Fertilisation: day 1

Fertilisation is the fusion of one sperm cell and one egg. Two nuclei are visible in the egg: a female pronucleus coming from the egg and a male pronucleus coming from the sperm cell.
We refer to this as a 2PN egg, where PN stands for pronucleus.
Both pronuclei will fuse and disappear until they are no longer visible. We then have a zygote.

All 2PN eggs are referred to as fertilised. We can see this event 16 to 18 hours after ICSI and 20 to 22 hours after IVF.

Even if we do not see any pronuclei (0PN), the egg may still divide and go on to form a good embryo. In that case, the egg will be considered fertilised at a later stage. Therefore, the final number of fertilized oocytes will be communicated at day 3.
The embryos developing from 0PN can be transferred.

Sometimes eggs have only one pronucleus (1PN). This may indicate egg activation without any involvement of a sperm cell in the fertilisation process. Those eggs are not considered fertilised.
In principle, embryos coming from 1PN eggs are not transferred.

Sometimes 1PN eggs evolve into 2PN, which indicates asynchronous appearance of the pronuclei (which may or may not have been observed in our evaluation). 1PN eggs may also have passed the 2PN stage already due to the asynchronous disappearance of the pronuclei.

If a 1PN egg still results in a good embryo, it can be considered as fertilised and the embryo can be transferred if no other embryos are available.

Eggs can also present themselves as 3PN. In the case of IVF, this may indicate entry of more than one sperm. In the case of ICSI, the explanation lies elsewhere.
However embryos coming from 3PN eggs are never transferred under any circumstances.

In the case of ICSI, it may happen that an egg does not survive the microinjection. The egg may be broken or degenerative and can no longer result in an embryo. This happens to about ten percent of injected eggs.

Only if all available egg cells are 3PN or if none of the eggs have survived the ICSI procedure, do we completely cancel the embryo transfer on day 1. We will inform you of this by phone.

In cases where we have only observed 1PN or 0PN, we wait until day 2 to make a decision, as the eggs may still start to divide, as they should. If this is not the case and the embryo transfer still needs to be cancelled on day 2, we will inform you by phone.

Why do I have a small proportion of fertilised eggs?

The average proportion of eggs that is fertilised after IVF is about 60% of the inseminated cumulus oocyte complexes (COCs). After ICSI, this proportion is about 75% of the injected mature eggs.

During IVF treatment, all COCs are inseminated, but we can only see the maturity of the eggs on day 1. It is therefore possible that there are proportionally more immature eggs that cannot be fertilised. Sometimes there are more 3PN eggs (where more than one sperm entered the egg).

If none of the eggs are fertilised (failed fertilisation), we will use ICSI in the next treatment cycle.
In the case of ICSI, failed fertilisation is often linked to the lack of available mature eggs. Some eggs are also very sensitive to the ICSI procedure and fail to survive the injection. On average, this is the case for about ten percent of injected cells. We are certain that the ICSI fertilisation percentage is linked to the quality and source of the sperm sample (sperm from ejaculate or surgically obtained sperm).


UPDATE 05/01/2024: Renovation works Brussels IVF

To increase the comfort and privacy of its patients, Brussels IVF is renovating its nursing unit (VP03) and the reception desk area at our Andrology Lab.

As part of these renovation works, from 8 January 2024, the reception desk of our Andrology Laboratory and its associated rooms to collect a sperm sample, will be temporarily located on the second floor of Brussels IVF (via route 980).

During your treatment, we guarantee the same quality of care but you might experience a reduced level of comfort due to the temporary location with limited space.
We will make every effort to minimise any inconvenience.

Consult the practical guidelines and the FAQ on the renovation works.