IVF Egg Maturity: Why Eggs May Not Fertilise

What Does Egg Maturity Mean in IVF?

During an IVF cycle, fertility medicines stimulate the ovaries to help several follicles grow at the same time. Each follicle may contain an egg. During egg retrieval, the doctor collects fluid from the follicles, and the embryology team examines it under a microscope to identify the eggs.

However, finding an egg is only the first step. Before an egg can usually be fertilised, it needs to reach a particular stage of development. This readiness is known as egg maturity.

After retrieval, the embryology team may classify the collected eggs into different groups based on their stage of development and appearance:

• Mature eggs (MII or M2): Eggs that have reached the stage generally considered ready for fertilisation.
• Immature eggs (MI or M1): Eggs that have progressed through some development but have not yet reached full maturity.
• Germinal vesicle (GV) eggs: Eggs at an earlier stage of development that have not completed the maturation process.
• Degenerated or abnormal-appearing eggs: Eggs that show signs of deterioration or features that may affect their suitability for treatment.
• Unclear or difficult-to-classify eggs: In some cases, the embryology team may not be able to determine the egg's stage or quality clearly.

The exact terminology and format can vary between embryology laboratories. The important point is that the number of eggs retrieved is not necessarily the same as the number of mature eggs available for fertilisation.

Your fertility specialist and embryology team can explain what your results mean and whether they suggest any changes to a future treatment plan.

What are M1, M2 and GV eggs?

These three terms describe where an egg stands in its final development. The table puts them side by side.

Why Are Some IVF Eggs Immature?

Eggs do not all mature at exactly the same pace. Even when an ultrasound shows several follicles of a suitable size, some eggs may still be developing when egg retrieval takes place. As a result, not every egg collected during an IVF cycle will necessarily be mature enough for fertilisation.

Several factors may contribute to immature eggs during IVF, including:

• Uneven follicle growth: Follicles may grow at different rates, meaning their eggs may not all be at the same stage of development when the trigger injection is given.
• Trigger timing: If the trigger is given before enough follicles have reached the intended stage, some eggs may not complete maturation in time.
• Smaller follicles: Eggs collected from smaller follicles may be more likely to be immature, although follicle size alone cannot confirm an egg's maturity.
• Individual ovarian response: Each person's ovaries respond differently to stimulation medicines, which can affect follicle growth and egg development.
• Age and ovarian reserve: These factors can influence the overall response to IVF treatment and may be considered when reviewing egg maturity results.
• PCOS or polycystic ovarian features: Some people with PCOS may have many follicles at different stages of development, which can affect the pattern of follicle growth during stimulation.
• Endometriosis or previous ovarian surgery: Depending on the individual's circumstances, these may influence ovarian function or response to treatment.
• Natural cycle-to-cycle variation: The number and maturity of eggs can vary between IVF cycles, even when similar treatment protocols are used.

Finding immature eggs does not automatically mean that something was done incorrectly during treatment. Egg maturity depends on several interacting factors, including individual biology, follicle development and the timing of the trigger injection.

Your embryology report is best understood as part of the complete IVF process. Your fertility specialist can review the number of eggs retrieved, how many were mature, fertilisation results and embryo development together to help explain the outcome and guide any future treatment decisions.

Does every follicle contain an egg?

No. An ultrasound shows follicles, but it cannot tell whether each one holds a retrievable egg. Some come back empty, and some give an egg that is not yet mature.

For this reason, IVF results make more sense as a chain of counts:

It is normal for the count to fall at each step, even in a cycle that went exactly to plan. What follows after fertilisation is covered in our page on embryo development and blastocyst culture.

Where Does the Trigger Shot Fit In?

The IVF trigger injection helps prompt the final stages of egg maturation before retrieval. The egg retrieval procedure is scheduled around the injection, commonly about 34–36 hours later, although the exact interval depends on the medication used and your clinic's protocol.

Before deciding when to give the trigger, your fertility specialist may consider:

• The size and number of developing follicles
• Your hormone test results
• How evenly the follicles have grown during stimulation
• How your ovaries have responded to the medicines
• The risk of early ovulation and ovarian hyperstimulation syndrome (OHSS)
• Your response and outcomes in any previous IVF cycles

The trigger injection supports final egg maturation, but it cannot guarantee that every retrieved egg will be mature. If you are concerned about the timing of your injection, ask your fertility team to review the prescribed injection time and egg retrieval schedule with you.

Read our guide to the trigger shot in IVF, including its timing and purpose, to understand this step in more detail.

Can Immature Eggs Mature in the Laboratory?

Some immature eggs may continue to mature after they are collected and placed in laboratory culture. This is sometimes called in vitro maturation after retrieval. However, not every immature egg will mature, and the likelihood varies depending on the egg and its stage of development.

An egg that matures later may not have the same fertilisation or embryo-development potential as an egg that was already mature at retrieval. Even if an egg reaches maturity in the laboratory, successful fertilisation and further development are not guaranteed.

The embryology team assesses whether continued culture or any further laboratory steps are appropriate. Not every immature egg can be used, but an immature egg is not automatically discarded in every case. The available options depend on the laboratory's protocols and the individual circumstances.

What Happens to Mature Eggs After Retrieval?

Once the embryology team identifies the mature eggs, the next step depends on your treatment plan, sperm results, and fertility history. Options may include:

• Conventional IVF: Prepared sperm are placed with the eggs in a laboratory culture dish, allowing fertilisation to occur without directly injecting a sperm into the egg.
• Intracytoplasmic sperm injection (ICSI): An embryologist injects a single sperm into each suitable mature egg. Learn more about ICSI treatment.
• Egg freezing: Suitable mature eggs may be frozen for future use when fertility preservation is the goal.
• Other laboratory approaches: Your specialist may recommend additional steps depending on your clinical circumstances and treatment plan.

The appropriate option depends on factors such as sperm quality, previous fertilisation results, the number of mature eggs available and the plan discussed with your fertility specialist.

Is a Low Egg Maturity Rate a Sign of Poor Egg Quality?

Not necessarily. Egg maturity and egg quality are related, but they are not the same thing. A lower-than-expected maturity rate may be associated with uneven follicle development, trigger timing, your response to stimulation or natural biological variation.

Egg quality is influenced by several factors, including age. A single maturity percentage cannot tell you the complete picture of egg quality or predict the outcome of an IVF cycle on its own.

For a more detailed explanation, read our guide to egg quality before IVF.

Questions to Ask Your Doctor and Embryologist

If your embryology report shows that some eggs were immature, these questions can help you understand the results and discuss the next steps:

1. How many eggs were retrieved during my cycle?
2. How many eggs were mature at the time of assessment?
3. What was my egg maturity rate, and how was it calculated?
4. Were any immature eggs kept in culture for further assessment?
5. How many mature eggs were fertilised successfully?
6. Was ICSI used, and why was it recommended for my cycle?
7. Would you consider any changes if I undergo another IVF cycle?

Your answers can help put the maturity results into context alongside fertilisation and embryo development.

Can the IVF Protocol Change in a Future Cycle?

If a cycle produces a higher-than-expected number of immature eggs, your fertility specialist may review the complete treatment cycle before deciding whether any adjustments are appropriate.

This review may include:

• Stimulation medicines: The medication dose and duration may be reviewed. See our guide to IVF stimulation protocols.
• Follicle development: The team may assess whether the follicles grew at similar rates or remained at different stages.
• Trigger type and timing: The medication and its timing may be reviewed in relation to follicle development and the retrieval schedule.
• Monitoring: Ultrasound findings and hormone results may help guide decisions in a future cycle.
• Underlying factors: Conditions such as PCOS or endometriosis, along with ovarian-reserve findings including AMH levels, may be considered where relevant.

A protocol change is not always necessary. Your fertility specialist should make recommendations based on the complete cycle report, your medical history and your treatment goals rather than one percentage alone.

Were your eggs mature, but fertilisation did not occur as expected? Read our article on why IVF eggs fail to fertilise to understand some of the possible reasons behind this next stage.

Final Takeaway

The number of eggs retrieved is only the beginning of your IVF laboratory report. Not every egg collected will be mature enough for fertilisation. Some may be immature because their follicles developed at different rates, while other factors related to your ovarian response and treatment cycle may also play a role.

M2 or MII eggs are generally considered mature and suitable for fertilisation. M1 or MI and GV eggs are at earlier stages of development and may not yet be ready. However, egg maturity alone cannot predict whether fertilisation or embryo development will be successful.

If your report shows fewer mature eggs than you expected, ask your fertility specialist and embryology team to explain the complete process, from follicle development and egg retrieval to maturity, fertilisation and embryo development.

You can also learn more about how IVF egg retrieval works and how follicle numbers relate to the eggs collected. Would You Like Help Understanding Your IVF Report?

Would You Like Help Understanding Your IVF Report?

Medical Disclaimer: This article is intended for general educational purposes and is not a substitute for medical advice or diagnosis. Your fertility specialist and embryology team should interpret egg maturity, fertilisation, and embryo development results in the context of your individual treatment and medical history.