The Science of Fertilisation in IVF
Fertilisation is the critical laboratory process in which sperm and egg unite to form an embryo. This takes place in our state-of-the-art embryology laboratory under highly controlled conditions — including precise temperature, pH levels, humidity, and air quality — that closely replicate the environment of the human fallopian tube. Every step is performed by experienced embryologists using advanced micromanipulation technology.
Fertilisation Methods
Conventional IVF (Insemination)
In conventional IVF, approximately 50,000–100,000 prepared sperm are placed around each mature egg in a culture dish. The sperm are allowed to penetrate and fertilise the egg naturally over a period of 12–18 hours. This method is typically used when sperm parameters (count, motility, and morphology) are within normal limits and there is no history of prior fertilisation failure.
ICSI (Intracytoplasmic Sperm Injection)
ICSI is a specialised technique in which a single high-quality sperm is selected and injected directly into the cytoplasm of a mature egg using a fine glass micropipette. This method bypasses the natural barriers to fertilisation and is recommended in the following situations:
Low sperm count or motility: When the total motile sperm count is insufficient for conventional insemination.
Abnormal sperm morphology: When a high proportion of sperm have structural abnormalities affecting their ability to penetrate the egg.
Surgically retrieved sperm: TESA, PESA, or micro-TESE samples require ICSI due to limited quantities.
Previous fertilisation failure: If a prior IVF cycle resulted in no or very low fertilisation with conventional insemination.
Frozen sperm: Thawed sperm may have reduced motility, making ICSI the preferred approach.
Preimplantation genetic testing (PGT): ICSI is required before PGT to avoid contamination from residual sperm DNA on the egg surface.
Split ICSI
In some cases, your embryologist may recommend a split insemination approach — where half of the mature eggs are fertilised using conventional IVF and the other half using ICSI. This strategy is sometimes used when sperm parameters are borderline, or when there is no prior fertilisation history to guide the decision. It allows the laboratory to assess how your eggs respond to each method.
Fertilisation Check
Approximately 16–18 hours after insemination or ICSI, the embryologist examines each egg under a high-powered microscope. Normal fertilisation is confirmed by the presence of two pronuclei (2PN) — one from the egg and one from the sperm — indicating that genetic material from both parents has been incorporated successfully.
Eggs that show no pronuclei (failed fertilisation), one pronucleus (abnormal), or three or more pronuclei (polyspermic fertilisation) are identified and excluded from further culture. The fertilisation rate — the percentage of mature eggs that fertilise normally — is typically 60–80% with conventional IVF and 70–85% with ICSI.
Factors That Affect Fertilisation
Several factors influence fertilisation success, including egg maturity (only metaphase II eggs can be fertilised), egg quality (which declines with age), sperm quality and DNA integrity, laboratory conditions, and the timing of insemination relative to egg retrieval. Our laboratory maintains the most stringent quality controls — including continuous environmental monitoring, HEPA-filtered air systems, and validated culture media — to optimise conditions for every patient.
Frequently Asked Questions
In conventional IVF, sperm fertilise eggs naturally in a dish. In ICSI, a single sperm is injected directly into each egg. ICSI is recommended when sperm quality is compromised, sperm is surgically retrieved, or previous IVF cycles had poor fertilisation. Your specialist will recommend the method most appropriate for your situation.
A normal fertilisation rate is typically 60–80% for conventional IVF and 70–85% for ICSI. For example, if 10 mature eggs are retrieved, you might expect 6–8 to fertilise normally. Not all fertilised eggs will develop into transferable embryos — further attrition occurs during the culture phase.
No. Only mature eggs (metaphase II stage) can be fertilised. Typically, 75–85% of retrieved eggs are mature. Immature eggs cannot be injected or inseminated successfully. Additionally, some mature eggs may fail to fertilise despite appearing normal.
Our embryology team will contact you by phone approximately 16–18 hours after the retrieval procedure — typically by mid-morning the day after your egg retrieval. You will be informed of the number of mature eggs, the number inseminated, and the number that fertilised normally.
Total fertilisation failure is uncommon but can occur, particularly with conventional IVF. If this happens, your specialist and embryologist will review the possible causes — which may include egg quality issues, sperm factors, or suboptimal insemination conditions — and adjust the approach for a future cycle, often switching to ICSI.
No. While ICSI significantly improves fertilisation rates in cases of male factor infertility, it does not guarantee that every egg will fertilise. Some eggs may be damaged during the injection process, and others may fail to activate despite successful sperm injection. Overall, ICSI produces normal fertilisation in approximately 70–85% of injected eggs.