A woman at age 20 has roughly 100,000 to 200,000 eggs remaining in her ovaries. That’s a significant drop from the approximately 2 million she was born with, but it’s still a large reserve, and her fertility is at or near its biological peak.
How Egg Count Changes Over Time
A baby girl is born with all the eggs she will ever have, typically around 2 million. Unlike sperm, which men produce continuously, eggs are a finite resource that depletes steadily from before birth onward. By adolescence, the count has already fallen to roughly 400,000. By age 20, estimates place the number somewhere between 100,000 and 200,000, depending on individual variation.
This loss continues regardless of what’s happening reproductively. Whether you’re pregnant, on hormonal birth control, or not ovulating for any other reason, eggs continue to disappear at the same rate. The process driving this is called atresia: a form of programmed cell death where the vast majority of egg-containing follicles simply break down and get reabsorbed by the body. Of the 1 to 2 million follicles present at birth, fewer than 400 will ever release a mature egg during ovulation. That means more than 99.9% of your eggs are lost to atresia rather than ovulation.
Why You Lose So Many Eggs
Each month, your ovaries recruit a batch of follicles (small fluid-filled sacs, each containing an immature egg) to begin developing. Hormonal signals select one follicle as the “winner” that matures fully and releases its egg at ovulation. The rest of that batch, sometimes dozens of follicles, undergo atresia and die off. This isn’t a sign of something going wrong. It’s a built-in regulatory system that controls how many follicles are in the active pool at any given time.
This means you lose far more eggs per month than just the single one you ovulate. The monthly loss from atresia dwarfs the one egg released during a cycle, which is why the total count drops so substantially between birth and early adulthood even though you’ve only ovulated a relatively small number of times by age 20.
Fertility at 20
Despite having lost the majority of eggs you were born with, age 20 is one of the most fertile points in a woman’s life. The chance of conceiving naturally in any given menstrual cycle is about 20 to 25 percent, which is as high as it gets. This is partly about quantity (you still have a large reserve) but also about quality.
Egg quality refers to whether an egg has the correct number of chromosomes. Eggs with the wrong number, called aneuploid eggs, are less likely to result in a healthy pregnancy. You might expect that younger women would have very low aneuploidy rates, but research analyzing over 15,000 embryo biopsies found that women aged 23 and younger actually had aneuploidy rates above 40%. This drops in the mid-20s before climbing again with age. So while your 20s are your most fertile years overall, peak egg quality may be slightly better in the mid-to-late 20s than at 20 itself.
What Happens After 20
From your 20s into your early 30s, egg count continues its steady decline, but fertility remains relatively strong. The more noticeable shift begins in the mid-30s. By age 37, the average woman has about 25,000 eggs left, and after 35 the rate of decline accelerates. Both the number and the chromosomal quality of remaining eggs drop more steeply from this point forward.
The decline isn’t a cliff edge at 35, though. It’s a gradual curve that steepens. A woman at 30 still has strong fertility. A woman at 36 hasn’t suddenly lost it. But the biological trend is real: each year after the early 30s, the odds of conceiving naturally in a given cycle decrease, and the risk of chromosomal abnormalities in eggs increases.
Factors That Speed Up Egg Loss
While age is the primary driver, certain factors can deplete your ovarian reserve faster than the typical timeline. Smoking is the only lifestyle factor with strong evidence linking it to accelerated egg loss, according to the American Society of Reproductive Medicine. Beyond that, the main culprits are medical rather than behavioral:
- Ovarian surgery: removing cysts or ovarian tissue reduces the physical reserve of eggs.
- Cancer treatments: chemotherapy and radiation can damage or destroy egg-containing follicles.
- Genetic conditions: certain disorders affecting the X chromosome can cause earlier depletion.
- Autoimmune conditions: some can trigger the immune system to damage ovarian tissue.
- Loss of an ovary: losing one ovary cuts the physical reserve, though the remaining ovary often compensates hormonally.
In some cases, women have diminished ovarian reserve with no identifiable cause. If you’re 20 and concerned about your egg count for any reason, a blood test measuring anti-Müllerian hormone (AMH) can give a rough estimate of your remaining reserve, and an ultrasound can count visible follicles. These tests don’t tell you about egg quality, but they can flag if your reserve is lower than expected for your age.