The Embryo Transfer


The culmination of an IVF cycle is the day of embryo transfer after several days and weeks, often of injections, scans, and appointments, following an egg collection and growing the embryos in the laboratory, usually the best blastocyst would then be transferred back to the womb.

Now, embryos can be transferred in the fresh cycle itself, or they may be stored and frozen for transfers of frozen embryo. This is an important and also quite a nerve-wracking day for patients. The patient though, however, is expecting an embryo to go back into the womb and will not be put to sleep in most cases. So you are awake, you’re asked to fill your bladder and under ultrasound guidance, the very best embryo will then be very carefully and gently passed through the cervix in a small catheter where it’ll be placed in the centre of the womb.

This then has a chance of success. The patient would leave the clinic and then just a couple of weeks later, take a urine pregnancy test to see if it’s been successful.

Fresh vs Frozen Transfer

When reaching the day of embryo transfer in a fresh cycle, this will often result in a conversation with the laboratory and clinical team to decide whether to transfer your best embryo in that fresh cycle. But there’s also often the option to transfer the embryo in a frozen cycle. This means not transferring an embryo at all, and in fact freezing the very best embryos, recovering from the cycle, and then coming back into the clinic to discuss putting the embryo back in a completely different cycle altogether, having prepared the womb, thawing out the embryo, and then transferring it. This is known as a frozen embryo transfer cycle.

But why would you transfer an embryo in a fresh cycle or a frozen cycle?

Well, there are a number of reasons. If a patient has had an over response to the medication during the cycle, then they will feel incredibly bloated. This is a condition called ovarian hyper stimulation, which occurs in around 3to 5% of IVF cycles. Transferring an embryo in that situation can in fact be quite dangerous for the patient and make their clinical condition worse.

Some patients may not have the opportunity to freeze embryos due to the embryo quality, and so there is not always a guarantee that embryos which reach blastocyst are suitable to be frozen, and this is because if you freeze an embryo of poor quality, it may well not survive that process. So an important discussion needs to take place with the team who have grown the embryos in the laboratory and your clinical lead to decide whether those embryos are suitable for transfer or even for freezing.

But certainly in a good clinic, whether laboratory is excellent than a high chance of blastocyst formation rate should result in hopefully surplus embryos, depending on the age of the patient, the egg reserve and the sperm parameters, which all factor in to overall embryo quality.

Which embryo to transfer?

More than half of patients starting an IVF cycle in the very best clinics will certainly have more than one blastocyst to transfer. This could of course, lead into a situation where more than one embryo is transferred, but in most cases, just a single embryo will be transferred and the surplus embryos will be frozen with consent from the patient.

But how do you choose which embryo to transfer? Well, embryos will be looked at down the microscope by the embryologist, and they’ll be given a morphological grade. These are usually ranging from A to C or in different grading systems. You’ll be given a guide as to what the highest quality embryo is and what the lowest quality embryo is.

Not every embryo that’s achieving blastocyst can in fact be frozen, as we’ve said already, but it’s important to have a detailed discussion with your team as to which embryo should be put back. Generally speaking, most clinics will advise that the embryo, which they believe has the best chance of creating a pregnancy to be transferred first and then freezing or vitrification of the remaining embryos.

When embryos are frozen, there is a course of risk that they may not survive that process, which would be discussed with you in the clinic.

How many embryos to transfer?

In some circumstances, there may be a decision that’s made with the clinical team to transfer more than one embryo. Now, historically in IVF when techniques weren’t as robust, and as science has improved and technology has changed over the years, we’ve moved from transferring several embryos at the very start of IVF to a situation in modern day where in fact, one embryo is transferred into the uterus in the vast majority of IVF cycles.

Certainly around 70 to 80% of IVF cycles in the UK are including a single embryo transfer. There are however reasons why you may transfer two embryos, and there are reasons why you may absolutely only transfer one embryo. If a patient has a uterine anomaly or which is a structural abnormality within the womb, or perhaps has sadly lost a baby very early on in pregnancy, then you may be advised just to transfer one embryo only into the womb.

If a patient is much older over the ages of 40 to 42, the chances of success are lower, and this is a point at which your clinician will discuss with you based on the quality of your embryos, whether it is in fact, better to transfer two embryos rather than one. But every time you transfer two embryos, there is an increased risk of multiple birth occurring.

Twin pregnancy is an unfortunate side effect of IVF and it’s really important that we’re not absolutely increasing that risk for you, and therefore in detailed discussion needs to take place about the safety of transferring a single or a double embryo.

A twin pregnancy could still occur following single embryo transfer. This is where the embryo then divides later on during its development to create identical twins. This is known as monozygotic twinning. However, the more common outcome from an IVF cycle is if double embryo transfer takes place and the two embryos themselves, both implant to create non-identical twins.

Twins following IVF are more common and the risk of twins to you in pregnancy can be higher, both to the babies and also to the mother. Mothers carrying twins may have a more complicated delivery, and the pregnancy course can certainly be more challenging. It’s therefore really important to have an informed discussion with the embryology and clinical team when making decisions about single or double embryo transfer.

The overriding objective of each and every IVF cycle should be to reach a live birth of a single healthy baby. Therefore, it’s really important at the point of embryo transfer to think very carefully about whether to undertake single or double embryo transfer. Of course, there are reasons to perhaps consider double embryo transfer, where we must also consider that risk of multiple births and the formation of twins, or even triplets inside the womb can lead to a very complex pregnancy.

So following embryo transfer, the patient would return home and within a couple of weeks, be undertaking a urine pregnancy test to see if the embryo transfer has worked. Then just two to three weeks later, after a positive test, the patient would hopefully come back into the clinic for a scan to see a very early six to seven week pregnancy.