Here’s a fact to bring out at your next family dinner: part of you once lived inside your grandmother.
Before your mother was born, her ovaries were already making eggs. One of those eggs would, decades later, become half of you. So while your grandmother was pregnant with your mom, she was also carrying the egg that became you. Three generations, nested like Russian dolls.
That’s just the opening chapter in one of biology’s stranger stories.
Most cells in your body are constantly being replaced. Eggs are different: you get your whole supply before you’re born, and that’s it.
The numbers are wild. A female fetus has about 6 to 7 million eggs at 20 weeks of pregnancy. By birth, that’s down to 1 to 2 million. By puberty, somewhere between 300,000 and 500,000 remain1.
How many actually get used? Over a lifetime of periods, only about 400 to 500 eggs are ever released1. The rest, well over 99 percent, quietly retire through a natural process of cell self-destruction. Think of it as a lottery where nearly every ticket expires unused.
The pace of loss picks up in the late thirties. By 50, fewer than 10,000 remain1. When the supply runs low enough, periods stop. That’s menopause.
If you have hundreds of thousands of eggs, why don’t you use them all at once? Because your body has a braking system. Each egg sits inside a tiny sac called a follicle, and those follicles make a hormone called anti-Müllerian hormone, or AMH. AMH holds most eggs back, so only a small batch starts developing each month. Without that brake, you could burn through your entire supply in a few cycles1.
AMH levels typically peak in your mid-twenties and fall as the egg supply shrinks1. With a weaker brake, fewer eggs are held back, so a bigger batch starts developing each month. Usually only one is still released; the rest fade away. The countdown to menopause speeds up.
Occasionally more than one egg does get released in the same month. That may be one reason twins are more common in women in their forties1.
When women in their forties use eggs donated by younger women, their chances of pregnancy are similar to those of younger women1.
In other words, the uterus holds up well with age. The biological clock people talk about is really an egg clock.
An egg’s chromosomes are held together by proteins called cohesins, a kind of biological glue. That glue is laid down before you’re born and is never replaced. The egg then waits, sometimes for four decades, for its turn to be released4.
Over that time, the glue wears thin. Researchers found that women aged 40 and over had noticeably less of these glue proteins in their eggs than women around 202. Another team found that in human eggs, the paired chromosome copies sit further apart as women get older, a sign the glue is loosening3.
When the glue fails, chromosomes can split the wrong way. Researchers now see this as one of the main reasons eggs are more likely to have the wrong number of chromosomes as we age3,4.
Eggs’ mitochondria, the tiny power plants inside cells, also decline and pick up small errors with age1. Scientists are still working out how much that matters.
Sperm production starts at puberty and keeps going for life, with fresh sperm made from stem cells that keep dividing. So sperm never face the forty-year-old-glue problem.
The trade-off is the photocopier. Every division carries a tiny chance of a copying error, and the errors add up over the years. A large Icelandic study found that each extra year of a father’s age adds about two new genetic changes to his child’s DNA5. The researchers say this helps explain why a father’s age matters for conditions such as autism and schizophrenia5.
For the number of chromosomes, though, eggs are the main story: more than 90 percent of extra-chromosome conditions trace back to the egg4.
In short, eggs hold on to decades-old glue, and sperm run a decades-long photocopier. Each ages in its own way.
Plenty of animals become less fertile near the end of their lives. But humans are unusual: we stop being fertile and then keep living for decades.
Among animals with backbones, the only others known to do this routinely are killer whales and short-finned pilot whales1. Why we evolved this way is still a mystery. Scientists have plenty of theories, but no firm answer.
Your eggs have been with you longer than almost anything else in your body. They were made inside your mother while she was inside your grandmother. They’ve waited decades, and most quietly bowed out without ever being called on. The countdown ends in menopause.
It’s not a design flaw. It’s one of the oldest and strangest stories your body carries.
If the countdown feels like it’s speeding up, our perimenopause check takes three minutes.