As a woman, you are born with all the eggs you have in your lifetime, typically 1–2 million. Over time, this healthy egg supply decreases in two ways. First, you lose eggs each month – about a thousand a month after puberty – and by age 35 you have only about 6 percent of your remaining egg count.

Many women believe that how many eggs they have left – their egg count – is the primary indicator of their fertility. But, while egg count is part of the equation, the true driver of fertility is egg quality, and the decline in egg quality is a natural, inevitable result of age. Declining egg quality and infertility come naturally with age. 

What is medically known as “egg quality” is not actually a fixed thing. Your egg quality indicates how prepared your eggs are to become healthy embryos once fertilized. In order to become a healthy embryo, your egg(s) must have the right number of chromosomes and the ability to combine with the chromosomes of the sperm. Also, they must have enough energy in order to grow and divide normally.

Your eggs develop before ovulation and they are affected by outside factors, so you want to keep your body as perfect as possible. Everything that you do affects your eggs, so the older you get the more environmental toxins your eggs have been exposed to and the more likely they are to be abnormal.

But all this does not mean there aren’t ways to keep your eggs healthy as you age. The best way is to start early. It is always good to improve your overall health as a way to improve your fertility potential. Maintain a healthy weight because being underweight or overweight can decrease your chances to conceive. Exercise, but not to a point of exhaustion—do yoga, Pilates, walking, light jogging or strength training instead of long-distance running.

If you’ve been trying to conceive for a while, you’ve probably heard a lot of talk about all kinds of ways to improve your egg quality. There is no treatment to improve egg quality; however, techniques such as In-Vitro Fertilization (IVF) allow the extraction of multiple eggs, hoping that some of them still reach the required quality. 

If you do not have any viable eggs after treatment, IVF may resort to egg donation, an excellent alternative to achieve your dream of having a baby at home, because with use of donor eggs, IVF success rates are not affected by a patient’s age, rather the age of the egg donor and quality of the egg are more significant.

If you have poor egg quality, then you have a higher rate of structural genetic abnormalities; however, by transferring chromosomally normal embryos, this technique maximizes the chance of embryo implantation, having a normal pregnancy and, ultimately, of taking a baby home.

Bad quality eggs can alter the reproductive process and make the difference between carrying a pregnancy to term or not. Poor quality eggs can also lead to chromosomally abnormal pregnancies and increase the risk of a genetic disorder in the baby.

So what causes poor egg quality? As from the age of 35 onwards, your pregnancy probability begins to decline significantly. Age is the most significant factor affecting egg quality in women; however, even as a young woman, you may have poor quality eggs as a result of conditions ranging from genetic problems, lifestyle, obesity and polycystic ovarian syndrome (PCOS) among others.

A blood test to measure your follicle stimulating hormone (FSH) blood levels can be performed on the 3rd -5th day of your menstrual cycle in order to obtain data on ovarian response and thus estimate the quality of your eggs. High levels of FSH can indicate low egg production and negatively affect their quality. Another blood test  called Anti Mullerian Hormone (AMH does exactly the same.

However, the best way to check whether your eggs have the ability to grow and develop properly is through an IVF cycle, which allows a direct egg observation followed by Preimplantation Genetic Diagnosis (PGT-A) , to detect chromosomal abnormalities in the embryo.

There’s no fertility test for egg quality. The fertility testing that exists, called ovarian reserve testing, tests for markers of your egg count, not your egg quality. However, some fertility tests can tell you how many eggs you have left, which can help predict how many eggs you’ll be able to retrieve during an egg freezing or IVF cycle.

The only way to know for sure if an egg is chromosomally normal is to attempt to fertilize it, and, if fertilization is successful, to perform a genetic test on the embryo; the egg quality of an unfertilized egg is not easily accessible. However, because errors in the eggs’ DNA happen naturally and inevitably over time, and because the impact of age on egg quality is consistent and universal, your age provides an accurate picture of what percentage of your eggs are normal.

With age there’s a decrease in egg quality, yes, you’re born with all your eggs, but for most of your life, they remain dormant in your ovaries, immature, until they’re called to action during a particular time when they will potentially be ovulated. Just before ovulation, these eggs go through a process of maturation and cell division.

Older eggs are more prone to errors during this division process; that’s why it’s more likely that older eggs, once ovulated, will contain abnormal DNA. Without healthy DNA, an egg can’t make a healthy baby. And as you age, more of your eggs become unhealthy.  

All women have some percentage of abnormal eggs. Women in their early 20s have about 20 percent abnormal eggs, while women in their 40s have upwards of 80 percent abnormal eggs. Most eggs with abnormal DNA, also known as “aneuploidy,” don’t fertilize at all, hence the relationship between fertility and egg quality. A small percentage may fertilize, but result in miscarriage, and an even smaller percentage may result in genetic disorders for the baby, such as Down syndrome. 

If you are in your 40s and you retrieve 10 eggs in an IVF cycle, it’s likely that only 10–20 percent of those eggs will be normal. If you retrieved 20 eggs in two IVF cycles, still only 10–20 percent of those eggs will be normal—but you’ll have more to work with because you retrieved more. This is one of the explanations for declining success rates as women age, even when using advanced reproductive technology like IVF. If there are very few healthy eggs to work with, the chance of success is low.

Fertility is driven by egg quality, and this is the bottom line. Both egg count and egg quality affect your fertility, but egg quality is really the boss here. Even when fertility medicine comes into play, the relationship between age, fertility, and egg quality affects chances of success. 

Fertility treatments like IVF use hormone medications to prompt the ovaries to produce multiple eggs in one cycle, but some of those eggs will still be abnormal (how many are abnormal will depend on your age). So while IVF can help increase the chances of finding some healthy eggs, it cannot make more of your eggs healthy. 

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