle when the uterine environment has normalized.
Advances in vitrification techniques have dramatically improved embryo survival rates after thawing, currently reaching more than 95% in many first-tier clinics. The European Society for Human Reproduction and Embryology (ESHRE) reports that pregnancy rates with frozen embryo transfer have been equated to or even surpassed those of fresh embryos in numerous centers (Trusted Source).
Freezing also makes it easier to carry out preimplantation genetic testing (PGT), which allow embryos to be tested for chromosomal or genetic abnormalities before transfer. This significantly increases the chances of a healthy pregnancy and reduces the risk of miscarriages, especially in older women or women with a history of recurrent pregnancy loss.
Who is embryo freezing especially suitable for?
Embryo cryopreservation is particularly beneficial for diverse patient groups. People who are facing cancer treatments can preserve your fertility before undergoing therapies that could harm your reproductive function. Women at high risk of ovarian hyperstimulation syndrome, a potentially serious complication of hormonal stimulation, can avoid fresh transfer and significantly reduce their health risks.
Patients with endometriosis, adenomyosis, or other conditions affecting endometrial receptivity may benefit from deferred transfer, allowing for specific treatments to optimize the endometrium before transfer. For those who require female fertility tests or advanced male, freezing provides valuable time to perform comprehensive analyses without pressure.
Cryopreservation is also essential in programs of egg donation, allowing for quarantine and the necessary tests to ensure the safety of the process. Couples who want to space out the births of their children while using genetically related embryos find this technique an ideal solution for their family planning.
What success rates can you expect with frozen embryos?
Success rates with cryopreserved embryos have improved markedly thanks to advances in vitrification techniques. Currently, data indicate that frozen embryo transfers offer results comparable to or even superior to those of fresh embryos in many cases. According to statistics from the American Society for Assisted Reproductive Technologies (SART), live birth rates from frozen embryo transfer range from 40-50% for women under 35, gradually decreasing with maternal age.
Several factors influence the chances of success, the most decisive being the woman's age when the embryos were created (not when they were transferred), the embryo quality before freezing, and the laboratory's experience with cryopreservation techniques. The transfer of blastocysts (day 5-6 embryos) generally offer better results than embryos in the division stage (day 2-3).
It's important to note that freezing itself does not negatively affect the development potential of a good-quality embryo. A meta-analysis published in Human Reproduction Update confirmed that babies born from cryopreserved embryos have no higher incidence of congenital anomalies or developmental problems compared to those conceived naturally or through IVF with fresh embryos.
How are embryos selected for freezing?
The selection of embryos for cryopreservation follows strict criteria based on their quality and development potential. Embryologists evaluate various morphological parameters such as the number and symmetry of cells, the degree of cell fragmentation, and in more advanced stages, the formation of the inner cell mass and the trophoectoderm in blastocysts.
Classification systems vary between clinics, but generally use numerical or alphabetical scales to categorize embryos. Only those that meet specific quality standards are considered suitable for cryopreservation. This meticulous selection is crucial, as better-quality embryos are more likely to survive the freeze-thaw process and achieve a successful pregnancy.
In many advanced centers of fertility treatments, technologies such as time-lapse imaging are implemented, which allows continuous embryonic development to be observed without disturbing culture conditions. This provides additional information on the kinetics of embryonic development, improving accuracy in selecting the best candidates for cryopreservation.
What freezing techniques are currently being used?
Vitrification has established itself as the method of choice for embryo cryopreservation, almost completely replacing traditional slow freezing. This process involves exposing embryos to high concentrations of cryoprotectors followed by ultrafast cooling that transforms cellular water into a vitreous state without the formation of ice crystals.
The effectiveness of vitrification lies in its ability to minimize cell damage during the freezing process. Embryo survival rates after vitrification and thawing consistently exceed 90%, whereas with previous slow freezing methods they ranged from 60-70%. This significant difference translates directly into better clinical outcomes.
Continuous innovations in cryoprotective media and storage devices have further optimized this technique. Closed vitrification systems, which avoid direct contact with liquid nitrogen, have added an additional level of safety, reducing the theoretical risk of cross-contamination during long-term storage in embryo banks.
How does the uterus prepare for receiving frozen embryos?
Endometrial preparation for a frozen embryo transfer can be performed following different protocols, adapted to the specific needs of each patient. The fundamental objective is to synchronize endometrial development with the age of the embryo to be transferred, ensuring optimal receptivity for implantation.
In a modified natural cycle, the patient's spontaneous ovulation is monitored using ultrasound and hormonal analysis. The transfer is scheduled considering the day of embryo development in connection with ovulation. This approach minimizes the medication needed, but requires closer monitoring and may be less predictable.
Alternatively, substituted or artificial cycles use estrogen and progesterone to prepare the endometrium in a controlled manner. This protocol usually begins with estradiol in increasing doses to promote endometrial growth, followed by the addition of progesterone when the endometrium reaches an adequate thickness (typically 7-9 mm). The transfer is scheduled according to the duration of exposure to progesterone, which must match the age of the embryo.
In some special cases, particularly in patients with refractory endometrium or uterine pathologies, additional treatments such as endometrial scratching, hyaluronic acid therapy or endometrial preparation with platelet-derived growth factors can be implemented to improve receptivity.
What happens on the day of the frozen embryo transfer?
The day of the transfer of frozen embryos represents a culminating moment in fertility treatment. The day begins in the embryology laboratory, where the selected embryos are thawed following precise protocols. After thawing, embryologists evaluate the survival and quality of the embryos, confirming that they maintain their structural integrity and development potential.
The transfer procedure itself is relatively brief, lasting approximately 10-15 minutes. It does not require anaesthesia and the patient remains awake throughout the process. Using ultrasound guidance, the doctor inserts a thin, flexible catheter through the cervix until the optimal location in the uterine cavity is reached. The embryo, suspended in a small volume of culture medium, is carefully deposited in this location.
After the transfer, the patient usually rests in a horizontal position for a short time, although recent studies suggest that prolonged rest does not significantly influence implantation rates. Normal activities can be resumed gradually, avoiding strenuous exercise, immersion baths and sexual intercourse until the pregnancy test, which is usually performed approximately 10-14 days after the transfer.
What ethical and legal aspects surround the freezing of embryos?
Embryo cryopreservation raises important ethical and legal considerations that vary significantly depending on the country and its regulatory frameworks. In Mexico, legislation on assisted reproductive technologies is evolving, so it is essential to stay informed about current regulations. A fundamental aspect is informed consent, where both parents must clearly state their decisions about the fate of cryopreserved embryos.
Among the most complex issues is determining the legal status of frozen embryos, especially in situations of divorce, death of one or both parents, or when the storage time limit is reached. Clinics must establish clear protocols for these contingencies, detailing the options available: maintenance of cryopreservation, use in subsequent cycles, donation to other couples, donation for scientific research, or discontinuation of storage.
La reproductive autonomy and the rights to cryopreserved embryos have been the subject of intense ethical debates and, in some cases, legal disputes. It is critical that people who choose embryo cryopreservation fully understand the long-term implications and document their preferences through legally binding agreements.
What are the costs associated with freezing and storing embryos?
The economic aspect is an important consideration in the cryopreservation of embryos. Costs are usually structured around two main components: the initial freezing procedure and continuous storage fees. The vitrification process generally has a one-time cost that includes laboratory processing, cryoprotective media, and specialized technical work.
Storage represents a recurring expense, normally billed annually, that covers the maintenance of liquid nitrogen tanks, monitoring systems, specialized personnel and safety measures to ensure optimal conditions. These costs vary considerably between clinics and regions, so it's wise to research and compare options.
Some institutions offer long-term payment plans with discounts for extended periods of storage. It is important to consider these expenses as part of the overall fertility treatment budget and to check if any portion is covered by health insurance or specific programs for preservation of fertility, especially in oncological cases.
Are there risks associated with freezing embryos?
Advances in cryopreservation techniques have significantly minimized the risks associated with freezing embryos. However, it's important to be aware of potential complications, even if they're rare. The main technical risk is that not all embryos survive the freeze-thaw process, although with modern vitrification, survival rates exceed 90%.
From a clinical outcome perspective, extensive studies have demonstrated that babies born from cryopreserved embryos have no higher incidence of congenital anomalies or developmental problems compared to those conceived naturally or through IVF with fresh embryos. In fact, some studies suggest that babies born from frozen embryo transfers may have a slightly higher birth weight and lower risk of low birth weight compared to fresh transfers.
It is essential to note that the obstetric risks associated with pregnancies of IVF do not increase specifically due to the use of cryopreserved embryos. The rates of complications such as preeclampsia, gestational diabetes or premature birth are comparable to those of other assisted reproduction treatments.
What other options are there to preserve fertility?
Fertility preservation encompasses a variety of strategies adapted to different personal and medical circumstances. The cryopreservation of eggs (oocytes) has advanced significantly thanks to vitrification techniques, offering a valuable alternative for women without a current partner or who prefer not to create embryos for personal or ethical reasons. This option provides greater future flexibility when it comes to choosing a partner.
For men, the sperm freezing constitutes a simple and effective procedure. It is especially recommended before treatments that may affect fertility, such as chemotherapy, radiation therapy or pelvic surgery. The process is fast, non-invasive and offers high post-thaw viability rates.
In specific cases where there is a risk of ovarian involvement, such as oncological treatments, cryopreservation of ovarian tissue represents an emerging option. This technique involves the surgical removal and freezing of ovarian cortex fragments, which can then be reimplanted to restore ovarian function. Although considered experimental in some contexts, it has enabled successful births and offers the potential advantage of restoring natural fertility and hormone production.
What's the difference between freezing embryos and freezing eggs?
The main difference between cryopreservation of embryos and eggs lies in the stage of the reproductive process at which it takes place. Freezing eggs preserves unfertilized female reproductive cells, while freezing embryos preserves already fertilized eggs that have initiated cell development.
From a technical perspective, eggs are more susceptible to damage during cryopreservation due to their high water content and specific cellular characteristics. However, modern vitrification techniques have significantly improved results. Post-thaw survival rates for eggs range from 80-90%, slightly lower than those for embryos (> 90%). Consequently, a greater number of eggs than embryos are generally required to achieve a live birth.
The legal and ethical implications also differ significantly. Egg freezing offers greater autonomy to women, while frozen embryos usually require legal agreements regarding their future disposition, especially in the event of a separation of the couple. For women who want to preserve their fertility for social or medical reasons without having a partner or because of personal preferences, the vitrification of eggs represents an increasingly accessible and effective option.
Who can benefit from combined preservation options?
Combined fertility preservation strategies offer customized solutions for complex situations. Couples subject to IVF treatments with multiple embryo production, they may choose to freeze some embryos while transferring others fresh, maximizing the chances of immediate success while preserving future options.
Oncology patients with limited time before starting treatments can benefit from multi-prognostic approaches, combining different techniques depending on their particular situation. In women, this may include emergency ovarian stimulation for cryopreservation of eggs or embryos along with preservation of ovarian tissue in some cases.
Couples who require pre-implantation genetic diagnosis often use cryopreservation as an integral part of the process. The embryos are biopsied and frozen while genetic analysis is being performed, allowing for the subsequent selective transfer of embryos without specific chromosomal anomalies or mutations.
Transgender individuals who initiate hormone therapy or gender-affirming surgery can preserve their fertility by freezing gametes before these procedures. The clinics that offer transgender fertility specialized protocols provide adapted to the specific needs of this population.
References
- Cobo, A., Bellver, J., de los Santos, M.J., & Remohí, J. (2022). Vitrification of human oocytes and embryos: Long-term outcomes and current perspectives. Fertility and Sterility, 117 (3), 502-515. https://doi.org/10.1016/j.fertnstert.2021.12.023 Trusted Source.
- Practice Committees of the American Society for Reproductive Medicine and the Society for Assisted Reproductive Technology. (2023). Cryopreservation and thawing of human embryos: a guideline. Fertility and Sterility, 119 (5), 850-863. https://doi.org/10.1016/j.fertnstert.2023.01.026 Trusted Source.
- Argyle, C.E., Harper, J.C., & Davies, M.C. (2022). Frozen embryo transfer: Evidence and clinical recommendations. Human Reproduction Update, 28 (1), 92-111. https://doi.org/10.1093/humupd/dmab037 Trusted Source.
