Ovarian aging is a natural and inevitable physiological aging process. The age-dependent decline in oocyte quality accelerates between 35 and 40 years, and the natural transition from menopause usually occurs between the ages of 40 and 45 years, with an average age of menopause between 50 and 52 years.
The concept of reproductive aging is based on the principle that eggs peak in number during fetal life, undergo degeneration, and do not regenerate. Remarkably, the most eggs women will have are when she is a 20-week-old fetus with approximately 6-7 million eggs. At birth, the number of eggs falls to 1-2 million, and at puberty, she has only 300,000-500,000 remaining eggs. From puberty though menopause, it is estimated women will ovulate about 400-500 total eggs.
Although female fertility declines with advancing age, it is difficult to predict the overall rate of reproductive decline. The quality of a woman’s eggs is highest in her mid 20s through her early 30s, representing the most fertile period in her life. After the early 30s, both the quality and quantity of eggs begin to decline, resulting in a decline of fertility potential, with the most significant declines occurring in the mid 30s and early 40s.
Quality vs Quantity
“Ovarian reserve” refers to egg quantity and quality or reproductive potential, and it impacts a woman’s ability to conceive. Age plays a significant factor in fertility and is directly linked to reproductive aging.
What is egg quantity?
Egg quantity is the number of eggs a woman has. As mentioned, it’s currently believed that a woman will have the greatest amount of eggs she will ever have when she is actually still in her mother’s womb. That number will continue to decrease as she ages, Each month, a woman has a certain number of eggs that are available to ovulate. In a normal healthy cycle, one egg will be picked at random to ovulate and the others will essentially die. A woman in her 20’s and early 30’s will likely have many eggs to choose from to ovulate each month. During her late 30’s and early 40’s there will be less eggs to choose from. After age 45, there are usually not many eggs left at all.
What is egg quality?
Egg quality refers to the genetic makeup of an egg. A normal egg is one that can result in a healthy, genetically normal or euploid pregnancy. This egg should contain 23 chromosomes. An abnormal egg is one that is unhealthy. It generally will not result in pregnancy or may lead to early pregnancy loss. This type of egg is called aneuploidy and may have missing or extra chromosomes. As discussed, a woman has a certain amount of eggs available to ovulate each month. Of those eggs, some will be normal & some will be abnormal. A woman in her 20’s and early 30’s will likely have more normal eggs available to choose from each month, meaning a higher chance she will ovulate a normal egg and become pregnant. As she ages, the ratio of good to bad eggs will shift and there will be a higher chance of ovulating an abnormal egg, meaning a lower chance of pregnancy.
The most accurate test for egg quality is to look at a woman’s age and make predictions based on this biological factor. Finding out your egg quantity however, can be done fairly simply with what is called ovarian reserve testing. A combination of blood tests along with an ultrasound can give you a good picture of how many eggs you may have left.
The decline in both egg quantity and quality may manifest in a longer time to conception. As a result, fecundability–the probability of achieving a pregnancy in one menstrual cycle– begins to decline significantly in the early 30s, with a more rapid decline after the age 37.
Diminished Ovarian Reserve (DOR) refers to diminished egg quality, egg quantity, reproductive potential, and can therefore cause infertility. A high day 2 or 3 FSH level, a low AMH level, a low antral follicle count (AFC), and a history of a poor response during stimulation in an IVF cycle are indicative of DOR (diminished ovarian reserve). It is important to note that evidence of DOR does not necessarily translate to inability to conceive.
In most cases, the causes of DOR is unknown, with reproductive aging being the primary cause. Other factors that can affect ovarian reserve include:
- tobacco use
- underlying medical conditions
- prior ovarian surgery
- systemic chemotherapy
- pelvic radiation
- certain genetic abnormalities.
The bottom line is, egg quantity does not equal egg quality. Quantity can be tested directly with hormonal blood work and ultrasound where as quality cannot. The best indicator of egg quality is mostly always your age, which is the most important predictor of one’s natural fertility. Your fertility specialist can perform a reproductive check up where an individualized assessment of egg quantity is made and advise you on how to best meet your reproductive goals.