Thawing of embryos (optional)

Last updated date: 31-Aug-2023

Originally Written in English

Thawing of Embryos (Optional)



After an in vitro fertilization (IVF) procedure, many couples choose to cryopreserve their frozen embryos. Due to concerns about ovarian hyperstimulation syndrome or excessive progesterone, they could freeze the embryos immediately after treatment before moving through with a transfer. Others save their embryos for later use or in case their first round is unsuccessful. In either case, each frozen embryo must be thawed in the same way before being used in a FET cycle. Many questions regarding the thawing process accompany the worry and excitement.

The opposite of freezing, embryo thawing entails bringing the embryos to ambient temperature before transferring them back into culture media at 37°C in a CO2 incubator. The embryos are then prepared for uterine implantation. On the day of the anticipated embryo transfer, the frozen embryos are reheated.

The thawing of an embryo takes about two hours. The freezing technique can occasionally cause damage to specific embryonic cells. Even some freeze damage to an embryo can result in a successful pregnancy, but the more damage an embryo has, the less likely it is to develop into a baby. Sometimes the embryo's cells sustain damage from all directions. The embryo in this instance won't be transferred.


What is Embryo Thawing

The process of bringing embryos back to their normal temperature involves removing the cryoprotectant molecules that allowed for preservation at -196°C and replacing them with intracellular water. This stimulates biological activity once more.


Is Thawing Process Safe and Successful?

Thawing Process

Researchers are aware that frozen embryos are priceless since they are the future offspring of the patients. Many patients are curious about the safety of the embryo warming procedure. Yes, it is the answer. Usually, frozen embryos are not harmed by the process. Furthermore, there is no proof that infants born from frozen and thawed embryos have any developmental or health problems.

Patients frequently wonder how many embryos make it through the freezing process and go on to become pregnant after being transferred. The quality of each embryo influences the outcome in part. Genetics and biology, which are outside the control of the laboratory, are what determines this.

The expertise and quality of the facility conducting the procedure, however, is another aspect that affects an embryo thaw's outcome. The laboratories adhere to the highest industry standards and demonstrate expertise in freezing and thawing operations. Because of this, the labs have excellent thaw survival rates, which increases your chances of having children. Get in touch with your doctor if you have any questions about IVF or conceiving with frozen embryos. You will see how prioritizing patients and science make motherhood possible.


Embryo Thawing Techniques

Embryo Thawing

The freezing method that was employed affects the embryo thawing procedure itself. Slow freezing and vitrification are the two types of cryopreservation. In the former, low doses of cryoprotectants and a progressive drop in temperature are used to create ice crystals in a controlled manner. Thus, during thawing, the physiological temperature is gradually and slowly recovered, and then the cryoprotectants are replaced with water.

On the other side, vitrification involves reducing the concentration of cryoprotectants and speeding up cooling to prevent the production of the aforementioned ice. Devitrification, therefore, involves a fast-heating process (from -196°C to 37°C in less than 1 second), which is followed by the necessary replacement of cryoprotectants with water.


Embryo Thawing Purpose

Embryo Thawing Purpose

  • Any excess embryos that were not transferred but reached the cleavage or blastocyst stage (day 5) after a couple has had an embryo transfer are vitrified, or frozen, to preserve them.
  • Increasing the chance of conception when new cycles fail or when parents desire more kids following a successful embryo transfer.
  • Supplying an alternative for partners who would transfer excessive numbers of embryos and run the risk of multiple gestational pregnancies.
  • Prevent embryo wastage by specifically freezing embryos for effective use.
  • Increasing by 10-30% the number of normal outcomes pregnancies per retrieval cycle. Numerous studies have assessed the children of frozen embryos. The outcome has generally been favorable, with no rise in birth malformations or abnormalities of development.
  • It makes it possible to transfer embryos with precaution.
  • Additionally, it establishes a cumulative pregnancy rate, raising the likelihood that patients will become pregnant following an IVF cycle overall.
  • Helping patients in managing problems like Ovarian Hyperstimulation Syndrome (OHSS), hence reducing or eliminating the danger of ovarian hyperstimulation syndrome.
  • Compared to having to repeat the ovarian stimulation necessary for a fresh embryo transfer (ET), it is less expensive and less invasive.
  • To increase pregnancy yields after just one cycle of ovarian stimulation.
  • No danger of ovarian hyperstimulation.


Do Freezing and Thawing Affect the Embryo

An embryo may occasionally need to be re-vitrified. Such circumstances occur when a couple agrees in writing to the thawing and transfer of two embryos but afterward changes their minds. The additional embryo in this case needs to be frozen for future use. It is possible to re-vitrify an embryo as long as it resists the thaw, keeps growing, and is of good quality. The embryo, however, is subject to the same rules and might not withstand a subsequent thaw. Every situation is unique, just as every embryo is. The technology and protocol for vitrification typically work quite well, and re-vitrification is often without issues.


When to Thaw the Embryo

Thaw the Embryo

On the day of the operation, the frozen embryo is typically thawed. The embryo could develop out of sync with the lining if it was thawed the previous night. The timing of everything makes it crucial that the instructions are followed, especially concerning consent and the required number of embryos to thaw.

Embryos of the highest overall quality are chosen first, then gradually lessen. It's not always true that the B or the less expanded is poor and won't take just because one has A quality and the other has B quality or one is more expanded than the other. It's critical to keep in mind that only frozen embryos that are likely to contribute to a possible viable pregnancy will be transferred.


Embryo Thawing Procedure

Frozen embryos

Embryo thawing is the procedure that allows frozen embryos to be used once more. The actual technique reverses the freezing process.

The best embryos are chosen for freezing and subsequent transfer because they have the best chances of resulting in pregnancy. That doesn't mean any particular embryo is superior to another. It's crucial to keep in mind that only the best embryos were chosen before the transfer and freezing procedures.

An embryologist removes the embryos from the freezer (where they are kept at -196 degrees Celsius) and warms them to body temperature. It takes two to three seconds for the complete frozen embryo to thaw. The possibility of ice formation in the embryo's cells during warming is avoided by rapid defrosting. Following that, the embryologist removes the cryoprotectant from the embryos and replaces it with the water that was removed during the freezing procedure.

This is accomplished by incubating the embryos in cryoprotectant concentrations that are decreasing while water concentrations are increasing. The embryos are injected through various solutions for 20 minutes. It is carried out before water completely replaces the cryoprotectant. The embryos can be prepared for transfer in less than 40 minutes after being taken out of the freezer. Embryo thawing is one of the most popular post-procedure and as technology advances, demand is only expected to increase in the future.


Embryo Thawing Risks

Risk of Pregnancies

Pregnancies that arise from embryo thawing don't run the risk of having congenital defects or other health issues. Preterm delivery, low birth weight, growth retardation, and perinatal mortality rates are reduced in frozen-thawed embryo outcomes research. The following are the primary risks of embryo thawing:

  • Damage caused by thawing to embryos.
  • Embryos that are not viable for thawing.
  • After thawing and implanting embryos, failure to become pregnant.
  • Preeclampsia and placenta accrete spectrum cases among other pregnancy-related medical conditions are on the increase.
  • Multiple births from more than one implanted embryo (twins or triplets).


Embryo Quality After Thawing

Embryo Quality

The number of viable or growing embryos on the day of transfer divided by the total number of thawed was used to calculate the thaw survival rate. The number of viable cells in an embryo after thawing was used by some researchers to quantify embryo survival. If more than 50% of the cells are viable, the embryo has survived.  A frozen embryo is said to have partially survived if at least 50% of its cells are still alive and to be atretic if all of its cells are dead.

Embryo morphology, or how the cells look to how much of them are fragmented, is one of the most important variables in determining whether an embryo will survive. When frozen, embryos with 2, 4, or 8 cells have a survival rate that is 5-10% higher than those with an odd number of cells.

According to certain studies, blastocyst survival rates were 88% and 85% for day 3 embryos, respectively. According to Veek et al., 76% of blastocysts managed to survive thawing. When thawed day 3 embryos and blastocysts were compared, Langley et al. found that blastocysts had a greater survival rate and later implantation rate.


Pregnancy and Live Birth Rate


The number of gestational sacs per number of transplanted embryos was used to define the implantation rate. When a fetus leaves the mother's body and exhibits signs of life, it is said to have been born alive. Early embryos that have undergone cryopreservation implant at the same rate as their fresh counterparts at the current level of ART. Cryopreserved embryos have consistently shown favorable results, with no rise in birth malformations or developmental anomalies. In contrast to embryo transfer during the same cycle as ovarian hyperstimulation, pregnancy rates are higher after frozen embryo transfer and perinatal outcomes are less impacted.

Frozen embryo transfer is available for a different cycle to focus on increasing the likelihood of successful implantation because it is thought that the endometrium is not ideally prepared for implantation after ovarian hyperstimulation.

Furthermore, just like embryo survival, complicated interactions also influence pregnancy rates, and only 7-10% of the predictive value can be observed and assessed. Fewer older women have frozen embryos, even though age is not a significant determinant. The number of surviving embryos transferred, the number of 100% surviving embryos transferred, and the morphology scores of the transplanted embryos are the most crucial criteria predicting pregnancy rates out of the roughly 20 factors analyzed. The frequencies of pregnancies delivered ranged from 5% (one bad quality embryo) to 36% (for excellent quality embryos).

For day 3 embryos and blastocysts, the implantation rate per number thawed was 12% and 14%, respectively. For day 3 embryos and blastocysts, the implantation rate per number of embryos that survived the thaw was 14% and 15%, respectively. For day 3 embryos and blastocysts, the implantation rate per transfer cycle was 44% and 38%, respectively.

According to Mary E.P. et al., there were 288 live births overall for day 3 embryos, including 175 singletons, 53 sets of twins, and 2 sets of triplets, for a live birth/thawed embryo rate of 10%. There were 37 singletons, 11 sets of twins, and 0 triplets among the blastocysts, resulting in 59 live births overall and a 12% live birth/thawed embryo rate.

According to the Genetics & IVF Institute in Washington, blastocysts (embryos that are cultured for 5 days as opposed to 2-3) are the exception. The larger embryos have unique needs for freezing without causing any harm. The rates of pregnancy and blastocyst cryo-survival have been challenging for several centers. With a newly developed protocol called vitrification with a closed system, there was a 62% transfer rate and a 35% pregnancy rate per transfer. Since excess blastocysts can be anticipated to produce pregnancy rates equivalent to embryos frozen two to three days following retrieval, this significant change now renders blastocyst transfer more attractive.



There is a significant probability that you will have more than one embryo after in vitro fertilization (IVF). In this situation, one of the healthy embryos will be transferred by your doctor. You can decide to freeze and keep the extra embryos to use them in subsequent pregnancy attempts. The embryologists at the fertility center will begin making preparations for the embryo thawing procedure as soon as you decide to try for a second child. The laboratory workers and embryologists are all skilled in freezing and thawing techniques. In these cutting-edge IVF labs, they will use the most recent, tried-and-true methods to give you a better chance of realizing your dream of becoming a parent with your frozen embryos.