ICSI: what it is, when it's used, and what it can't fix

ICSI (pronounced "ik-see") stands for intracytoplasmic sperm injection. Instead of letting sperm swim to the egg in a dish, an embryologist picks up a single sperm in a very fine glass needle and injects it directly into the centre of a mature egg. It was invented in the early 1990s for couples where sperm could not fertilise eggs on their own, and it is now used in more than half of IVF cycles worldwide.

What happens

1. A few hours after collection, the fluffy cells around each egg are gently removed so the embryologist can see which eggs are mature. Only mature eggs can be injected.
2. The prepared sperm are placed in a thick fluid that slows them down. The embryologist chooses one that looks normal and is moving, immobilises it by pressing its tail, and draws it into the needle.
3. Under a high-power microscope, using tiny joystick controls, the egg is held still by gentle suction on one side and the needle is passed through its shell and outer membrane on the other. The sperm is released inside.
4. The egg goes back to the incubator. Fertilisation is checked the next morning, exactly as with conventional IVF.

The whole procedure takes a minute or two per egg. It requires steady hands and a lot of practice; it is one of the skills embryologists train longest for.

When ICSI is used

• Sperm problems — low count, poor movement, abnormal shapes, or sperm retrieved surgically from the testis. This is the reason ICSI was invented and where it clearly helps.
• Previous fertilisation failure with conventional IVF.
• Frozen or few eggs, or when eggs will be genetically tested (to avoid stray sperm contaminating the test).
• Clinic policy — some clinics use ICSI for everyone. This is a matter of debate.

What ICSI does not do

ICSI solves one problem: getting a sperm into an egg. It does not improve egg quality, does not make embryos more likely to reach blastocyst, and does not raise pregnancy rates when the sperm was fine to begin with. Several large studies show that for couples without a sperm problem, conventional IVF works as well as ICSI. If your clinic recommends ICSI, it is reasonable to ask why in your case.

What it can't guarantee

Even with ICSI, not every egg fertilises — around 70–80% is typical. The sperm can be placed inside, but the egg still has to "activate" and the two sets of genetic material still have to combine properly. When fertilisation fails despite ICSI, the reason is often in the egg or the sperm's contents rather than the technique.