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IVF and ICSI Explained with Consultant Embryologist Lyndon Miles

IVF and ICSI Explained with Consultant Embryologist Lyndon Miles
Authored by
Shaun Greenaway

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Conventional IVF places prepared sperm and eggs together so fertilisation can occur in the laboratory. ICSI is an additional technique in which an embryologist injects one sperm into a suitable mature egg. ICSI may help when sperm parameters are significantly abnormal, sperm have been surgically retrieved or previous IVF produced failed or very low fertilisation.

What is the main difference between IVF and ICSI?

Both treatments use the same wider IVF process: ovarian stimulation, egg collection, laboratory culture and embryo transfer or freezing. The difference is how the sperm and egg are brought together at the fertilisation stage.

  • Conventional IVF: Thousands of prepared sperm are placed with each egg
  • ICSI: One selected sperm is injected into each suitable mature egg
  • Conventional IVF: A sperm must interact with and enter the egg
  • ICSI: The injection bypasses some steps involved in entering the egg
  • Conventional IVF: Often appropriate when sperm parameters and treatment history support it
  • ICSI: Considered for significant male-factor problems, surgically retrieved sperm or previous failed or very low fertilisation
  • Conventional IVF: Fertilisation is not guaranteed
  • ICSI: Fertilisation is still not guaranteed

ICSI stands for intracytoplasmic sperm injection. It is performed as part of an IVF cycle, not as an entirely separate course of treatment.

When does NICE recommend considering ICSI?

The 2026 NICE fertility guideline says to consider ICSI:

  • When the partner producing sperm has abnormal semen parameters, taking the severity into account
  • When a previous IVF cycle resulted in failed fertilisation or a very low fertilisation rate

NICE also says ICSI should be offered when surgically retrieved sperm or previously frozen eggs are used. It recommends not using ICSI for non-male-factor fertility problems when semen parameters are normal.

This matters because ICSI is sometimes presented as automatically more advanced or successful than conventional IVF. The appropriate choice depends on the reason for treatment. ICSI can address some barriers to fertilisation, but it cannot correct poor egg quality, guarantee embryo development or guarantee a baby.

What happens to the semen sample in the laboratory?

The exact procedure varies with the sample and laboratory, but the journey normally includes:

Identity and consent checks

The clinic verifies the sample, request, consent and treatment plan. Witnessing or electronic systems are used at critical stages to reduce the risk of identification errors.

Initial assessment

The laboratory records relevant features and assesses whether enough usable sperm are available for the planned treatment. The treatment sample may not receive the same full report as a diagnostic semen analysis, so ask what will be measured and reported.

Sperm preparation

Embryologists separate motile sperm from seminal fluid, cells and debris using an appropriate preparation method. The goal is to obtain a working sample for conventional IVF or ICSI, not to make an abnormal sample “normal”.

Selection for conventional IVF or ICSI

In conventional IVF, prepared sperm are placed with the eggs. In ICSI, the embryologist examines a prepared sample under a microscope and chooses an individual moving sperm with suitable visible features for injection.

Visible selection cannot reveal every aspect of sperm genetics or guarantee that a sperm will produce a healthy embryo. It is one laboratory step within a much larger biological process.

How does fertilisation happen in conventional IVF?

Prepared sperm and eggs are incubated together under controlled conditions. A sperm must interact with the cells around the egg, bind to and pass through its outer layer, and fuse with the egg membrane.

The following day, the embryology team checks for signs of normal fertilisation. Not every mature egg fertilises. A failed or low fertilisation rate can happen even when semen measurements appear normal because conventional semen analysis does not measure every sperm function or egg factor involved.

What happens during ICSI?

The embryologist first removes surrounding cells so the egg’s maturity can be assessed. Only suitable mature eggs can normally be injected. One selected sperm is immobilised and drawn into a fine injection pipette, then placed inside the egg.

ICSI can help the sperm overcome difficulty entering the egg, but the egg still needs to activate and begin the biological steps of fertilisation. Some eggs may be unsuitable for injection, some can be damaged during the procedure and some will not fertilise normally.

The HFEA describes ICSI as effective for male infertility but notes that it carries additional procedural risk, including possible egg damage during preparation and injection.

Does ICSI choose the “best” sperm?

An embryologist can select a sperm based on movement and visible appearance at the magnification used. This does not prove that it is genetically normal or biologically superior in every way.

Be careful with claims about enhanced sperm-selection methods. The 2026 NICE guideline says not to use intracytoplasmic morphologically selected sperm injection, known as IMSI, as an adjunct to ICSI, and not to use physiological ICSI, known as PICSI, in preference to standard ICSI.

If an extra sperm-selection technique is offered, ask:

  • Why is it recommended for us?
  • Is it part of standard treatment or a paid add-on?
  • What is its HFEA evidence rating?
  • Does the evidence show more live births, not only a laboratory change?
  • Are there additional risks or costs?

What happens when sperm are surgically retrieved?

When no usable sperm are present in the ejaculate, sperm may sometimes be collected from the epididymis or testicle. Procedures include PESA, TESA, TESE and micro-TESE, depending on the cause and clinical plan.

Surgically retrieved sperm are usually used with ICSI because the number, maturity and movement of sperm may be limited. The embryology and surgical teams coordinate collection, tissue examination, sperm preparation, freezing and use with the eggs.

The possibility of finding sperm depends on the diagnosis and procedure. A retrieval attempt does not guarantee that usable sperm will be found. Ask what the contingency plan is for the eggs if retrieval is unsuccessful.

Read Understanding Surgical Sperm Retrieval and Micro-TESE Explained before discussing the options.

How are fertilised eggs monitored?

After the fertilisation check, embryos are cultured in controlled incubators. The laboratory records development at agreed stages. Depending on the clinic and treatment plan, an embryo may be transferred or frozen at the cleavage stage or after reaching the blastocyst stage.

Not every normally fertilised egg becomes a blastocyst. Development can stop at any stage, and this does not mean either partner caused the outcome.

Some clinics use time-lapse incubators that photograph embryos without removing them for each observation. This can provide additional laboratory information, but patients should check the current HFEA add-on rating before paying extra on the assumption that it improves the chance of a baby.

What does embryo grading mean?

Embryo grading records visible developmental features. At the cleavage stage, this can include cell number, symmetry and fragmentation. At the blastocyst stage, it can include the degree of expansion and the appearance of the inner cell mass and trophectoderm.

NICE recommends evaluating embryo quality at cleavage and blastocyst stages using the ARCS and UK NEQAS Embryo Grading Scheme.

Grading helps the laboratory decide which embryo may be most suitable for transfer or freezing. It is a ranking tool, not a diagnosis or guarantee. An embryo with a high grade may not implant, while an embryo with a lower grade can sometimes result in a healthy birth.

Ask the embryologist to explain:

  • Which grading system the clinic uses
  • What your embryo’s grade describes
  • Whether it is suitable for transfer or freezing
  • How much weight the clinic gives the grade alongside developmental timing and your circumstances

How does a clinic decide which embryo to transfer?

The decision considers embryo development, grade, the age of the person providing the eggs, treatment history and the risks of multiple pregnancy.

NICE generally recommends single-embryo transfer when a top-quality blastocyst is available and says no more than two embryos should be transferred in any cycle. Transferring more embryos can increase the chance of twins or a higher-order multiple pregnancy, which carries health risks for the pregnant person and babies.

Suitable embryos not transferred may be frozen with valid consent. A “full cycle” of IVF includes the ovarian-stimulation episode and the transfer of any resulting fresh and frozen embryos, rather than only the first fresh transfer.

What information should men receive from the embryology team?

Men can feel reduced to producing a sample while updates focus entirely on eggs and embryos. You are entitled to understandable information, while respecting both partners’ consent and privacy.

Ask for an explanation of:

  • The semen sample and whether the planned fertilisation method changed
  • How many eggs were collected and how many were mature
  • How many mature eggs underwent conventional IVF or ICSI
  • How many showed normal fertilisation
  • How many embryos continued developing
  • Which embryo was selected for transfer and why
  • Which embryos were suitable for freezing
  • Any unexpected event and its implications for future treatment

Numbers often fall at each stage. Ask the clinic to prepare you for that attrition before treatment so updates are easier to understand.

Questions to ask before agreeing to IVF or ICSI

  • Why are you recommending conventional IVF or ICSI in our case?
  • How severe are the semen abnormalities, and have they been confirmed?
  • What happens if the treatment-day sperm sample is different from expected?
  • Should we freeze a backup sample?
  • If sperm retrieval is planned, what is the contingency plan if no sperm are found?
  • What fertilisation rate does the clinic expect in patients like us, and how uncertain is that estimate?
  • Which embryo grading system do you use?
  • Are any sperm-selection or embryo-monitoring add-ons being proposed?
  • What will the add-ons cost and what does the HFEA say about the evidence?
  • Who can explain the embryology report after the cycle?

The most important thing to remember

ICSI changes how sperm enters the egg; it does not make the rest of reproduction certain. Ask why the laboratory method is appropriate for your diagnosis, what happens if the plan changes and how fertilisation and embryo-development results will be explained. Good information helps both partners participate in treatment decisions.

The original webinar with consultant embryologist Lyndon Miles covers the laboratory journey from sperm collection to embryo transfer. Any wording presented as his personal opinion or quoted explanation should be checked against the recording and approved before publication.

For a wider treatment overview, read IVF for Men, or join The Male Fertility Hub community for peer support.

Sources

This article provides general information and does not replace advice from your embryologist, fertility clinic or another qualified healthcare professional.