Before in vitro fertilisation takes place at the assisted fertility laboratory, the semen sample is prepared, enabling us to separate the healthy mobile sperm from dead cells or non-sperm cells. This enables us to select the sperm with the best conditions for fertilisation. There are two ways in which fertilisation can be carried out. The difference between these is the way in which the male and female gametes are fused.
This method allows for a degree of natural selection: only the strongest and healthiest sperm will be able to achieve fertilisation.
If the quality of the sperm is not adequate, this process will not be able to successfully complete. There won’t be any embryos or, if there are, they won’t be viable because they won’t have the quality needed to achieve pregnancy.
ICSI is an exceptional step, but it does not guarantee fertilisation (fusion of the nuclei).
The aim of the biomedical team is to obtain high quality embryos. As such, they will choose one of these techniques estimating which one can give the best chances of success for each individual case. The cause of infertility for the couple (if it is known) and the characteristics of both gametes will be taken into consideration. For example, if the egg has a thicker zona pellucida, it is more difficult for the sperm to penetrate it, and for that reason, the support that ICSI can offer may be necessary to achieve pregnancy. Likewise, if the sperm has low motility (asthenozoospermia) or if there are sperm-egg recognition problems, it would be difficult to achieve fertilisation through IVF.
If the gametes are good quality, IVF may provide enough help to achieve pregnancy. It is also a simpler and more natural method. Even with an immature egg, for which ICSI has poor chances of success, conventional IVF can allow the egg the reach a higher level of maturity and create an embryo. Many professionals prefer to use the most natural and least invasive method to achieve pregnancy, and it is for this reason that conventional IVF is still frequently used in assisted reproduction.
Although it is possible to evaluate which technique will be most suitable beforehand, the final decision will be made in the laboratory once the eggs and sperm can be observed. As such, fertilisation that was initially planned as IVF can be changed to ICSI and vice versa.
If you have a high number of eggs, some clinics will be able to recommend a “mixed” or “combined fertilisation” cycle. Some eggs would be fertilised by IVF and others through ICSI.
The important thing in this entire process is to have the appropriate professional advice so that you can get answers to any questions you have and understand the nature of the decisions being made.