In female fertility, egg quality can be an important factor that determines the likelihood of conceiving. Egg quality is often an even bigger factor in fertility than the number of eggs a woman has left. However, like egg count, egg quality is something that declines naturally with age. Unlike sperm, eggs aren’t produced throughout a woman’s life.

What does egg quality mean? 

A woman is born with all the eggs she will ever have in her life. However, this doesn’t mean that all of those eggs are ready to be released from the ovaries to have a chance of being fertilised. Instead, eggs will remain in a ‘dormant’, immature state until their time comes. In the days before ovulation, part of the fertile window, eggs undergo a maturation process that involves cell division and prepares them to be fertilised. 

Naturally, as a woman loses eggs every month, her total number of healthy eggs - also known as euploid eggs - will decrease over time. That number will also go down due to the ageing of her eggs - the older the egg, the more likely it is that an error will occur in the maturation process. This results in an egg containing abnormal DNA, referred to as an aneuploid egg. Aneuploid eggs can’t develop into healthy babies if fertilised.

Regardless of age, it’s expected that all women will have some amount of eggs that will become abnormal or aneuploid during maturation. However, that number increases with age. Women in their early 20s are expected to have around 20% aneuploid eggs, while women in their 40s will likely have more than 80% aneuploid eggs. This age-based decline in egg quality is universal, according to studies.

Poor egg quality, or aneuploidy, has a significant impact on fertility. The majority of aneuploid eggs don’t result in fertilisation, which can severely impact a couple’s chances of conceiving. On the other hand, those that do fertilise are more likely to result in miscarriage or birth defects. This is why older mothers are more likely to have a miscarriage or give birth to a child with Down’s syndrome. 

What causes low egg quality? 

All eggs which haven’t yet gone through the maturation process remain in the ovaries, unaffected from everyday toxins and illnesses. This protects them from the majority of things which normally affect our health. However, there are a few significant exceptions to this. 

Smoking can have a big impact on your fertility as a whole, not just your egg quality. Women who smoke generally have fewer eggs in total, a lower percentage of which are healthy. Additionally, women who smoke reach menopause four years earlier on average than women who don’t. Women who are trying to conceive should consider quitting smoking, or at the very least reducing their smoking habit to give themselves the best chances of developing normal, euploid eggs.

Another thing that can affect your egg quality is exposure to toxic substances such as those used in chemotherapy. Although the procedure can be lifesaving for cancer patients, it poses a significant risk to the likelihood of future conception. For this reason, many women choose to freeze their eggs prior to undergoing cancer treatment. This way, they stand a far better chance of conceiving through IVF afterwards. 

Finally, there is evidence to suggest that endometriosis can have an impact on egg quality. While it’s not clear exactly how endometriosis alters egg quality, it has been noted that, in IVF where eggs are donated from a woman with endometriosis, successful fertilisation is far less likely than when eggs are donated by a woman who doesn’t have the condition. 

Does AMH indicate egg quality? 

AMH is an abbreviation for anti-Mullerian hormone, a biological compound released by the follicles of the ovaries. In fertility tests, it is often used as an indicator of egg count - the more AMH detected, the more working follicles you have and so the more eggs you’re likely to have. 

AMH testing doesn’t tell doctors anything about the genetic health of your eggs, nor whether they’re likely to become aneuploid during maturation. Fortunately, the decline of egg quality due to age is relatively predictable, so fertility specialists will likely have a good idea of your egg quality based on your age and lifestyle.

Does diet affect egg quality? 

When trying to conceive, people are understandably often eager to try any and all fertility treatments in the hope that they’ll result in a successful pregnancy. However, it should be stressed that there is no evidence to suggest that traditional fertility treatments or lifestyle alterations (except those mentioned above) can reduce the likelihood of aneuploidy. 

Things such as yoga, exercise and diet are said to help fertility, and to some extent, they can. Taking folic acid supplements can assist the development of a healthy embryo. Similarly, meditation can improve mental health, having a positive impact on fertility. However, these have no proven direct effect on egg quality. 

However, diet may have an indirect effect instead. Mitochondrial health is a key factor in egg quality - if the mitochondria can’t power the cell properly it can’t multiply or grow normally. Although the resulting aneuploidy can’t be reversed, mitochondrial health can be improved so that future eggs are less likely to become abnormal during the maturation process. 

As well as eating a healthy, balanced diet and getting a range of vitamins, women who are trying to conceive should also consider taking L-carnitine. This helps to boost mitochondrial energy production and, alongside providing the ovaries with sufficient nutrients and supplements, can help to make it more likely a maturing egg will be euploid. 

Why does egg quality decrease with age?

All cells age. As with humans, the ageing process in cells results in things starting to wear out and go wrong, which can lead to a decrease in overall functionality. This is why many of the cells in our bodies are replaced over and over. Egg cells, or oocytes, age just like any other type of cell, but these aren’t replaced. The eggs you have at age 30 are the same cells you had when you were born - just older and not always functioning as well as they once did. 

Unfortunately, once an egg has become abnormal due to an error in the maturation process, there is nothing that can be done to return it to a healthy state. This, paired with the fact that ageing is a natural process that can’t be prevented, means that it’s very hard to counter the effects of poor egg quality.