- ‘Embryo transfer’ refers to the transplantation of a mammalian preimplantation embryo into the reproductive tract of a recipient female so that it may implant and continue to develop to birth.
- Mammalian embryos of many species can develop in vitro from fertilization to the blastocyst stage (approximately 100 cells), but at this point, they must implant in the uterus in order for embryogenesis to proceed normally.
- Embryo transfer thus refers to a step in the process of assisted reproduction in which embryos are placed into the uterus of a female with the intent to establish a pregnancy.
- This technique (which is often used in connection with in vitro fertilization (IVF)), may be used in humans or in animals, in which situations the goals may vary.
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History of Embryo Transfer
- The first successful embryo transfer was performed in 1890 in the rabbit.
- However, the techniques of embryo transfer were not perfected and applied to a large number of mammalian species until the 1950s and 1960s, when methods for the efficient in vitro culture of preimplantation embryos were also developed.
- In 1978, this work culminated in the first birth of a human from a transferred embryo, which had been conceived by in vitro fertilization.
Conditions for Embryo Transfer
- IVF and embryo transfer are needed in cases where natural fertilization is not an option or has difficulty occurring during conditions such as ovulation disorders, damage to fallopian tubes, endometriosis, premature ovarian failure, uterine fibroids, genetic disorders, impaired sperm production, etc.
Types of Embryo Transfers
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 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 although it is highly debated.
- Assisted hatching (AH) before Transfer:
Assisted hatching is performed in the laboratory just before embryos are transferred into the woman’s uterus. Using a laser, the embryologist makes a tiny, precise opening in the shell of each embryo after which it is transferred.
Procedure of Embryo Transfer
- 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 doctor chooses the best embryo to transfer to the womb.
- Embryos are placed in a soft catheter and are injected in the uterine cavity through the cervix.
- For a pregnancy to begin, the embryo must then attach itself to the wall of her womb or uterus (implantation).
The transfer of preimplantation embryos may be accomplished either surgically, by making an incision through the abdominal wall and exposing the uterus, or non-surgically, by gaining access to the uterine lumen through the vagina, the latter being more common.
- The doctor inserts a speculum into the woman’s vagina to keep the vaginal walls open.
- Using ultrasound for accuracy, the doctor then passes 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.
- Embryo transfer is most commonly performed after 72 hours (day 3 post retrieval).
- ‘Blastocyst transfer’ is generally performed at 120 hours (day 5 post retrieval).
Risks Involved in Embryo Transfer
- There are minimal risks associated with the embryo transfer procedure.
- Risks 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 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.
Importance of Embryo Transfer Technique
- Transferring a fertilized embryo to a woman’s uterus is an important part of the in-vitro fertilization process. The objective of embryo transfer is to facilitate conception following fertilization from the in vitro fertilization procedure.
- The ability to produce live young, or even mid-term fetuses, from isolated preimplantation embryos, depended historically on the development of embryo transfer techniques.
- Embryo transfer used extensively in research with laboratory animals involves the removal of the developing embryo from the reproductive tract of one female and transferal to another.
- A technique commonly used for the production of transgenic and knockout mice, embryo transfer may also be used for the production of pathogen-free colonies.
- Experiments involving separation of maternal and fetal genetic effects are possible with this technique.