Only mature eggs can be used for IVF. If an egg is mature it means it has completed the first phase of DNA division and it is ready to be fertilised by a sperm. Around 80% of the eggs collected from the ovaries are mature, so how do we know which are ready to be fertilised and what can we do with immature eggs?

Growing eggs for IVF

Women are born with hundreds of thousands of immature eggs in their ovaries. Once they reach puberty the menstrual cycle begins and the store of eggs starts to reduce. Every month several eggs are chosen and they all start developing but only one becomes dominant. The dominant egg is released at ovulation in hopes of being fertilised naturally and the other eggs which started growing with it are now lost.

IVF drugs allow for control over the ovary so we can rescue the eggs that would normally be lost each month. The aim is to grow multiple eggs simultaneously so they all reach full maturity at the same time. As the eggs develop, the maturity is monitored by the size of the fluid-filled follicle each one grows in - the larger the follicle the more likely it is that the egg is mature. However, this requires careful timing because if the follicle gets too large the egg will be ovulated and lost.

There is only one kind of egg that can make an embryo.

Depending on the size of the follicle and the quality of the egg, a wide range of different egg types can be collected for IVF including:

  • Mature eggs
  • Immature eggs
  • Abnormal eggs
  • Empty shells
  • Giant eggs

The ONLY type of egg that can be fertilised is a mature egg.

Immature eggs come in two varieties:

  1. Germinal vesicle eggs - these are very immature eggs which come from smaller follicles. They are easily identified by a large nucleus in the centre of the egg.
  2. Metaphase 1 (M1) eggs - These are eggs which have started the maturation process but have not been able to complete it. The large nucleus is no longer visible in the centre of the egg but the polar body hasn't been pushed out yet either. M1 eggs are considered immature but occasionally they can spontaneously mature in the lab after they have been collected from the ovaries. They can be injected with sperm if this happens but they are less likely to make successful embryos.

It might sound crazy but IVF labs actually don't aim to have 100% of eggs mature, the average is around 80%. It may be frustrating to have some eggs which are essentially wasted but it is important that we see some immature eggs in IVF. This is because we need to make sure the all the fluid from the larger follicles containing mature eggs has been drained. If 100% of the eggs were mature it would leave us wondering whether any suitable eggs had been left in the ovaries.

What happens during egg maturation?

Egg maturation is a complicated process which relies on a series of events occurring inside the egg at strict time points. The key to maturation is making sure that the egg removes half of its chromosomes to make room for the sperm's chromosomes which arrive at fertilisation. For this to happen, the egg’s DNA undergoes replication followed by two rounds of division known as meiosis 1 and meiosis 2.

36 hours before the egg collection is scheduled you will be asked to take a 'trigger injection'. The trigger mimics a surge of hormones which occurs naturally in the body to cause an egg to be ovulated. All the eggs which have been growing simultaneously will be ovulated around 38 hours after the trigger injection so the egg collection procedure must be carefully timed to make sure the eggs aren't lost. The trigger causes the eggs to progress through the final stages of maturation known as meiosis 1.

When the egg has completed meiosis 1, the maturation process is complete. In the lab we can confirm that this has happened by looking for a small sphere on the edge of the egg called the polar body. This tiny sphere contains half of the eggs chromosomes which have been discarded to make room for the sperm's chromosomes. 

The egg completes the second phase of meiosis during fertilisation after the sperm has entered the egg.

mature egg

If you are having ICSI you can find out whether your eggs are mature within a couple of hours. If you are having traditional IVF you will have to wait until the next day.

Immediately after eggs have been retrieved from the ovaries they are surrounded by a huge mass of support cells called the cumulus cells. This makes it impossible to identify a polar body to confirm whether the egg is mature or not. The cumulus cells will be stripped away a couple of hours after the egg collection if the plan is to inseminate them using ICSI. This means that you can find out the number of mature eggs on the same day as the eggs collection. However, the supportive cumulus cells need to stay in place for traditional IVF insemination so you will not be able to find out about egg maturity until the following day.

There are other factors that can be used to indicate maturity immediately after egg collection, but these are much less reliable. For example, the appearance of the supporting mass of cumulus cells changes from being dark and tightly compressed around immature eggs, to fluffier and more expanded once the egg has matured. This can therefore give a quick and easy impression of whether the eggs are mature or not. Don’t be afraid to ask your embryologist what the eggs looked like as they were being collected.

Egg maturing

Rescuing immature eggs

The majority of IVF clinics discard immature eggs. However, a technique called in vitro maturation (IVM) aims to continue the maturation process in the lab after the eggs have been removed from the ovaries.

The technique hasn't quite been perfected yet but in the very near future it could allow us to make use of all the eggs collected instead of having to discard around 20%. Watch this space.