Fertilisation failure, where sperm does not successfully fertilise the egg during IVF, can be an unexpected challenge. This occurs even when both eggs and sperm appear normal under conventional assessments. In most cases, failed fertilisation suggests underlying biological abnormalities that are not easily detected through routine fertility tests.

The incidence of fertilisation failure is approximately 10–15% in traditional IVF cycles and around 3% in ICSI cycles. A deeper understanding of fertilisation mechanism and contributing factors can help in addressing this issue effectively.

How Does Fertilisation Happen?

Fertilisation is a complex process that begins when sperm binds to the outer protective layer of the egg. Enzymes released from the sperm’s head break down this layer, allowing the sperm to penetrate and enter the egg. Once inside, the sperm’s genetic material (DNA) fuses with the DNA of the egg, triggering a cascade of cellular changes necessary for successful fertilisation.

In IVF, two primary techniques are used to fertilise the egg:

• Traditional IVF – The egg is placed in a culture dish with at least 20,000 sperms, allowing fertilisation to occur naturally. This method closely mimics natural conception, with a fertilisation rate of approximately 50–70%.
• Intracytoplasmic Sperm Injection (ICSI) – A single sperm is selected and injected directly into the egg using a microneedle. This method bypasses many barriers to fertilisation and achieves a higher fertilisation rate of around 60–80%.

Despite the use of these advanced techniques, some eggs still fail to fertilise, requiring further investigation into potential causes.

Reasons Why Eggs Do Not Fertilise

Identifying the cause of fertilisation failure is essential before proceeding with another IVF cycle. Some of the most common reasons include:

• Poor egg quality – Eggs that are not fully mature or have chromosomal abnormalities may fail to fertilise. This issue is often linked to advanced maternal age, diminished ovarian reserve, or ovarian dysfunction.
• Poor sperm quality – Sperm with structural defects, low motility, or high levels of DNA fragmentation may struggle to penetrate the egg or complete fertilisation. Severe sperm abnormalities significantly reduce fertilisation success.
• Unidentified causes – In many cases, the eggs and sperm appear normal under microscopic examination, yet fertilisation does not occur. This suggests that certain functional deficiencies in either the sperm or the egg may be affecting the process, but they remain undetectable through standard laboratory assessments.

Determining the exact reason for fertilisation failure is challenging, but thorough evaluation of previous IVF cycles can provide insights for refining future treatment strategies.

Treatment Options for Fertilisation Failure

For patients who experience failed fertilisation, adjustments in treatment protocols can enhance the chances of success in subsequent IVF cycles. Some of the possible solutions include:

• Switching to ICSI – If a patient previously underwent traditional IVF and experienced fertilisation failure, ICSI is often recommended for the next cycle. By directly injecting a selected sperm into the egg, ICSI increases the likelihood of successful fertilisation.
• Advanced sperm selection techniques – Technologies such as Physiological ICSI (PICSI) and Intracytoplasmic Morphologically Selected Sperm Injection (IMSI) enable the selection of the most functionally competent sperm, improving fertilisation rates.
• Oocyte activation treatment – Some eggs fail to activate after sperm entry, preventing normal fertilisation. Calcium ionophore treatment can be used to enhance oocyte activation, particularly in cases where previous ICSI cycles resulted in poor fertilisation rates.
• Use of donor eggs or sperm – If significant egg or sperm abnormalities are identified, the use of donor gametes may be considered as an alternative approach to improve fertilisation and overall IVF success rates.

Meet Our Fertility Specialists

Dr Chua Ka-Hee

Consultant Obstetrician & Gynaecologist, Fertility Specialist

MBBS (Singapore), MRCOG (UK)