Is ICSI more likely to succeed than conventional IVF?

ICSI improves fertilisation rates specifically in cases of male factor infertility, but it doesn't necessarily improve overall pregnancy rates for everyone. Your clinic can explain whether it's likely to help in your particular situation.

What if none of my eggs fertilise?

This is distressing and uncommon but does happen. Your clinic will usually review the cycle with you afterwards to consider whether ICSI, different sperm preparation, or other adjustments might help in a future attempt.

Can abnormally fertilised eggs still be used?

Eggs with abnormal fertilisation, such as three pronuclei, aren't normally used for transfer because of the risk of chromosomal abnormalities. Clinics discard these rather than culture them further.

How soon will I know if any embryos are developing well?

Clinics typically give updates on days 2 or 3, and again closer to transfer if you're going to day 5. Ask your clinic when to expect calls, since practice varies.

Fertilisation: what happens in the lab

Written by Zayn Miah & R KhanomLast updated 17 September 2026Not clinically reviewed

Short answer: On the day of egg retrieval, collected eggs are fertilised either by mixing them with prepared sperm in a dish, or through ICSI, where a single sperm is injected directly into each mature egg. The embryology team checks the following morning for signs of normal fertilisation, and typically only some of the eggs collected will fertilise successfully.

Two ways to fertilise eggs

Conventional insemination places thousands of prepared sperm around each egg in a dish, letting fertilisation happen much as it might naturally. ICSI instead involves an embryologist selecting a single sperm and injecting it directly into the centre of a mature egg using a fine needle under a microscope.

Why ICSI is sometimes used instead

ICSI is generally recommended when sperm count, movement or shape are reduced, when previous cycles had unexpectedly poor or no fertilisation, or when using frozen or surgically retrieved sperm. Your clinic will explain their reasoning if they recommend it for you.

Checking maturity first

Not every egg collected is mature enough to fertilise. The embryologist assesses each egg shortly after collection, and only mature eggs are used for ICSI. With conventional insemination, immature eggs are simply left with the sperm and may or may not mature and fertilise on their own.

What happens overnight

Eggs and sperm, or injected eggs, are placed in an incubator that mimics conditions inside the body, including temperature and gas levels. Fertilisation is checked around 16 to 18 hours later.

Signs of normal fertilisation

A normally fertilised egg, now called a zygote, shows two distinct structures called pronuclei, one from the egg and one from the sperm. Eggs showing zero, one or three pronuclei are not considered normally fertilised and aren't usually used for transfer.

It's normal for not every egg to fertilise, and fertilisation rates vary between individuals and cycles. A lower than expected number doesn't necessarily mean anything is wrong, but it's a fair question to ask your clinic to talk through.

What the clinic tells you and when

Most clinics call with a fertilisation update the morning after retrieval. This is often an anxious phone call, and it's worth having pen and paper ready to note down the numbers and any questions you want to ask.

From fertilised egg to embryo

Day 1: fertilisation is confirmed, and the fertilised egg is now called a zygote.
Day 2 to 3: the embryo divides into multiple cells.
Day 5 to 6: some embryos reach the blastocyst stage, with distinct cell types forming.
Time-lapse imaging incubators, used by some clinics, allow continuous monitoring without removing embryos to check them.

Questions worth asking

How many of my eggs were mature, and how many fertilised normally?
Would you recommend ICSI for us, and why?
Do you use time-lapse imaging, and does it change how decisions are made?

Questions people ask next

Is ICSI more likely to succeed than conventional IVF?: ICSI improves fertilisation rates specifically in cases of male factor infertility, but it doesn't necessarily improve overall pregnancy rates for everyone. Your clinic can explain whether it's likely to help in your particular situation.
What if none of my eggs fertilise?: This is distressing and uncommon but does happen. Your clinic will usually review the cycle with you afterwards to consider whether ICSI, different sperm preparation, or other adjustments might help in a future attempt.
Can abnormally fertilised eggs still be used?: Eggs with abnormal fertilisation, such as three pronuclei, aren't normally used for transfer because of the risk of chromosomal abnormalities. Clinics discard these rather than culture them further.
How soon will I know if any embryos are developing well?: Clinics typically give updates on days 2 or 3, and again closer to transfer if you're going to day 5. Ask your clinic when to expect calls, since practice varies.

Words on this page

Sources

IVF: what is IVF and how does it work?, HFEA (UK fertility regulator). Link checked 17 September 2026.
Guidelines and patient information, ESHRE. Link checked 17 September 2026.

Written by patients, not clinicians. This page has not yet been reviewed by a named fertility professional, so it explains published guidance and points you to the original source instead of giving medical advice. It is not medical advice. Decisions about your treatment belong with your own clinic. Read our editorial policy and corrections policy.