Common Questions About a Woman’s Egg Supply Through Her Lifetime

When you talk about female fertility, there are many questions that come up in relation to a woman’s eggs, more specifically, “how many eggs does a woman have?” While most of us are pretty connected with our bodies, there are many such questions that come up when discussing a woman’s egg supply. While asking such questions are good, the answers to such question vary from person to person and are highly dependent on what time in the woman’s life we are talking about. Since there is so much of natural variation between women, it is important to be aware about your own egg reserve, egg quality, and quantity. Let’s look at some of the common questions about a woman’s egg supply throughout her lifetime.

An immature egg is known as an oocyte. Oocytes stay inside follicles, which are fluid-filled sacs that contain the immature egg.^{(1, 2, 3)} Now oocytes rest in these follicles in your ovaries until they start to mature and grow. The oocyte grows up to become an ootid and then develops into an ovum (plural ova), which is a mature egg. Egg cells in the body are constantly degenerating and going through a process of cell death known as atresia.^{(4, 5)}

When a female reaches puberty, a new batch of eggs is selected every day to start developing. The number of eggs that leave the ‘waiting area’ every day depends on the exact age of the woman. When a woman moves into the peak reproductive years, nearly 30 to 40 eggs will leave this waiting area and start developing every day. This means a thousand eggs will develop every month. Of these, though, only one-tenth will enter the menstrual cycle, and just one egg will develop fully and reach the ovulation stage.⁽⁶⁾

Since many eggs start developing but die off before they reach ovulation, a woman goes through nearly 300,000 to 400,000 eggs during their reproductive years but only actually ovulates 300 to 400 eggs. An average woman starts to run out of eggs and begins menopause around the age of 52 years. Egg quality, though not directly related to quantity, is also correlated. As a woman continues to age, both the egg quality and quantity begin to witness a decline.⁽⁷⁾

Now that we are aware of some of the basic facts about a woman’s eggs let us look at some of the common questions.

Yes, female babies are born with all the egg cells they are ever going to have in their lifetime. It surprises many people to learn that the body will not make any new egg cells during a woman’s lifetime. This has been an accepted fact for a long time. However, reproductive biologist John Tily conducted some research in 2004 that had initially found new egg stem cells in mice. Nevertheless, Tily’s theory has been majorly refuted by the wider scientific community, though there is still a small group of researchers who continue to pursue this line of research.⁽⁸⁾

The peak number of eggs that a woman will ever have during her lifetime develop at around 20 weeks of gestation, meaning before being born and while in the mother’s womb. At this point, women have around six to eight million eggs. And no new eggs will ever be made after this point on.

When a fetus is in the early stages of development, a female has approximately six million eggs already.⁽⁹⁾ The number of these eggs, or oocytes, is steadily reduced so that when a baby girl takes birth, she only has one to two million eggs.

It is natural to wonder that since the eggs are already present in the body, what stops the menstrual cycle from starting right from the birth of a baby girl. The menstrual cycle stays on hold until a girl child reaches puberty. The stage of puberty begins when the hypothalamus in the brain begins to produce gonadotropin-releasing hormone (GnRH).^{(10, 11)} In turn, the gonadotropin-releasing hormone boosts the pituitary gland to start producing follicle-stimulating hormone (FSH). It is the follicle-stimulating hormone that kick starts the development of eggs and leads to an increase in estrogen levels. With so much going on inside the body, it is quite common for women to experience mood swings during this time.

Menstruation typically begins around two years after the breast bud appears, which is the little bit of tender tissue that develops into a breast. The average age for this is usually 12 years, but for some, it can start as early as eight years. Many even experience it by the age of 15.⁽¹²⁾

On reaching puberty, a girl has anywhere between 300,000 to 400,000 eggs. Before puberty, over 10,000 eggs die every month.⁽¹³⁾

After puberty, the number of eggs that die off every month starts to decrease gradually. After the start of the menstrual cycle, a woman begins to lose around 1,000 immature eggs every month. This comes to around 30 to 35 immature eggs every day.

While scientists still are not sure about why this happens, it is known that this process is not influenced by most of the factors that we can control. For example, it is not influenced by our hormones, pregnancies, nutritional supplements, birth control pills, health, or even the intake of chocolate.

One exception to this is smoking. It is important to note that smoking can accelerate egg loss. In some cases, traditional and certain chemotherapies can also aggravate egg loss. ⁽¹⁴⁾

Once the follicles mature, they start becoming sensitive to the hormones of your menstrual cycle. Even though several follicles mature, only one single egg is the ultimate winner. Only one egg goes on to reach the stage of ovulation. This is usually the case. There are exceptions to his rule, and these exceptions can sometimes lead to the birth of fraternal twins.

It is estimated that by the time a woman reaches 32, her fertility levels start to slowly decline, decreasing more rapidly after the age of 37.⁽¹⁵⁾

By the time a woman reaches the age of 40, she is just about down to only three percent of her pre-birth egg supply.⁽¹⁶⁾

While there is no one rule for all women about how many eggs there will be left, once you hit your 40s, it is likely that the egg supply will decline further. Of course, there are some factors, like smoking, which might mean you will have even fewer eggs left as compared to another woman who does not smoke.

Studies have shown that the average woman has less than a five percent chance of conceiving and getting pregnant per cycle after reaching her 40s. The average age of menopause is 52 years.⁽¹⁷⁾

If you calculate these numbers, you will find that only around 25,000 eggs are left in the ovaries once by the time you reach 37 years of age. At this time, you still have about 15 years left until you reach menopause. Of course, some may hit menopause earlier, and some will hit it later.

Just before you hit ovulation each month, your eggs start to divide. The older eggs are more prone to experience some errors during this division process. This makes it more likely that they will contain certain abnormal chromosomes. This is why the risk of having a baby with Down’s syndrome or other developmental abnormalities goes up as you age. As the years go by, the eggs either get ovulated or discarded. Only the older, lower-quality ones remain behind.

Once the supply of viable eggs runs out, your ovaries stop making estrogen, and you enter the phase of menopause. The exact age at which this happens depends on the precise number of eggs you were born with. For example, if you were born with a larger number of eggs, you could be amongst a small percentage of women who are able to have biological children naturally, even in their mid or late 40s.

If you are having trouble having a baby, you now know what could be the possible reasons. If you are concerned that you are aging and the quality of your eggs might decrease, then you may consider the option of freezing your eggs, also known as elective fertility preservation (EFP), or oocyte vitrification. Many women who opt for elective fertility preservation are motivated by their biological clock. Others may be about to begin chemotherapy or radiation treatments that may have an impact on their fertility. However, in such cases, freezing your eggs before chemotherapy is not considered to be an elective preservation. It is known as medically indicated fertility preservation.

Many women remain worried about their chances of having a baby with frozen eggs. However, studies indicate that the chances of having a child with frozen eggs are better if you freeze the eggs before you turn 35.⁽¹⁸⁾

There are also other reproductive technologies that you can turn to achieve a pregnancy in your later years. In vitro fertilization, for example, allows women to become pregnant in their 40s and even in their 50s. However, in vitro fertilization using your own eggs might not be a viable option for an infertile woman who has already crossed 40. However, taking donor eggs from younger women can allow older women in their 40s and 50s to achieve a successful pregnancy.⁽¹⁹⁾

If you are struggling to get pregnant, you should talk to your doctor at the earliest about various fertility options and understand how a woman’s fertility changes over time.

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