Why is female fertility referred to as a ‘biological clock’?
Egg number and egg quality decrease over the course of a woman’s lifetime, impacting natural fertility. However, because these terms are often mistakenly used interchangeably, it is important to define and differentiate egg quantity from egg quality.
What is the female biological clock?
Of all the factors involved in establishing a healthy pregnancy (sperm, the reproductive tract, and eggs) the only time sensitive aspect are the eggs in a woman’s ovaries. Women are born with a finite number of eggs, which cannot be regenerated or modified. As a result, the number and quality of eggs decrease over the course a woman’s lifetime, impacting natural fertility and odds of successful fertility treatment. There are many misconceptions about the biological clock, with the terms ‘egg quantity’ and ‘egg quality’ often being used interchangeably. We’re here to set the record straight, dispel common myths, and provide you with everything you need to know about egg quantity and quality.
What is egg quantity and how does it change over time?
‘Egg quantity’ refers to the amount of eggs left in a woman’s ovaries, at a given point in time. At birth, there are 1-2 million eggs, with this number gradually decreasing throughout early reproductive life. Over time, the rate of decline in egg quantity speeds up, usually becoming more accelerated at age 35 and beyond. Once the egg count approaches less than 1000, menstrual cycles start to become irregular and eventually stop, resulting in menopause. For reasons that are not always clear, some women may be born with an overall lower egg quantity to start with, whereas others may experience an earlier than expected acceleration in the rate of loss of eggs.
How is it tested?
- Every month a subset of the eggs stored inside the ovaries are recruited ‘out of hiding’ to the surface of the ovary, becoming visible on ultrasound. In a natural cycle where ovulation takes place, one of these eggs will be randomly selected to be the lucky one that gets released and has a shot at being fertilized by sperm. The rest of the recruited eggs are discarded and cannot be recycled to be used again in a future cycle. The size of the recruited pool of eggs is always in proportion to how many total eggs are stored away in the ovary. In addition, fertility treatments/medications can only impact the recruited eggs and cannot reach the eggs stashed in the ovary, to be used in future cycles. When doctors perform an ultrasound and count the number of eggs seen on the screen, it provides a ball park estimate of how many potential eggs can be stimulated and retrieved during an IVF cycle. A total count of at least 8 follicles (follicles=fluid filled spaces that each contain a single egg) is considered synonymous with having ‘normal’ ovarian reserve.
- Performing bloodwork to assess certain hormone levels can provide a relative indicator of egg quantity.
AMH (Anti-mullerian hormone) is produced by the cells that line the follicles. The higher the egg quantity, the higher the AMH level. In general, levels less than 1ng/ml indicate reduced egg quantity. AMH levels can be tested at any time in a woman’s menstrual cycle and regardless of whether they are using hormonal forms of birth control.
Follicle stimulation hormone (FSH), is another hormone level that can be measured to indicate egg quantity. It is produced by a gland in the brain, called the pituitary, in an effort to stimulate the ovaries to produce estrogen. When the FSH level is high, it indicates a low egg count as the brain is pumping out more of this hormone to try and stimulate the ovaries. FSH should only be tested on day 2 to 4 of menstruation as the level can fluctuate throughout the course of the cycle. In addition, FSH level will be falsely lowered in anyone taking estrogen-containing birth control.
What does ‘egg quantity’ tell me about my fertility?
A common misconception is that all women with high egg counts are very fertile. However, studies have definitively shown that egg quantity does not predict that chance of naturally conceiving a pregnancy. In an unmedicated, natural menstrual cycle, a single egg is typically ovulated. Therefore, the odds of becoming pregnant is not a ‘numbers’ game but more a function of the quality of that single egg that is ovulated. The number of eggs overall correlates to the number of eggs recruited in a given cycle, and available for treatment purposes. Therefore, egg quantity may provide doctors with information on whether a patient is an ideal candidate for treatments such as IVF, which rely on being able to access and utilize an adequate number of eggs.
What is egg quality?
Just like owning anything (i.e. cars, appliances, etc.) for decades, without the ability to change or replace parts, would lead to breakdowns over time, the quality of eggs diminishes as women age. Eggs become more prone to accumulating imbalances in the amount of DNA and the odds of ovulating a genetically abnormal egg increase with aging. An egg with the wrong number of chromosomes, if fertilized, will give rise to an embryo with the incorrect amount of DNA. This most often leads to the embryo not having the proper programs to guide normal growth and development- with most genetically abnormal embryos not implanting at all, leading to a negative pregnancy test that month. Sometimes an abnormal embryo will start to implant and THEN stop growing – this is the most prevalent cause of miscarriage. Some of these genetic errors can result in live birth of a child with significant health problems (i.e. Down’s syndrome).
How does egg quality change over time?
No one is immune to egg quality issues. Even in a woman’s 20s to early 30s, approximately 25-30% of the embryos that form from their eggs are anticipated to have genetic errors. This proportion rises to ~50% by age 37-38 and to over 70% at age 40 and beyond. This is the main reason why it is increasingly rare to hear of women over the age of 45 years old conceiving children using their own eggs (unless using eggs/embryos that they froze when they were younger).
How can it be tested?
There is no way to directly test a woman’s egg quality or understand what proportion of a woman’s eggs would give rise to genetically abnormal embryos. Unlike egg quantity, no blood test or ultrasound can allow us to assess egg quality on an individual basis. We know that most genetic errors found in embryos are derived from egg quality. The genetic content of the embryo is less tied to the age of sperm, as men are able to regenerate new sperm cells over time. Nowadays, many patients who undergo IVF treatment, opt to have their embryos genetically tested via preimplantation genetic testing (PGT). These embryo quality tests may provide an indirect assessment of egg quality.
How do you turn back or slow the biological clock?
There is no known way to ‘turn back the clock’ and regenerate the number of eggs or reduce the proportion of eggs that will give rise to abnormal, unhealthy embryos. However, healthy lifestyle choices, such as avoiding cigarette smoking – which is known to accelerate decline in egg quantity and increase the risk of premature menopause, can help to SLOW the biological clock. In addition, exposure to radiation or chemotherapy can accelerate decline in egg quantity. Women have the option to preserve their fertility, in general, and in advance of these types of treatments, by freezing eggs and/or embryos, to halt the biological clock.
The bottom line is, egg quantity does not equal egg quality. Quantity can be tested directly with hormonal blood work and ultrasound; quality cannot. The best indicator of egg quality is your age – which is the most important predictor of one’s natural fertility. You can always schedule a reproductive check-up with a fertility specialist to get a better understanding of where your fertility stands, where an individualized assessment of egg quantity is made and counseling on how to best meet your reproductive goals is provided.