In order for an egg to allow a sperm to fertilise it and start developing into an embryo, it first has to be activated. In some women with previously unexpected failed fertilisation, even when advanced techniques such as ICSI are used, the eggs may be stimulated in subsequent cycles to activate by using chemicals called calcium ionophores.

There is currently conflicting evidence regarding this technique. Although egg activation may improve fertilisation rates, there are not enough studies demonstrating the effectiveness or safety. Therefore, this treatment is only offered in selected cases and is therefore not recommended for routine use.