Baby girls enter the world with enough of them to populate perhaps 40 small cities. A dozen or so years later, the first will make a debut of its own. And in the months and years to come, others will appear regularly, sometimes greeted with relief, other times with disappointment, perhaps most often with a touch of annoyance.
Abdullah Pope/Agence France-Presse — Getty Images
Not women's eggs, obviously.
Now, for women in the baby boom generation, they may be coming more sporadically, or not at all, signaling unmistakably that one time of life is over, and another begun. But what happened to all those eggs?
When girls are born, they have about two million eggs in their ovaries, nestled in fluid-filled cavities called follicles. That may sound like a lot, but consider that months earlier, when they were still in utero, they may have had as many as six or seven million eggs. Those eggs are still immature, and the proper name for them, by the way, is oocytes (rhymes with: nothing).
The first eggs to bite the dust were those in the fetus, which waste away. And by the time a girl reaches puberty, most of her remaining eggs have also deteriorated and been reabsorbed. If that sounds ominously like something from a "Star Trek" episode about the Borg, imagine if all those eggs had to take the customary path out of the body.
Even with the Great Egg Disappearance, girls enter puberty with many more than they will use, 300,000 or more. Each month, the body produces a hormone, FSH, which stimulates the follicles to prepare an egg for maturation and release.
With eggs backed up like bowling balls on a busy Saturday night at the lanes, the ovaries can afford to be a little wasteful, and as many as several dozen follicles are called into action. Then a single mature egg — usually, anyway — gets the tap on the shoulder and begins its travels to the uterus.
As for the maturing eggs that didn't make the grade, there is no second chance. But they do not go out on their own. "Each month you probably lose a thousand or so," said Dr. James T. Breeden, president of the American Congress of Obstetrics and Gynecology. "There's just a natural death of them."
For all the eggs a woman begins with, in the end only about 400 will go through ovulation. While men produce sperm throughout their lives, over time the number of eggs declines, and they disappear with increasing frequency the decade or so before menopause. Those that remain may decline in quality. "When you have a thousand or less within the ovaries, you're thought to have undergone menopause," said Dr. Mitchell Rosen, the director of the Fertility Preservation Center at the University of California, San Francisco.
It's true that women make far more eggs than they end up using, but men should not pass judgment. "They produce millions of sperm, millions," Dr. Rosen said. "The whole process is not the most efficient in the world."
Questions about aging? E-mail boomerwhy@nytimes.com
Booming: Living Through the Middle Ages offers news and commentary about baby boomers, anchored by Michael Winerip. You can follow Booming via RSS here or visit nytimes.com/booming. You can reach us by e-mail at booming@nytimes.com.
This article has been revised to reflect the following correction:
Correction: February 5, 2013
An earlier version of this article incorrectly described estrogen levels at the time of ovulation. They rise, rather than fall.
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