Embryo grading and freezing

To ensure that you have a good chance of getting pregnant, it is important that the best quality embryo is selected for transfer and all remaining good quality embryos are frozen and stored for use if the first transfer is not successful.


When are embryos transferred?

Embryos can be transferred into your womb at different stages of their development. This may help decide which embryo has the best chance of carrying on developing to establish a pregnancy.

  1. Sometimes embryos can be selected on day two or three of their development (known as cleavage stage embryos).
  2. Others may be left to develop for four or five days until they reach what is called the blastocyst stage.
  3. Embryologists use their training and experience to assess embryos and select those most likely to implant in the womb and result in a pregnancy.

They do this by applying a system of grading, using criteria for early stage embryos 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.

It is very important that the best quality embryo is selected to be transferred, and that any other good quality embryos that remain are frozen for further treatment. This can avoid the need for repeated drug stimulation, egg retrieval, sperm collection and fertilisation. For more information see the HFEA website.

Clinic policies on freezing embryos

Clinics have their own policies on freezing embryos, and your clinician must discuss this with you. This information is particularly important if you are likely to have a single embryo transfer. Some clinics may freeze a single good quality embryo. Other clinics may have a policy that recommends you have a certain number of good quality embryos to freeze to give you the best chance of success.

Did you know?

A number of recent studies have shown that, although overall pregnancy rates were similar when a single blastocyst was transferred to the womb compared with double blastocyst transfer, up to 47% of pregnancies after multiple blastocyst transfer were twins.

The expert says...

"Where a top-quality blastocyst is available, use single embryo transfer."

NICE guidelines - Fertility problems assessment and treatment, February 2013

The expert says...

"…new research has shown that the single embryo transfer of a good quality blastocyst during IVF treatment can not only increase a woman’s chances of falling pregnant, but it also has the desired effect of decreasing the risk of a multiple birth."

Professor Philip Steer, editor-in-chief of BJOG: An International Journal of Obstetrics and Gynaecology.

Time-lapse imaging

Recent improvements in technology have lead to the development of a new type of system called 'time-lapse' which allows the embryos to be constantly monitored within an incubator.