Embryo freezing is a procedure that allows people to store embryos for later use. A person can also freeze eggs, which are not fertilized. An embryo forms after fertilization and after the cells start to divide.
The first successful pregnancy resulting from freezing a healthy embryo took place in the 1980s. Since then, many people have frozen embryos and used them later.
A person may decide to store an embryo if they hope to become pregnant in the future, to donate to others, for medical research, or for training purposes.
The process begins by using hormones and other medications to stimulate the production of potentially fertile eggs. A doctor then extracts the eggs from the ovaries, either for fertilizing in a lab or for freezing.
Successful fertilization may lead to at least one healthy embryo. A doctor can then transfer the embryo to the womb, or uterus. If the treatment is successful, the embryo will develop.
Fertilization often results in more than one embryo, and the doctor can freeze and preserve the remaining embryos.
According to an article published in the journal Human Reproduction, the medical community uses the term embryo from the moment that cells divide after fertilization until the eighth week of pregnancy.
Before freezing can take place, people need to create suitable embryos. To do so in the laboratory, doctors must harvest and fertilize some eggs.
First, the person will take hormones to make sure that ovulation happens on schedule. They will then take fertility medications to increase the number of eggs that they produce.
In the hospital, a doctor will extract the eggs, using an ultrasound machine to ensure accuracy.
A person may wish to freeze their eggs. Or, they may wish to use them at once to become pregnant. In this case, the doctor may recommend in vitro fertilization (IVF) or intracytoplasmic sperm injection (ICSI).
During IVF, the doctor exposes the eggs to sperm and leaves them in a laboratory for fertilization to take place. After this, the correct term for a fertilized egg is an embryo.
An embryologist will monitor the development of the embryos over the next 6 days, after which they may choose a suitable embryo for implantation.
During ICSI, the doctor extracts the eggs and injects a single sperm directly into an egg.
They may do this if there is a problem with the sperm or if past attempts at IVF have not resulted in fertilization. The doctor may use one embryo and freeze the others.
The main aim of freezing embryos is to preserve them for later use.
The biggest challenge is the water within the cells. When this water freezes, crystals can form and burst the cell.
To prevent this from happening, the doctor uses a process called cryopreservation. It involves replacing the water in the cell with a substance called a cryoprotectant.
The doctor then leaves the embryos to incubate in increasing levels of cryoprotectant before freezing them.
After removing most of the water, the doctor cools the embryo to its preservation state. They then use one of two freezing methods:
Slow freezing: This involves placing the embryos in sealed tubes, then slowly lowering their temperature. It prevents the embryo’s cells from aging and reduces the risk of damage. However, slow freezing is time-consuming, and it requires expensive machinery.
Vitrification: In this process, the doctor freezes the cryoprotected embryos so quickly that the water molecules do not have time to form ice crystals. This helps protect the embryos and increases their rate of survival during thawing.
After the process of freezing is complete, the doctor stores the embryos in liquid nitrogen.
The process of thawing an embryo after cryopreservation has a relatively high success rate, and research suggests that women who use thawed embryos have good chances of delivering healthy babies.
According to a comparative study and review published in 2016, babies born after cryopreservation showed no increase in developmental abnormalities. However, the medical community still requires more long-term follow-up studies.
Some research indicates that, compared with slow freezing, vitrification increases an embryo’s chance of survival, both at the freezing stage and during thawing.
Any complications or side effects of embryo freezing usually occur when the doctor is extracting the eggs.
Common side effects of extraction tend to be mild and temporary.
- cramping or bloating
- feeling full
- changes in vaginal discharge
- overstimulation of the ovaries
In theory, a correctly frozen embryo can remain viable for any length of time.
The embryos remain in sealed containers at temperatures of -321ºF. At this temperature, almost no biological processes, such as aging, can occur.
There are examples of successful pregnancies resulting from eggs that people have stored for up to 10 years. No long-term research into embryo freezing exists because doctors have only been carrying out the procedure since 1983.
Some countries regulate the length of time that a person can store their embryos. Freezing and storage are expensive, and each clinic has its own rules about what happens if a person can no longer use their embryos or keep them frozen.
A study published in the International Journal of Reproductive Biomedicine looked at the results of over 1,000 instances of embryo transfer involving either fresh or frozen embryos.
The researchers found no statistical difference between the types of embryos, in terms of pregnancy rates or fetal health. The authors pointed out that people could use frozen, but not fresh, embryos for additional transfers in the future.
Other studies suggest that transferring frozen embryos may be more effective.
Results of research published in 2014 indicate that frozen embryo transfer might lead to a higher rate of pregnancy and better outcomes for both the woman and baby.
Embryo freezing may be a better option for certain groups, such as:
- people with genetic disorders that affect reproduction
- people who will soon undergo chemotherapy
- people who take medications that affect fertility
- same-sex couples and other LGBTQ+ people who wish to have children
Some hospitals offer services specifically for the LGBTQ+ community.
People who are approaching an advanced reproductive age and who are not yet ready to have children may also benefit from freezing embryos for later use.
In a freeze-all cycle, a doctor extracts an embryo, freezes it, and stores it.
People may benefit from this process if they have a higher risk of ovarian stimulation syndrome. This is a rare and potentially dangerous condition that can arise when a person is receiving stimulating hormones to increase egg production.
To reduce the risk of this condition, a doctor may recommend freezing the embryo and transferring it when the ovaries are no longer stimulated.
A doctor can also use a freeze-all cycle to test an embryo for a genetic disorder. This involves removing a few strands of DNA and testing the chromosomes.
Embryos with a standard set of chromosomes are more likely to transfer successfully. Prescreening can ensure that future offspring have a lower chance of developing genetic diseases.
Embryo freezing is relatively safe and often leads to a successful pregnancy and delivery.
Possible risks include a slightly higher chance of genetic abnormalities in the child. It is important to note that, because this is a new procedure, the long-term health effects are not yet clear.
Also, women who become pregnant at a later age have a higher risk of complications.
Finally, treatment can be costly. However, it offers a wider range of options for people who wish or need to delay pregnancy, or who may otherwise be unable to conceive.