This is an in vitro fertilisation procedure similar to IVF, where the egg and sperm are also fertilised in the laboratory.
With ICSI, the same preparatory procedure is used as with standard IVF. The woman will be given fertility drugs to stimulate her ovaries to develop many eggs. The eggs are collected and the sperm are retrieved from the male partner for fertilisation in the laboratory.
In some cases, men with almost complete failure of sperm production or a blockage may require a small sperm retrieval operation (see PESA and MESA).

In the laboratory under high-powered microscope, a single live sperm is very carefully selected and injected into the fluid centre of the egg. A holding pipette stabilises the egg from one side and from the opposite side a thin, hollow needle containing the selected sperm is inserted into the egg. After the procedure, the eggs are placed into cell culture overnight and observed for signs of fertilisation. Any resulting embryos are transferred into the uterus as with IVF a few days later.

ICSI is a major advance in treating severe male infertility. It is a relatively new technique, first used in 1992. In most cases of male infertility the sperm are unable to penetrate the outer coating (zona pellucida) of the egg and so fertilisation through natural conception or IVF is not possible. With ICSI, very few sperm are required and the ability of the sperm to penetrate the egg is unimportant. The fertilisation rate per egg using ICSI is about 70%.

ICSI may be recommended where the male partner has abnormalities in the number, quality or function of the sperm. It may also be advised in cases where previous conventional IVF treatment resulted in failed fertilisation of eggs or a low fertilisation rate.