The Patient's Guide to Embryology

Everything you need to know about egg maturity

Around 20% of the eggs collected from the ovaries are immature, so how do we know which are ready to be fertilized and what can we do with immature eggs?

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 fertilized by a sperm.

Growing eggs for IVF

Women are born with hundreds of thousands of immature eggs in their ovaries known as primordial follicles. Once the menstrual cycle begins, eggs start to mature and one gets ovulated every month. IVF drugs allow multiple eggs to grow and mature simultaneously and the aim is for them all to reach full maturity at the same time. Maturity is monitored by measuring the size of the fluid-filled follicle that each egg grows in – the larger the follicle, the more likely it is that the egg is mature. Growing multiple follicles requires careful timing because if one gets too large, the egg will be ovulated and lost, but if the follicles don’t get large enough, the eggs inside will be immature.

Types of eggs

Many different types of egg can be collected from the ovaries but only mature eggs are able to be fertilized. The type of egg depends on the size of the follicle, the quality of the egg and the equipment used.

Some of the egg types seen in IVF labs include:

  • Mature eggs
  • Immature eggs
  • Abnormal eggs
  • Out-of-zona eggs
  • Empty shells
  • Giant eggs

Mature eggs

Egg maturation is a complicated process which relies on a series of events occurring inside the egg at specific time points. The key to maturation is making sure that the egg removes half of its chromosomes to make space for the sperm’s chromosomes which arrive at the point of fertilization. For this to happen, the egg’s DNA replicates then divides over two phases known as meiosis I and meiosis II.

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. It takes 36 hours to complete the final stages of egg maturation but the eggs will be ovulated after around 38 hours. This means that the egg collection procedure must be carefully timed to make sure the eggs aren’t ovulated, but have had enough time to mature.

Mature eggs contain a small sphere on the edge of the egg called a polar body as seen below. This tiny sphere contains half of the eggs chromosomes which have been discarded.

Immature eggs

There are two types of immature eggs commonly seen in IVF:

  1. Germinal vesicles (GV eggs) – these are very immature eggs which come from small follicles. They are easily identified by a large nucleus in the centre of the egg.
  2. Metaphase I (MI) 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 small, spherical polar body hasn’t been pushed out yet either.
    • MI eggs are considered immature but it is possible for them to spontaneously mature in the lab or be encouraged to mature artificially.

How many mature eggs should I expect?

IVF labs don’t actually aim for 100% of eggs to be mature, the average is around 80%. It may be frustrating to have some eggs which cannot be used for treatment, but it is important to see some immature eggs because it ensures that no mature eggs have been left in the ovaries.

The average number of eggs collected is 8-12 but this number will depend on your age, medical history and how your ovaries respond to the hormone treatment.

Don’t be afraid to ask

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 which make it impossible to confirm whether the egg is mature or not. Eggs being inseminated with ICSI will need to have the cells removed within a few hours and this means that you can find out the number of mature eggs on the same day as the egg collection. For traditional IVF, the supportive cells need to stay in place so you won’t be able to find out about egg maturity until the following day.

Rescuing immature eggs

Many clinics discard immature eggs as they cannot be fertilized, however, some clinics attempt to artificially mature the eggs in the lab after they have been collected. In some cases, immature eggs are collected intentionally to avoid hormone stimulation treatment. The technique is known as in vitro maturation (IVM) and it aims to continue the maturation process which usually happens in the ovaries by incubating the eggs with a special nutrient solution for around 24-48 hours.

The technique hasn’t quite been perfected yet meaning that the eggs are less healthy than eggs which mature in the ovaries, but IVM is being researched and improved all the time and could become common practice in the near future.

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