Embryology FAQs

For answers to questions such as what is EEVA, ICSI, blastocyst and IVF embryo grading, click the sections below.

We aim to answer all your queries about embryology in person but, in the meantime, here are some of our most commonly asked questions.


ICSI – injecting a single sperm into each mature egg – is usually recommended if the sperm quality is not good enough to fertilise your eggs using standard IVF (mixing the sperm and eggs in a dish). This may have been identified by semen analysis before you started IVF treatment, but is sometimes discovered on the day of your egg collection. ICSI may also be performed if you have had low or complete failure of fertilisation during a previous treatment.

Only mature eggs are able to be fertilised, so these are the ones used for ICSI. At Bourn Hall Clinic, we expect 75-80% of eggs to be mature, but everyone is different.

Fertilisation and embryo grading

For patients having IVF, the average fertilisation rate at Bourn Hall Clinic is around 60%. Some people will exceed this, and for others the percentage will be lower. Unfortunately, for a small number of patients, no eggs will fertilise.
If you are having ICSI, the average fertilisation rate is 65% of those eggs that were suitable for ICSI. It is possible that none of your eggs will fertilise, but this is rare.

Embryo grading is based on the size and shape of the cells within them, as well as the amount of fragmentation of cells seen within the embryo. When your eggs are fertilised (day one), they will not have cleaved (divided) and are, therefore, only a single cell. This means that we do not assign these embryos a grade until the following day (day two) by which time they should have cleaved.


We recommend extended culture to the blastocyst stage for any patient who has four fertilised eggs or more. Extended culture is used to help select the best embryos from the group for your transfer. You can request to have extended culture even if you have less than four embryos; however, your chance of having blastocysts available at transfer may be reduced. Of course, you may decide not to try extended culture – instead you could opt for a day two or day three transfer. Our embryologists are here to talk it through with you.

Approximately 60% of all embryos in extended culture in our laboratory will successfully reach the blastocyst stage. However, not all of the blastocysts will be of top quality.


In order for an embryo or blastocyst to have a realistic chance of surviving the freezing and thawing procedures, it must have developed to the right stage and be of suitable quality. Some embryos won’t make it to the correct stage of development or they may be of substandard quality, which is why we might not be able to freeze them.

We recommend that we thaw all your frozen embryos, as it is likely that some of them will not survive the procedure. The more embryos you have available for culture after thawing, the more likely it is that you will have some good quality blastocysts at the time of transfer.

This depends on the stage of your embryos (when they were frozen) and on how many embryos you wish to be transferred. If you have cleaved embryos (frozen on day two or day three) or blastocysts (frozen on day five or day six) then we will thaw your embryos until there are enough surviving to meet the required amount for transfer.


If your embryos were frozen at the pronucleate stage (frozen on day one), then we will continue to thaw them until we have double the number that you need for transfer. For example, if you want two embryos transferred, we will thaw embryos until we have four surviving. This is because this stage of embryo still has to divide, and hopefully it will allow you to have two good quality embryos available for transfer.
Not all embryos or blastocysts will survive the freezing and thawing procedure, however. Our current thawing survival rate is approximately 80%.

Donated sperm

Donor sperm from our sperm bank will be assigned for your treatment. You will be offered a choice of donors based on the physical characteristics that you have specified, and other factors such as treatment type and blood group.


You will be informed of the physical characteristics of the donor, and you are entitled to know any non-identifying information that they have given. Once you have decided upon the donor, we will reserve sufficient samples for a minimum of five treatment cycles.


Eeva – Early Embryo Viability Assessment – is a new, non-invasive assessment test that uses time-lapse imaging of your embryos. It is designed to improve treatment outcomes by providing more detailed information about your embryos that will enable us to predict embryo viability more accurately. This helps our embryologists select those embryos with the highest potential at an early stage. You can find more information here.