Transferring a fertilized embryo to a woman's uterus is an important part of the in vitro fertilization process.
There are some things to expect during the embryo transfer process, as well as some risks and precautions to consider. This article takes a look at how the process works and who can benefit from embryo transfers.
Contents of this article:
What is an embryo transfer?
IVF involves removing a woman's egg from her ovaries and fertilizing it with sperm in a laboratory.
An embryo transfer is the last part of the in vitro fertilization (IVF) process. During IVF, fertility medications are used to stimulate the ovaries into releasing healthy eggs.
These eggs are then removed from a woman's ovaries and fertilized in a lab. Once the fertilized eggs have multiplied, the embryos are transferred to the woman's uterus. For a pregnancy to begin, the embryo must then attach itself to the wall of her womb or uterus.
When embryo transfer is needed
IVF and embryo transfer is needed in cases where natural fertilization is not an option or has difficulty occurring. There are many reasons for embryo transfer, including:
- Ovulation disorders: If ovulation is infrequent, fewer eggs are available for successful fertilization.
- Damage to Fallopian tubes: The Fallopian tubes are the passageway through which the embryos travel to reach the uterus. If the tubes become damaged or scarred, it is difficult for fertilized eggs to safely reach the womb.
- Endometriosis: When tissue from the uterus implants and grows outside of the uterus. This can affect how the female reproductive system works.
- Premature ovarian failure: If the ovaries fail, they do not produce normal amounts of estrogen or release eggs regularly.
- Uterine fibroids: Fibroids are small, benign tumors on the walls of the uterus. They can interfere with an egg's ability to plant itself in the uterus, preventing pregnancy.
- Genetic disorders: Some genetic disorders are known to prevent pregnancy from occurring.
- Impaired sperm production: In men, low sperm production, poor movement of the sperm, damage to the testes, or semen abnormalities are all reasons natural fertilization may fail.
Anyone who has been diagnosed with these conditions may consider IVF and embryo transfer an option.
What to expect before, during, and after an embryo transfer
Around 2 or 3 days before the embryo transfer, the doctor will choose the best eggs to transfer to the womb.
There are many processes available to aid selection, though non-invasive methods such as metabolomic profiling are being tested. Metabolomic profiling is the process of selecting the most beneficial eggs based on a number of different factors. This could limit the need for invasive procedures in the future.
These eggs will then be fertilized in a lab and left to culture for 1-2 days. If many good quality embryos develop, the ones that are not going to be transferred can be frozen.
The process of an embryo transfer
The final part of IVF is the embryo transfer.
The embryo transfer process is similar to the process for a pap smear. The doctor will insert a speculum into the woman's vagina to keep the vaginal walls open.
Using ultrasound for accuracy, the doctor will then pass a catheter through the cervix and into the womb. From there, the embryos are passed through the tube and into the womb.
The process is usually pain free and rarely requires any sedatives. Some women may feel discomfort as a result of having the speculum inserted or from having a full bladder, which is required for ultrasound. The process is short, and the bladder can be emptied immediately after.
After the embryo transfer
A follow-up appointment 2 weeks later to check if the embryo has implanted will show if the transfer was successful.
After the procedure, women may experience some cramping, bloating, and vaginal discharge.
Types of embryo transfer
In order to extract and fertilize the eggs during IVF, doctors generally follow the same process every time. Once fertilization has occurred, there are a few different options available for embryo transfer:
Fresh embryo transfer: Once eggs have been fertilized, they are cultured for 1-2 days. The best embryos are chosen to transfer directly to the woman's uterus.
Frozen embryo transfer: Any healthy embryos that were not used in the first transfer can be frozen and stored for future use. These can be thawed and transferred to the uterus.
Blastocyst embryo transfer: If many healthy embryos develop after the fertilization, it is common to wait to see if the embryos develop into blastocysts. According to a study in the Indian Journal of Clinical Practice, blastocyst embryo transfer has a higher success rate than the standard embryo transfer on day 3. However, another recent study suggests that it may pose risks later in pregnancy and should not always be recommended.
Assisted hatching (AH): A study in the Reproductive Biomedicine Online found that the process of assisted hatching - weakening the outer layer of the embryo before it is transferred to the uterus - does not improve pregnancy and implantation rates in women who are having fresh embryos transferred. The researchers noted, however, that women having frozen embryos implanted do benefit from having their embryos treated in this way.
How many embryos are transferred?
There are still differences in practice as to how many embryos are transferred into the woman's uterus. In many cases, only one fertilized embryo is transferred to the uterus, while other doctors believe that two fertilized embryos increase the chances of a successful pregnancy.
According to guidelines set out by the International Journal of Gynaecology and Obstetrics, the number of fresh embryos to be transferred varies according to the woman's age and outlook. In many cases, no more than two embryos will be used. For women under the age of 35 with an excellent chance of pregnancy, doctors will consider using just one embryo.
A recent study posted in Fertility and Sterility showed that single embryo transfer in women less than 38 years of age reduces the risk of multiple births, yet does not seem to affect live birth rates. This is important to note, as many doctors recommend using multiple embryos to ensure pregnancy. This research indicates that multiple embryos may not be necessary.
When the woman's chances for pregnancy seem to be low, doctors may opt to use a technique called heavy load transfer (HLT), in which three or more embryos are transferred to the uterus. According to a study in Facts, Views & Vision in Obgyn, HLT should be recommended in patients with a poor natural outlook, as it is likely to boost pregnancy rates to an acceptable level.
Success rates of embryo transfers
Studies suggest there may only be a small difference in success rates between fresh and frozen embryos used in IVF.
The rate of success for embryo transfers may vary based on the transfer method used.
According to a study posted in the International Journal of Reproductive Medicine, there is no statistical difference between using fresh and frozen embryos. Embryo transfers using fresh embryos had a 23 percent pregnancy rate, whereas frozen embryos had an 18 percent pregnancy rate.
The study showed that frozen embryos could also be used for additional embryo transfers where fresh embryos could not. If the chance for pregnancy is low, doctors may consider freezing additional embryos for a second attempt at embryo transfer at a later date.
Individual success rates can vary and may depend upon the cause of infertility, ethnic backgrounds, and genetic disorders.
Risks and precautions of embryo transfers
The risks of embryo transfers themselves are very low. These risks are mostly related to increased hormonal stimulation, causing an increased risk such as a blood clot blocking a blood vessel.
The woman can also experience bleeding, changes in her vaginal discharge, infections, and complications of anesthesia if it is used. The risk of a miscarriage is about the same as in natural conception.
The greatest risk of embryo transfer is the chance of multiple pregnancies. This occurs when multiple separate embryos attach to the uterus. This may increase the risk of stillbirth and children born with disabilities, and is more common in pregnancies due to IVF than natural conception.
Writen by Jon Johnson