The quality of the eggs depends on the reproductive age and more importantly on the number of eggs remaining in the ovaries. The number of eggs does not increase or even stay the same during reproductive years, but constantly declines until menopause. By menopause, all eggs have been exhausted and ovulation no longer occurs. The average age of menopause in the US is 51, unless the woman is a smoker or has other risk factors.
A woman is born with all the eggs she will have for the rest of her reproductive years and that number is estimated to be between 1-2 million at birth. There is a constant decline in the egg reserve and most of the eggs following birth are lost in the first 10-12 years, down to approximately 400,000 at puberty. At age 35-37, the total number of eggs remaining in the ovaries is estimated to be 25,000 and less than 500 at the time of menopause.
A single egg is released from the ovary each month through the process of ovulation. In order to mature an egg and ovulate, a number of eggs develop at the same time and the “best egg” gets selected in the first part of the menstrual cycle. It is during the process of egg recruitment and development that the eggs are lost each cycle. If there is one egg released each month, one would assume that there would be approximately 400 eggs lost during a lifespan (12 ovulations per year over 35-40 years). However, the group of eggs that start growing each cycle to generate the “best egg” tends to disappear within that cycle. This is also true in women who are pregnant, on birth control pills, taking Lupron, breastfeeding or has irregular or no menstrual cycles at all. It is believed that the loss of eggs in the ovaries is independent of any events or exposure to medications and cannot be stopped. The loss of egg reserve can be accelerated by a number of factors including smoking, exposure to toxins, drugs, surgery, poor lifestyle and endometriosis.
The consequences of decreased egg reserve are decreased fertility and increased rate of miscarriages. Women in their 20s are at low risk for infertility and miscarriage compared to someone in their 40s based on their egg reserve. Once the egg reserve starts declining, the quality of the eggs becomes poorer and the eggs tend to be chromosomally abnormal, even though ovulation and periods continue regularly without any symptoms. In other words, quantity does not affect the quality until a threshold level is reached which is generally observed in mid 30s. After age 35-37 the quality is affected based on lower quantity or decreasing egg reserve.
Each cell in the body should have 46 chromosomes (44 + XX in women and 44 + XY in men) including the egg. Prior to fertilization and just before the egg is extruded from the ovary (process of meiosis), the chromosome number of the egg has to be brought down to 23 to accommodate the other 23 coming from the sperm to make 46 for the embryo to be healthy. During this process called meiosis, the chromosomes of the egg may separate unevenly, leaving a chromosome number in the egg that is other than 23. In such a case, the egg is chromosomally abnormal and thus the embryo, regardless of the sperm quality. The outcome is either no implantation and no pregnancy or a miscarriage in the first trimester.
It is not well documented in medical literature whether all the eggs are normal at birth, get exposed to environmental factors over the years and become abnormal. Some eggs are abnormal at birth and tend to be selectively released later in life. Most likely, the ovaries and thus the eggs are exposed to a number of factors through life and become abnormal over time or during meiosis, similar to other systems in our body. Unfortunately, there is currently no medical or surgical treatment to reverse such a process to create new eggs. Having said that, most recent research suggests that there may be stem cells in human ovaries that may actually give rise to new egg development. More research and data will reveal whether this is indeed true or not, but the idea of growing new eggs in the ovaries is always exciting and stimulating. It would also be promising for those women who have low egg reserve.