Embryo transfer is the process of putting the embryos that are growing in the lab back into the uterus. The period during which the embryos remain in the lab is one where important procedures are undertaken and crucial decisions are made. While growing the embryo is the responsibility of the embryologist, depositing them in the uterus is the job of the clinician.
Embryo transfer can be done during any one of the four stages of embryo development
- Day 2 (cleavage stage)
- Day 3 (cleavage stage)
- Day 5 (Blastocyst stage)
- Day 6 (Blastocyst stage)
Embryo transfer to be done at which stage of the development?
The first decision which is jointly taken by the embryologist and the clinician is about the stage at which the embryo should be transferred. A number of factors such as the quality of embryos, the number of attempts at IVF the patient has already had among other factors are taken into consideration before taking this decision. The preferred time for transfer is usually between Day 2 and Day 6 of the embryo development. The question as to which is the best day for embryo transfer is still under debate. Day 5 transfers may be ideal since in natural conception cycles, embryos typically implant on Day 5 or 6 after ovulation. According to this theory, Day-5- or Day-6-blastocyst embryo transfers may be preferable due to the ideal uterine environment conditions at this stage. Embryos transferred at the blastocyst stage often have a higher implantation rate (meaning that they are more likely to “stick” and develop into a healthy pregnancy).
How many embryos need to be transferred?
The next big decision is the number of embryos to be transferred. This decision is taken jointly by the clinician, embryologist and the patient. Keeping in mind, the fact that each embryo is a potential baby, factors such as patient history, age, number of attempts are taken into account. Generally, one or two embryos are selected or transferred to avoid multiple pregnancies. Remember, it is just not about conceiving BUT about a heathy pregnancy and taking home a healthy baby. You might ask what if I have eight embryos and I transfer two good embryos to the uterus. Will the other six embryos go to waste? The answer is no, not at all. The solution to this is vitrification of the good, surplus embryos.
Success of the embryo transfer depends on:
- Using a strict grading criterion and selecting the best embryos
- Each transfer done should be the best, there is no point saving the best embryos. If the clinician feels that the endometrium (uterus lining) is not ready, she will freeze the embryos rather than transferring and wasting them.
As is evident, both the embryologist and the clinician play important roles leading to the success of the embryo transfer. The embryologist follows a strict grading criterion and selects the best embryo for transfer and the clinician decides if the endometrium is ready or not. A frozen embryo transfer (FET) is a procedure where embryos that were cryogenically-stored (frozen) from a previous IVF or donor egg cycle are thawed and transferred to a uterus as part of an in-vitro fertilization (IVF) cycle.
Embryologist: “We will be transferring two good embryos for you.”
Patient: “I just need some more information regarding the embryos. Can I know the gender and weight of the embryos?”
Embryologist: “We can only grade the embryos as per their morphology. It is impossible for us to know the gender and weight at this stage. Also, that shouldn’t matter as long as you are having a healthy child.”
Before we move to Frozen Embryo Transfer (FET) it is critical to understand one more technique, know as Vitrification,Freezing or Cryopreservation.
What is Vitrification or Cryopreservation?
Cryopreservation is the freezing of biological matter to preserve it for future use, and it is utilized as a method of fertility preservation. It involves the cooling and storing of sperms, eggs, and embryos at very low temperatures to maintain their viability. A woman’s eggs, a man’s sperm, or a fertilized embryo are all capable of being cryopreserved. When the woman or the couple is ready, the eggs or the embryo, as the case may be, are thawed (warmed), fertilized, and transferred to the uterus. This procedure allows an individual or a couple to protect their biological reproductive material at a time it is the healthiest, before age or health conditions decrease their quality or quantity. Once the materials are preserved, they may be used in future in ART to achieve a pregnancy. Scientifically, these eggs and embryos can be frozen for multiple years without any detrimental effect on them. However, the ICMR guidelines suggest that they can be frozen for up to 5 years.
A couple may opt for vitrification for any one of the following reasons:
- Ill health. For example, if a patient is to undergo cancer treatment that may damage their reproductive abilities and cause degenerative reproductive system disorders.
- Prioritization of career, educational, or personal goals.
- Present financial and emotional unpreparedness to bear a child
- Cancellation of embryo transfer due to a number of reasons
- Preservation of “leftover” embryos from an IVF treatment for use at a later date if the present cycle doesn’t work or for another baby
Vitrification is done by washing the oocytes or embryos in media within a stipulated time, once the washing is completed the embryo or eggs are loaded on a loading device and submerged immediately into liquid nitrogen. They are stored in liquid nitrogen at all times (-196c). The number of embryos or oocytes that can be frozen on one straw depends on the stage of their development.
- Cleavage stage – Not more than three embryos should be vitrified per straw
- Blastocyst stage – Their diameter is bigger; not more than two should be vitrified on one straw
- For oocytes – Not more than four oocytes should be frozen on one straw
Essentials for a good vitrification program
- Embryologists trained in vitrification
- Vitrification of only good embryos
- Adherence to the predefined number of embryos or oocytes that should be frozen on each straw
There is currently no evidence that cryopreservation negatively impacts the outcome of IVF or the health of the baby.
Clinician: “We have transferred a single embryo. However, you have two additional embryos which are good. Would you like to cryopreserve them? You need to let us know as soon as you can as we need to do it as quickly as possible as there is a protocol to be followed here.”
Patient: “Doesn’t cryopreservation mean keeping the embryos in the fridge?”
Clinician: “No, cryopreservation requires a protocol and special skills. The embryos are stored in liquid nitrogen at a temperature of -196c, not a normal fridge.”
Frozen Embryo Transfer (FET)
Frozen embryo transfer is the transfer of embryos which were frozen from a previous cycle. Just like a normal embryo transfer, the clinical prepare the uterus of the woman undergoing treatment. Once the endometrium lining is ready the clinician will schedule the patient for her embryo transfer.
Your embryologist will take the embryos out of liquid nitrogen and thaw (melt) them to bring them back to 37 degree celsius. Remember, the temperature in liquid nitrogen is -196 degree celsius so in the process of thawing they are warmed several degrees in 10-12 minutes. The embryos are loaded in the catheter and handed over the clinician, they then gently deposit the embryos in the uterus.
FET like a fresh embryo transfer can be done on Day 2, Day 3, Day 5 of embryo development. The only difference between a fresh embryo transfer and FET is that the embryos in case of FET were frozen and have to be thawed before depositing in the uterus. The process of doing the embryo transfer and preparing the lining in both cases remain same.
Frequently Asked Questions (FAQ)
Which is better FET or Fresh Embryo Transfer ?
The decision on whether to do a fresh or frozen embryo transfer depends on the clinician, He/She will take the call depending on your Endometrium lining, Hormonal status and on the number of follicles that were growing in your ovaries. If your ovaries are bulky or you had many follicles they will most likely freeze all your embryos to prevent a condition known as OHSS (Ovarian Hyper Stimulation Syndrome)
Is Anaesthesia required for Embryo transfer?
Most cases do not require anaesthesia. However, most clinicians will do a mock transfer before your actual transfer and will be able to assess your individual case. Some women require mild anaesthesia because of apprehension or a medical condition.
Is Embryo transfer Painful ?
Embryo transfer is not a painful procedure, it is a quick and skilful step. However, the procedure needs to be done on a full bladder, so the sensation to urinate may be the only discomfort you may feel
Why is it important to do Embryo transfer on a full bladder?
A full bladder helps change the angle of the uterus to make the transfer easier and helps your clinician see the catheter clearly with trans-abdominal ultrasound to place the embryo perfectly.
I vitrified three embryos last year? Can we check how they are doing ?
Once vitrified, we will know about their survival only when they have been thawed for the procedure. Though, if vitrified. properly they will remain in the same state or grade for years.
I had vitrified two embryos when I was 32. Now I am 38 years old and I want to thaw the embryos and transfer them. Will my age have an impact on the embryos ?
No even though you are 38 years old, your embryos are still of a younger woman as they were vitrified earlier. Your current age will have no impact.