Step 4: Fertilisation

IVF treatment

Step 4 of IVF treatment is fertilising the egg in the laboratory. A fertilised egg is called an embryo.

We prepare the sperm sample by putting the normal and moving sperm in an electronically tagged test tube.

In an IVF cycle, we put the sperm and egg(s) together in a carefully labelled and electronically tagged dish.

Sometimes, we inject each egg individually with a single sperm. This is called intra-cytoplasmic sperm injection or ICSI. We put the sample into a dish labelled and tagged with your name and unique number.

We leave the dishes in an incubator overnight. This machine keeps the eggs at the correct temperature and allows them to be fertilised with the sperm. 

Intracytoplasmic sperm injection (ICSI)

You might have ICSI as part of your IVF treatment. We might recommend ICSI if:

  • you have a low sperm count (fewer sperm than usual)
  • the sperm do not move normally (poor mobility)

Your treatment is exactly the same. The difference is how the eggs are fertilised. In an ICSI cycle, we inject each egg individually with a sperm. This helps the sperm inside the egg and gives the best chance of fertilisation. 

Your doctor explains if this is the right treatment for you. 

Safety and security

Our checking and witnessing guidelines make sure that the eggs, sperm and embryos used in your treatment belong to you. An electronic tagging system adds to the security.

We often ask you to give your name and date of birth. This helps to ensure accuracy.

Fertilisation and embryo development

The next morning, the embryologist (a fertility treatment scientist) carefully examines each egg to see if it is fertilised.

We call you the day after your egg collection and tell you how many eggs have fertilised. We also explain when to come in for embryo transfer.

Eggs that have fertilised are called embryos. As embryos develop, their cells divide.

  • By day 2, an embryo should have 2 to 4 cells.
  • By day 3, an embryo should have 6 to 8 cells.
  • By day 5, an embryo has more than 100 cells.

We transfer embryos to the womb 2, 3 or 5 days after fertilisation.

Freezing spare embryos

It is possible to freeze embryos for later use. However, this only happens in 40% of cycles.

Embryos with good potential to attach to the lining of the womb (implant) should reach a stage called 'blastocyst'. We freeze good quality blastocysts on day 5 or 6 of development. If you have 1 or 2 good quality embryos to transfer and spare embryos that are suitable for freezing, you have an above average chance of conception (getting pregnant).

We select the embryos that are frozen carefully to give you the best chance of pregnancy. Currently, about 85% of our blastocysts and our early stage embryos survive being frozen and thawed. 

Embryos are frozen at an extremely low temperature. This makes sure that they do not deteriorate over the number of years that they are stored. Even if your first IVF cycle results in a baby, you might want to use your frozen embryos to have another baby at a later date.

Read more about frozen embryo transfer.

If you do not have spare embryos

Most people do not have any spare embryos to freeze.

Your NHS funding might state that you can have 1 initial cycle, and 1 to 2 cycles using your frozen embryos. However, if your first cycle does not result in spare embryos to freeze, there is not any NHS funding available for more cycles. This is because there are no frozen embryos to transfer. 

If no eggs fertilise

Rarely, in about 1 in 20 cycles (5%), none of the eggs fertilise and there are no embryos for transfer. This is of course disappointing. We offer you the earliest available appointment with a senior doctor. You can then talk about the cycle and your future treatment options. We also offer you an appointment to see one of our counsellors.

Resource number: 2687/VER4
Last reviewed: July 2023
Next review: July 2026

Trusted Information Creator. Patient Information Forum

Contact us

If you have any questions about IVF, please contact our assisted conception unit (ACU).

Phone: 020 7188 2300

Email: [email protected]

Do you have any comments or concerns about your care?

Contact our Patient Advice and Liaison Service (PALS)

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