Women’s eggs begin dying before birth and continue declining every day until menopause, which typically occurs around age 50. A female fetus actually has the most eggs she’ll ever have at about 20 weeks of gestation, carrying roughly 6 to 7 million. By the time she’s born, that number has already dropped to about 1 to 2 million. By puberty, only 300,000 to 400,000 remain. This lifelong decline isn’t something that starts at a certain age. It’s happening constantly, from the womb onward.
How the Egg Supply Changes With Age
The drop in egg count follows a predictable curve, though the exact numbers vary from person to person. At birth, you start with 1 to 2 million eggs. By puberty, roughly 300,000 to 400,000 survive. By age 40, the average is around 25,000. At menopause, fewer than 1,000 remain, and those are no longer sufficient to sustain a menstrual cycle.
The rate of loss isn’t steady. It accelerates noticeably after age 35. Before that point, eggs are still being lost every day, but the pace picks up in the mid-to-late 30s and continues increasing through the 40s. This acceleration is one reason fertility specialists often describe 35 as a meaningful threshold, not because fertility suddenly drops off a cliff, but because the decline in both egg quantity and quality shifts into a higher gear.
Why 99% of Eggs Never Get Ovulated
Of all the eggs a woman is born with, less than 1% will ever be released during ovulation. Over a reproductive lifetime, that works out to roughly 400 to 500 eggs. The remaining 99% die through a process called follicular atresia, which is essentially the body breaking down and reabsorbing follicles (the tiny fluid-filled sacs in the ovaries that contain eggs).
This isn’t a malfunction. It’s a normal, hormonally regulated process. Each month, a group of follicles begins developing, but typically only one “wins” and releases a mature egg. The rest are reabsorbed. This selection process happens cycle after cycle, steadily reducing the total reserve. In the early stages of life, particularly right after birth and during puberty, the body clears large numbers of eggs through a cellular recycling process. Later, a different form of programmed cell death takes over as the primary driver. Both are normal biological housekeeping.
Egg Quality Declines Alongside Quantity
The number of eggs isn’t the whole story. Egg quality, meaning the likelihood that an egg has the correct number of chromosomes, also drops with age. Chromosomal errors in eggs are a major cause of miscarriage and conditions like Down syndrome.
Up until about age 35, the rate of chromosomal abnormalities in eggs is relatively low, under 5% for most types of errors. After 35, those rates climb significantly. The type of error changes too. In younger women, when a chromosomal mistake does happen, it tends to involve whole chromosomes failing to separate properly. In older women, smaller pieces of chromosomes (individual chromatids) are more likely to missegregate. Chromosomes 15, 16, 21, and 22 are the most susceptible to these errors, which is why trisomy 21 (Down syndrome) becomes more common with maternal age.
This means that even if a woman in her early 40s still has thousands of eggs remaining, a larger proportion of those eggs will carry genetic errors that make a healthy pregnancy less likely.
What Speeds Up Egg Loss
Several factors can accelerate the natural timeline of egg depletion. Smoking is one of the most well-documented. On average, smokers reach menopause more than a year earlier than nonsmokers, reflecting a faster rate of egg loss over time.
Chemotherapy is far more dramatic. Certain cancer treatments can wipe out the equivalent of about 10 years’ worth of ovarian reserve. Chemotherapy drugs kill eggs directly through programmed cell death, but they also damage the blood supply to the ovaries, which primordial follicles (the smallest, most dormant eggs) depend on to survive. There’s also evidence that chemotherapy triggers a chain reaction: by destroying actively growing follicles, it removes the hormonal signals that keep dormant eggs in a resting state, causing a wave of premature activation that further depletes the reserve.
Other factors linked to faster depletion include ovarian surgery, radiation therapy targeting the pelvic area, and certain autoimmune conditions. Genetics also play a significant role. The age at which your mother or older sisters reached menopause is one of the stronger predictors of your own timeline.
Birth Control Does Not Preserve Eggs
A common belief is that hormonal birth control, by suppressing ovulation, should “save” eggs and delay menopause. The logic seems intuitive: if you’re not ovulating, you’re not using up eggs. But the math doesn’t support it. You only ovulate about 400 to 500 eggs in a lifetime. The overwhelming majority of egg loss happens through the background process of follicular atresia, which continues regardless of whether you’re on the pill.
Large studies confirm this. Women who used oral contraceptives for 10 or more years had no difference in the timing of menopause compared to women who never used them. Birth control prevents ovulation, but it does not slow down the constant, underlying attrition of the egg reserve.
What Happens at Menopause and After
Menopause is defined as going 12 consecutive months without a period. It typically occurs around age 50 and is driven by the egg supply dropping below roughly 1,000 follicles, a number too low to sustain regular hormonal cycles. At that point, the ovaries can no longer produce enough estrogen and other hormones to trigger monthly ovulation and menstruation.
Interestingly, the ovaries don’t go completely silent the moment menopause begins. Some residual follicular activity can persist in the first year or so after the final period, occasionally producing hormone levels similar to premenopausal ranges. But this activity is sporadic and fades. The eggs that remain are too few and too aged to restore fertility, and they gradually disappear entirely in the years that follow.