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Decisions to make about your embryos

During IVF, clinics need to make various decisions about your embryos, including how many to transfer to your womb and when to transfer them. Find out more about the different decisions you and your clinic will make about your embryos.


What decisions will my clinic and I need to make about my embryos?

During treatment your clinic will talk to you about various decisions that need to be made regarding your embryos. These include:

  • selecting only the best quality embryo(s) for treatment
  • whether to put one, two or (very rarely) three embryos back in the womb
  • at what stage of development to transfer your embryo(s) back to the womb.

How do clinics decide which embryos are the best quality?

Embryos vary in quality – those that are of the best quality are more likely to implant in the womb and lead to a pregnancy. If you have more than one embryo, your embryologist will use their training and expertise to select the best quality embryos using criteria such as:

  • the number of cells present
  • how fast the cells are dividing
  • whether the cell division is even
  • whether there are any fragments of cells present – this means that some cells have degenerated.

Rarely there may be no good quality embryos and then your doctor will need to make a judgement about whether they think any of your embryos can realistically continue to a healthy pregnancy.

How do clinics decide how many embryos to put back in the womb?

If you have more than one good quality embryo available, it’s now best practice for most women to have only one embryo put back in the womb and freeze the others (called an elective single embryo transfer or eSET). This is to reduce the chance of you having a multiple birth, which can pose serious risks to the health of both mum and babies, including babies that are more likely to be premature or of low birth weight.

Find out more about the risks of fertility treatment

In some cases a clinic may decide it’s appropriate for women to have more than one embryo put back. This is typically for older women who are less likely to be successful overall and are therefore less likely to have two embryos successfully implant in the womb.



Are success rates lower for women who have elective single embryo transfers?

No. For most women having IVF, transferring one embryo is just as successful as transferring multiple embryos and your chance of having a multiple pregnancy is much lower.

Our latest data shows that in 2017, the average pregnancy rate for eSET was 37.5%, compared to a 36% pregnancy rate for multiple embryo transfers. By contrast, when having a single embryo transfer, the chance of a multiple pregnancy is less than one in 50, compared to just less than one in three for multiple embryo transfers



Bear in mind that sometimes the highest quality embryo is selected for eSET, which can have a slight impact on birth rates.

For most women eSETs are far safer and just as effective as multiple embryo transfers, which is why we recommend them.

When are embryos transferred back to the womb?

Embryos can be transferred to the womb at two different stages of their development.

  • Cleavage stage embryos are selected on day two or three of their development
  • Blastocyst stage embryos are selected on day four or five of their development. 

In the UK, roughly half of women have a cleavage stage transfer and the other half have a blastocyst transfer.

What’s the difference between blastocyst and cleavage transfers?

The difference is that because blastocyst embryos have been able to develop for longer in the lab and they’re more fully formed, it’s sometimes easier for the embryologist to select the embryos that are most likely to implant in the womb. This means that blastocyst transfers tend to have higher birth rates.

However, not all embryos that are left to the blastocyst stage will survive and in some cases a couple could have no embryos available to transfer to the womb. There’s no way of knowing if the embryo had been transferred at the earlier cleavage stage, whether it could have continued to a successful pregnancy.

Are pregnancy rates affected by whether you have a blastocyst or cleavage transfer?

Yes, our Fertility Trends report for 2014 shows pregnancy rates are higher for blastocyst transfers than for cleavage transfers.

Read our Fertility Trends report 2014

Are multiple pregnancies affected by whether you have a blastocyst or cleavage transfer?

Yes, multiple pregnancy rates are higher for double blastocyst transfers compared to double cleavage transfers. The risk of having a multiple pregnancy with either a blastocyst or cleavage transfer is reduced to less than 2% if you have only one embryo put back.

Read our Fertility Trends report 2014

Why are some women less likely to have blastocyst transfers than others?

Some women or couples may be less likely to have a blastocyst transfer because the embryologist is concerned they’ll end up with very few or no embryos if a lot of the embryos don’t survive to the blastocyst stage.

This includes women who are producing fewer normal healthy eggs, older women and couples with only one or two embryos. Generally you’re more likely to be recommended a blastocyst transfer if you’re a younger women with a good chance of getting pregnant.

What are some of the new techniques being used to select and transfer embryos?

Your clinic may offer you various add-ons, which they claim can help them to select the best quality embryos or ensure an embryo successfully transfers and implants in the womb. These include:

  • Time-lapse imaging
  • Embryo glue
  • Freeze-all cycles
  • Pre-implantation genetic screening (PGS)
  • Assisted hatching.

For an explanation of what these and other add-ons involve and our assessment of how effective they are, view our treatment add-ons page.

Where can I find clinics offering blastocyst transfers?

More and more clinics are now offering blastocyst transfers so you should contact your shortlist of preferred clinics to find out if blastocyst is available.

Choose a fertility clinic



Review date: 4 September 2021

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