What is embryo freezing?
Embryo freezing, also known as embryo cryopreservation, is the process of freezing and storing fertilized eggs (embryos) that are produced during in vitro fertilization (IVF) to be used for a future pregnancy. The eggs collected during an IVF cycle are fertilized in the laboratory using the partner’s or a donor’s sperm. The resulting excess, viable, high-quality embryos that are not transferred during the IVF cycle are cryopreserved for later use.
Several factors can contribute to a patient deciding to freeze embryos, including:
- Those wishing to delay pregnancy for personal reasons such as focusing on their career or life events.
- A couple or individual undergoing IVF may desire to freeze untransferred embryos that can be used for later pregnancy attempts without having to undergo another ovarian stimulation and egg retrieval cycle.
- Women or men with a cancer diagnosis where potential, fertility-damaging cancer treatment is planned that will likely decrease future pregnancy success post-treatment.
- Patients who choose to donate embryos to other couples or individuals (third party reproduction), or donation for medical research purposes.
The process of embryo freezing and storage
The process begins with the female patient receiving hormone injections to stimulate her ovaries. After 9-11 days, a reproductive endocrinologist retrieves the eggs with the patient under mild sedation. The mature eggs collected during an IVF cycle are fertilized in the lab using either partner or donor sperm.
Depending on the age of the patient and embryo quality, embryo(s) are then transferred into the uterus during the same ovarian stimulation cycle (termed a fresh embryo transfer) to achieve pregnancy. Any remaining, high-quality embryos (blastocysts) are then frozen. In the event that a fresh embryo transfer is not desired or planned, it is possible to freeze all high-quality embryos for later use.
Vitrification is the preferred freezing (cryopreservation) process. This is a form of rapid freezing where the embryos are frozen quickly to avoid crystallization, which can damage the cell and decrease embryo quality. Another method for freezing embryos (not commonly used in most laboratories today) is a slow freezing method.
Following the freezing process the embryos are stored in liquid nitrogen where they will be viable in a frozen state indefinitely. All of the embryo’s biological activity is on hold, to resume once the embryo(s) are thawed for future use.
Compared with the slow freezing method, vitrification increases an embryo’s likelihood of survival during both the freezing phase and subsequent embryo thaw, which can improve implantation and overall pregnancy rate.
Preimplantation genetic testing of frozen embryos
Patients may elect to have preimplantation genetic testing done on embryos to determine if genetic disorders such as problems with chromosome numbers and single gene disorders are present. These can cause implantation failure, miscarriage or birth defects in a baby. This testing is usually performed on embryos before initial cryopreservation, and less frequently on frozen embryos that had not previously been tested.
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What is a frozen embryo transfer (FET)?
A frozen embryo transfer is the second part of the IVF procedure that helped create the embryo. This process involves thawing the frozen embryo(s) and transferring the embryo(s) into the uterus to accomplish pregnancy. The amount of time the frozen embryo was stored does not impact IVF success rates. Cryopreserved eggs and embryos have the potential to remain viable indefinitely.
The most common reason a patient may choose a frozen embryo transfer is that the patient has excess high-quality embryos from a previous IVF cycle. Research shows that babies born using FET or a fresh cycle embryo transfer are not more likely to experience health and developmental problems than babies born naturally. Studies in this regard are ongoing.
Embryo freezing benefits
Freezing the embryos created after retrieval during IVF enables a woman or couple to have multiple opportunities at a future pregnancy from a single IVF retrieval and fertilization cycle. Patients who prefer to delay having a child may benefit from freezing embryos for use at a later time. Although there is no guaranteed success, the freezing of all high-quality embryos for transfer in future cycle(s) may lead to an overall increase in pregnancy rates.
Embryo freezing can also promote the prospect of transferring one embryo for implantation. Known as elective single embryo transfer (eSET), this is the preferred form of frozen embryo transfer. It reduces the chance of having twins or more, called a multiple pregnancy, which carries increased health risks for the mother and child. Couples may be more likely to choose to transfer just one embryo if they have additional frozen embryos to use should the first transfer not result in a child.
Another benefit is that it can reduce the need for repeat ovarian stimulation cycles with their inherent risks from egg retrieval and ovarian stimulation. Embryos can also be frozen while waiting for test results from preimplantation genetic testing to determine the best embryo(s) to transfer.
Embryo freezing risks & considerations
There is a risk of damage to embryos in the freezing and thawing processes. It is estimated that about 5% of embryos do not survive the thaw process. As mentioned previously, sometimes the process can damage cells in an embryo, in which case the embryo would not be suitable for transfer. If this unlikely event occurs, any excess embryo(s) available can then be immediately thawed so the patient can undergo the planned embryo transfer.
Unused frozen embryos can bring up ethical issues for some couples and individuals when they have more embryos than they plan on using. Options can often include donation to other infertile couples, donation of embryos for research or choosing to discard them, which can be a difficult choice for some. Cost of storing frozen embryos is another consideration. Our doctors discuss these issues with all patients.