Embryo Freezing 2024 | Purpose, Methods, and Results
What is embryo freezing?
Embryo freezing, also known as embryo cryopreservation, is a process used to freeze and store embryos for future use.
An embryo is an egg that has been fertilized by a sperm cell. This process is a way to help individuals achieve pregnancy and preserve fertility.
Why freeze embryos?
Embryos are often frozen after individuals undergo treatments to attempt pregnancy. Examples include in vitro fertilization (IVF) and intracytoplasmic sperm injection (ICSI).
These procedures fertilize eggs with sperm, and sometimes additional embryos are produced. Freezing these extra embryos may be chosen for future use if:
- The embryo transfer to the uterus is postponed or canceled after the egg has been fertilized.
- You wish to delay IVF to a later time.
- You want an option in case early fertility treatments fail.
Embryo freezing is also used to preserve fertility. For example, a woman diagnosed with cancer may want to preserve fertilized eggs before starting chemotherapy or radiation if the treatment could affect her ability to conceive.
What is the difference between egg freezing and embryo freezing?
Embryo freezing (cryopreservation) involves freezing a fertilized egg. Fertility programs may also offer egg freezing, which involves freezing unfertilized eggs.
Is IVF more successful with frozen embryos?
Scientists have yet to agree on whether pregnancy has a better chance with fresh or frozen embryos. Researchers continue to study this topic.
Steps of embryo freezing
The process of freezing embryos involves several stages, starting with stimulating the woman’s ovaries and ending with transferring the frozen embryos to the uterus. Here are the steps in detail:
Egg retrieval
The embryo freezing process begins with retrieving eggs from the woman. This is done by administering hormonal treatments to stimulate ovulation at a precise time, followed by fertility medications to increase the number of eggs produced. The doctor then retrieves the eggs using an ultrasound-guided device for accuracy.
Egg fertilization
The eggs are fertilized by one of two methods:
- IVF: Eggs are exposed to sperm in the lab until fertilization occurs, and embryos are formed. These embryos are monitored for the next six days, and healthy embryos are then frozen.
- ICSI: A single sperm is injected directly into an egg. This method is used when there is a sperm issue or previous IVF failure.
Embryo freezing
The goal of freezing embryos is to preserve them for future use. The biggest challenge is the water inside the cells, as it can freeze and form crystals that can burst the cell, leading to the loss of the embryo. To prevent this, water is replaced with a cryoprotectant before freezing using one of two methods:
- Slow freezing: Embryos are placed in sealed tubes, and their temperature is gradually lowered, which prevents the aging of the embryo cells and reduces the risk of damage. However, this method takes a long time and requires expensive equipment.
- Vitrification: Embryos are rapidly frozen using liquid nitrogen, preventing ice crystals from forming and protecting the embryos, thus increasing their survival rate during thawing.
Transferring embryos
When the parents wish to use frozen embryos, the embryos are thawed and transferred to the mother’s uterus using one of two methods:
- With hormonal treatments: The mother receives estrogen for two weeks to build up the uterine lining, followed by progesterone injections or suppositories to prepare the uterus to receive the embryo. The embryo transfer is scheduled based on when progesterone treatment starts and the stage at which the embryo was frozen.
- Without hormonal treatments: The doctor monitors the woman’s natural cycle and waits for ovulation. The embryo is then transferred to the uterus three to five days after ovulation, depending on the stage of embryo freezing.
After the transfer of thawed embryos, a blood pregnancy test is performed about 10 days later to confirm whether the procedure was successful.
What happens before embryo cryopreservation?
You must provide your consent for embryo freezing. Your doctor will give you consent forms to read and sign. The paperwork should include details such as:
- The number of embryos to be frozen.
- The storage duration (typically 10 years).
The doctor may also help you decide which embryonic stage is best for freezing one or more embryos. Stages include:
- Cleavage stage: When a single cell divides into four to eight cells, around 72 hours post-fertilization.
- Blastocyst stage: When the single cell has divided into 200 to 300 cells, five to seven days post-fertilization.
What happens during embryo cryopreservation?
There are two methods for freezing embryos: vitrification and slow freezing.
In vitrification, fertility specialists:
- Add a cryoprotectant (CPA) to the embryos. CPA acts like an antifreeze and protects the cells from ice crystals.
- Place the embryos directly into tanks containing liquid nitrogen at -321°F (-196.1°C).
What happens after embryo freezing?
When frozen embryos are needed later, fertility specialists:
- Remove the embryos from the liquid nitrogen.
- Slowly return them to normal temperature.
- Soak them to remove the cryoprotectant (CPA).
- Use the embryos according to the required procedure.
What are the benefits of embryo cryopreservation?
Embryo freezing can help individuals conceive later in life if they face current barriers such as:
- Aging
- Infertility issues.
- Social/personal reasons, such as pursuing higher education or professional demands and planning to delay pregnancy for several years.
- Treatments that may damage fertility (e.g., chemotherapy or pelvic radiation for cancer).
Are there risks or complications with this procedure?
Embryo freezing does not pose risks of birth defects or health problems for the resulting pregnancy. Studies on frozen and thawed embryos show lower rates of premature birth, low birth weight, growth restriction, and perinatal mortality.
The main risks associated with embryo cryopreservation include:
- Damage to embryos during the freezing process.
- Embryos that are not viable for freezing.
- Failure to achieve pregnancy after thawing and transferring the embryos.
- Increased rates of medical complications during pregnancy, such as preeclampsia and placenta accreta spectrum.
- Multiple births if more than one embryo is transferred (twins or triplets).
Is frozen embryo transfer successful?
Frozen embryo transfer occurs when the embryo is thawed and implanted into the woman’s uterus. This procedure is often successful, but success rates vary significantly based on many factors, including:
The overall health of both parents.
The mother’s age at the time of egg retrieval.
The presence of fertility issues such as endometriosis, fibroids, or uterine polyps.
The success or failure of previous fertility treatments and pregnancies.
Your doctor will help you understand the factors that may affect your chances of success.
What if I don’t use the frozen embryos?
If you don’t use the frozen embryos, you can:
Dispose of them. The fertility clinic will remove them from the freezer and allow them to thaw, making them non-viable.
Donate them to research.
Donate them for educational purposes (e.g., to help future fertility specialists).
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