During a typical ovulation, one egg is released. Out of the million-plus eggs you’re born with, your body selects a single egg each menstrual cycle, releasing it from one ovary into the fallopian tube. Over an entire reproductive lifetime, only about 400 eggs will ever be released.
Why Only One Egg Wins
Each cycle doesn’t start with just one candidate. In the days leading up to ovulation, a group of small follicles (fluid-filled sacs that each contain an immature egg) begin growing in response to a hormone called FSH. This initial group is sometimes called a “cohort,” and it can include anywhere from a handful to over a dozen follicles.
As these follicles grow, one pulls ahead. Around the time a follicle reaches 8 to 11 millimeters in diameter, a selection process kicks in. The leading follicle ramps up its production of estrogen, which signals the brain to dial back FSH. That drop in FSH is enough to starve the smaller, less developed follicles of the hormone they need to keep growing. The dominant follicle, however, has already become more sensitive to FSH and can thrive even at lower levels. It’s a self-reinforcing advantage: the winner gets stronger while the competition fades.
What Happens to the Other Follicles
The follicles that don’t make the cut undergo a process called atresia. They gradually break down and get reabsorbed by the body. This isn’t a sign of anything going wrong. It’s a normal, ongoing process that happens at every stage of follicle development, from the earliest dormant eggs to nearly mature ones. The vast majority of eggs you’re born with will be eliminated this way rather than ever being ovulated.
To put the numbers in perspective: you start life with roughly one to two million eggs. By puberty, that number has already dropped to a few hundred thousand. By age 40, the average is around 25,000. The rate of loss accelerates after 35. Of all those eggs, only about 400 will ever mature and be released during ovulation. The rest are quietly reabsorbed over the decades.
When Two Eggs Are Released
Sometimes the body releases more than one egg in a single cycle, a phenomenon called hyperovulation. One study estimated that about 20% of people who menstruate have the capacity to hyperovulate, though it doesn’t necessarily happen every cycle. When two eggs are released and both are fertilized, the result is fraternal (non-identical) twins.
Several factors make hyperovulation more likely. Genetics play a strong role, which is why fraternal twins tend to run in families on the mother’s side. Age is another factor: people in their 30s and 40s sometimes have higher FSH levels, which can stimulate more than one follicle to reach maturity. Stopping hormonal birth control can also temporarily cause a hormonal surge that leads to multiple eggs being released.
Hyperovulation is also the principle behind fertility treatments. Medications used during IVF or ovulation induction deliberately raise FSH levels to encourage multiple follicles to mature at once, sometimes producing 10 or more eggs in a single cycle. Outside of medical intervention, releasing more than two eggs naturally is rare.
How Ovulation Timing Works
The single egg released during ovulation has a short window of viability. Once it leaves the ovary, it survives for about 12 to 24 hours. If it isn’t fertilized in that time, it breaks down and is shed along with the uterine lining during your next period.
Ovulation typically happens around day 14 of a 28-day cycle, but this varies. Cycles anywhere from 21 to 35 days are considered normal, and ovulation generally occurs about 10 to 16 days before the start of your next period. Tracking signs like changes in cervical mucus, a slight rise in basal body temperature, or using ovulation predictor kits can help you estimate when that single egg is released.
Does Each Ovary Take Turns?
There’s a common belief that ovaries alternate neatly, left one month and right the next. The reality is less orderly. Studies show ovulation is somewhat random, with a slight tendency to favor the right ovary. It’s possible to ovulate from the same side several cycles in a row. If one ovary has been removed, the remaining one can ovulate every cycle on its own, maintaining normal fertility.