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In an eggshell...
- Mature eggs have a polar body, and 23 chromosomes, but immature eggs are either in germinal vesicle or MI stage
- Mature eggs are ready for fertilisation, but immature eggs, even if fertilised, have a higher chance of forming abnormal embryos
- In egg freezing cycles, mature eggs are preferred for freezing
“We’ve got 15 eggs, congrats, but 10 are mature, so we’ll go ahead and freeze them”
This is actually something you might hear in your egg-freezing cycle. And yes, I’ll get to the main question.
“What about the other 5?”
At this point, you’ll likely hear that they’re immature eggs, or have some defects, making them the “odd ones out” from the freezing batch.
But let’s get to the former. You may wonder: Why aren’t immature eggs frozen? And what really is the difference between mature and immature eggs?
The answer? We’ll let our embryologist describe it all, below!
The Basics of Egg Maturation: A Recap
Did you know women are born with all the eggs they'll ever have?
It's true! At birth, we have about 1-2 million immature eggs stored in tiny sac-like structures called follicles within our ovaries.
These eggs sit dormant in our ovaries, like they're hibernating, until puberty arrives and our hormonal cycles kick in. Then the action really starts!
Every month, our hormones recruit a batch of follicles to wake up and start developing. It's a bit like a competition - several follicles begin to grow, but most don't make it to the finish line. Many undergo cell death, while usually just one follicle becomes dominant enough to house a mature egg.
When you think about the numbers, it's pretty amazing - we're born with over a million eggs, but only about 400-500 will ever complete the journey to ovulation during our lifetime. The rest are lost through this natural selection process, or as we age.
This race to release the mature egg happens with every menstrual cycle. During the final stage, our body produces a surge of Luteinizing Hormone (LH), which triggers the egg to complete its maturation and detach from the follicle wall.
Then it's released into the fallopian tube - what we call ovulation. That's where it waits for a possible meeting with sperm for fertilisation.
So each month, this incredible process ensures that typically just one mature, viable egg gets its chance at potential fertilisation.
What is a Mature Egg?
A mature egg, also known as a metaphase II (MII) oocyte, has completed the first meiotic division and is ready for fertilisation.
In our natural cycle, this is the stage at which it’s ready to be released by the follicle.
Essentially, all the cells in our body have 46 chromosomes, but a mature egg cell will have only 23 chromosomes.
This is because the goal of the egg cell is to undergo fertilisation with a sperm that has 23 chromosomes, ultimately forming the zygote, which has 46 chromosomes.
Here’s how we can identify a mature egg:
- They typically have a standard size of 110-120 micrometres (0.11-0.12 mm) in diameter, excluding the zona pellucida.
- Presence of the First Polar Body: This is a small cell between the outer membrane and the inner membrane of the egg cell. This is produced when the oocyte undergoes a cell division, pushing out the polar body (smaller cell) that has the other ‘23 chromosomes’.
- Developed features: The zona pellucida, a glycoprotein layer surrounding the egg cell, and the cumulus cells (that surround the egg) are more developed in mature oocytes.
- Clear perivitelline space: Mature eggs show a distinct, visible space between the cell membrane and zona pellucida.
- Sunburst appearance: When retrieved from the ovaries, embryologists usually see cumulus cells surrounding the egg. When the egg is mature, these cells typically spread out, giving it a ‘sunburst’ appearance
What is an Immature Egg?
An immature egg has not undergone the necessary cell division to reduce its chromosome number from 46 to 23. This means it cannot be fertilised by sperm, but even if it were to be, it might not produce a viable or normal embryo.
Immature eggs can exist in two forms:
- Germinal Vesicle (GV): Consider this the “pre-teen” phase 👶. At this stage, the egg cell is larger, has an intact nucleus (which is visible too).
- MI Oocyte: Consider this the “teen” phase of the egg cell 🚶. This form is more mature than a germinal vesicle but still lacks a polar body.
Can Eggs Mature After Retrieval?
Yes, immature eggs can mature in the lab through a process known as in vitro maturation (IVM). This is where MI or GV eggs (the pre-teen and the teen phase) are deliberately collected and matured in the lab under controlled conditions and specific IVM- media.
The catch? Definitely the fact that this is planned beforehand. For IVM, the ovarian stimulation protocol, the media used, and the egg freezing cycle length also differ.
But- there is a particular type of IVM that can be applied to normal cycles, known as rescue IVM. Usually, any immature eggs in a normal cycle are discarded, but rescue IVM attempts to mature the MI and GV eggs retrieved either for a few hours or overnight.
Essentially, these eggs are ‘rescued’ and then fertilised using ICSI (Intra cytoplasmic sperm injection).
However, this method is not routinely practised as studies do not have solid evidence of its efficiency. There are studies that have transferred a mix of mature and rescue IVM eggs, making it harder to draw a solid conclusion.
What’s the Difference Between Mature and Immature Eggs?
The major differences between mature and immature eggs lie in their genetic content, viability and fertilisation potential. These are also the same reasons why mature eggs are preferred for freezing over immature ones during your egg-freezing cycle:
What are the Factors that Affect Egg Maturation?
Several factors can influence the number of mature eggs retrieved in a cycle. Let's go through some important ones:
1. Age
As women age, both the quantity and quality of their eggs decline. This process accelerates, especially past ages 35-38, leading to an increased likelihood of retrieving immature or fewer number of eggs.
A study also found egg immaturity to be higher in women aged 41 and above, due to age-related reserve decline and ovarian ageing.
But surprisingly, egg immaturity was also found to be frequent in the <35 year age group as well. In this case, it could be due to more number of smaller follicles developing, chances of not responding to the egg freezing trigger, or in some cases, genetic causes or premature ovarian failure.
2. Ovarian Reserve
Our ovarian reserve is an estimate of the number of eggs available in our ovaries. This is usually estimated based on the results of tests such as AMH (Anti-Mullerian hormone) and AFC (Antral Follicle Count).
Women with a higher ovarian reserve often produce more follicles during stimulation, which can lead to a greater number of eggs being retrieved. However, this may be a mix of mature and immature eggs, and not just mature, per se.
For example, while PCOS patients often produce a higher number of oocytes, they may be immature, or the quality of these oocytes and embryos can be lower, due to hormonal imbalances and other factors.
3. Controlled Ovarian Stimulation Protocol
The type and dosage of medications used during ovarian stimulation plays a vital role in egg maturation.
A well-tailored protocol can optimise egg development, while inappropriate dosages (which is quite rare) or inaccurate medicine storage or application may lead to issues with egg maturation.
For instance, someone with PCOS has a risk of OHSS. In such cases, a change in protocol is needed to maximise the mature eggs from their egg freezing cycle while keeping the risk minimal.
4. Timing of Trigger Injection
The timing of the human chorionic gonadotropin (hCG) or egg freezing trigger shot is key for ensuring that eggs reach maturity before retrieval.
The egg freezing trigger shot is usually given 36 hours before the egg collection surgery. If the trigger is administered inaccurately, too early or too late, it may result in immature eggs during the egg collection surgery.
The key here is to monitor the effect of the trigger shot via hormone levels right before surgery. This can help understand whether the trigger has worked, thus increasing the chances of retrieving mature oocytes.
5. Underlying Health Conditions
Conditions such as polycystic ovary syndrome (PCOS), endometriosis, obesity, elevated or reduced hormone levels, such as thyroid, or autoimmune diseases, etc., can affect egg quality and maturation.
For instance, Hashimoto's disease is an autoimmune disorder that leads to an underactive thyroid (hypothyroidism). Inadequate levels of thyroid hormone can affect ovarian function and the development of follicles, thus potentially affecting egg development and maturation as well.
Hence, it’s important to assess your medical history and tailor your medication protocol accordingly when there’s an underlying medical condition.
6. Individual Stimulation Response
Regardless of how well the protocol is tailored, there are differences in how every individual responds to stimulation medication. In some cases, there might be slow or uneven response, leading to immature eggs at the time of egg retrieval.
Think Egg Freezing, Think Amilis
Choosing the right support and knowing the right information can make your egg freezing cycle a lot less of a maze.
That’s why at Amilis we decided to make the egg freezing journey accessible and affordable for all. And part of it is providing the right resources and support.
In fact, when you opt to start your fertility journey with Amilis, here’s what you get:
- Free consultations with top fertility clinics in the UK
- Free consultations with expert doctors
With zero waiting times, you get to consult expert specialists and counselling at clinics. And if you’re just starting out your fertility journey, you also get:
- The option to book an AMH test (at just £80) or a full hormone fertility test (at just £130) anytime, at your convenience
- A personalised report on your hormone levels and what it could mean for your fertility
- A free 1-1 call with our team to figure out the next steps (and to get any of your questions, answered!)
When you opt to use our free resources or book our tests, you also get a stellar support team (that’s us 👋) guiding you along at every step!
Figuring out where to get started? Book a free chatl, or take our personalised fertility quiz to know more!
Frequently asked questions
Can immature eggs mature in the lab?
Yes, it is possible for immature eggs to mature in the lab. This can happen in two ways. Either via IVM or in vitro maturation, where immature eggs are retrieved on purpose and matured using IVM media. The second way is rescue IV,M which aims to “rescue” immature eggs during a normal cycle by maturing them for a couple of hours or overnight before ICSI or fertilisation.
What percentage of eggs retrieved are mature?
On average, about 70-85% of retrieved eggs are typically mature. However, this percentage can vary widely depending on several factors. Age plays a significant role, with younger women often having higher maturity rates. The stimulation protocol used, medical history, timing of trigger injection and individual response to fertility medications also impact maturity rates.
What are some reasons for eggs not maturing?
Eggs may fail to mature for various reasons. Poor response to ovarian stimulation medications is a common cause, where follicles grow but eggs inside don't complete maturation. Genetic factors can affect a woman's egg quality and ability to mature properly. Underlying health conditions like PCOS (Polycystic Ovary Syndrome) can lead to higher rates of immature eggs. Additionally, the timing of the trigger shot before egg retrieval is crucial - if given too early or too late, it may affect egg maturity rates.
How many mature eggs can I expect after retrieval?
The number of mature eggs after retrieval varies widely between individuals. On average, women under 35 might expect between 8-15 mature eggs, while those over 40 might expect 2-8 mature eggs. Your individual response to stimulation medications significantly impacts this number. Some women are "high responders" and produce many eggs, while "low responders" may produce fewer. Your doctor can provide a more personalised estimate based on your age, AMH levels (a hormone that indicates ovarian reserve), and other fertility markers.
