One sperm. One egg. Fertilised.

In conventional IVF, prepared sperm and eggs are placed together in a dish and fertilisation is left to happen on its own. ICSI removes that uncertainty entirely. An embryologist selects one healthy, motile sperm, immobilises it, and injects it directly into the centre of a mature egg using a glass needle finer than a human hair, under high magnification.

The reason this exists is simple: fertilisation involves several steps where sperm can fail. It must reach the egg, bind to the outer shell, penetrate it, and fuse with the inner membrane. When sperm count is very low, motility is poor, or shape is abnormal, one or more of those steps may never happen — and in a conventional IVF dish the result is no fertilised eggs at all, discovered only the following morning.

ICSI bypasses every one of those hurdles. It is why azoospermic men whose sperm has to be retrieved surgically can still father genetically their own children: even a handful of viable sperm cells is enough, because only one is needed per egg.

It is worth setting expectations honestly. ICSI reliably solves the fertilisation step, and typically achieves fertilisation in around seven to eight of every ten mature eggs injected. What it does not do is improve egg quality or guarantee a good embryo. A fertilised egg still has to divide correctly and reach blastocyst. ICSI is a precise answer to a specific problem — sperm failing to fertilise — not a general upgrade to IVF, and it is recommended when there is a reason for it rather than by default.