What does 'egg quality' actually mean?

The first thing to understand is that an egg (or oocyte) is simply a single cell. Like almost every other cell in our body, it has two main components:

• a nucleus, which contains the chromosomes — the genetic information (DNA);
• a cytoplasm, which surrounds the nucleus and contains all the cellular machinery that keeps the cell alive and functioning, including mitochondria, proteins, RNA molecules and many other specialised structures called organelles.

When doctors talk about “egg quality,” we're actually referring to the health and function of both of these components.

Genetic quality

The first is genetic quality — whether the egg contains the correct number of chromosomes. A healthy mature egg contains 23 chromosomes. After fertilisation, these combine with the 23 chromosomes from the sperm to create an embryo with the normal complement of 46 chromosomes.

If the egg has too many or too few chromosomes, the embryo will usually also have an abnormal chromosome number (aneuploidy). This is one of the commonest reasons why embryos fail to implant, miscarriages occur, or, less commonly, babies are born with chromosomal conditions such as Down syndrome.

Unfortunately, genetic quality is influenced primarily by age. As women get older, the process of chromosome separation during meiosis becomes increasingly prone to errors. Despite many claims made online, no supplement has been shown to reverse this biological process or make an older egg genetically younger.

Cytoplasmic quality

The second component refers to everything inside the egg apart from its chromosomes — its mitochondria, energy production, proteins, messenger RNA, enzymes and other cellular machinery that allow the egg to complete maturation, fertilise successfully and support the earliest stages of embryo development.

One way to think about it is this: the chromosomes are the blueprint. The cytoplasm is the machinery that reads the blueprint and powers the construction. Both are essential.

An egg can have excellent cytoplasmic quality and still be chromosomally abnormal. Likewise, a chromosomally normal egg still depends on healthy mitochondria and a well-functioning cytoplasm to fertilise normally and support embryo development until the embryo takes over with its own genome.

In other words, both matter — but they are not the same thing.

Understanding this distinction also explains why different treatments aim to achieve different things. While no treatment has been proven to restore the genetic quality of ageing eggs, some interventions — including CoQ10 — are thought to support mitochondrial function and, therefore, the egg's cytoplasmic competence. That is a very different goal from changing the chromosomes inside the egg.

Scientists often use the term oocyte competence rather than simply egg quality, because an egg's ability to create a healthy embryo depends on both its chromosomal integrity and its cytoplasmic competence.