A woman typically releases one egg per month. That single egg is the result of a selection process that starts with a much larger group of follicles, and understanding how that process works explains a lot about fertility, age, and why the number sometimes varies.
One Egg From Many Candidates
Each menstrual cycle, a cohort of follicles (small fluid-filled sacs that each contain an immature egg) begins to mature inside the ovaries. Research using daily ultrasound monitoring shows that a group of follicles measuring 2 to 5 millimeters gets recruited from a continuously available pool every cycle. From that initial group, one follicle outcompetes the rest and becomes the “dominant” follicle. It grows rapidly from about 5 millimeters to roughly 20 millimeters in diameter, while the egg inside matures to become capable of fertilization. The remaining follicles in the cohort break down and are reabsorbed by the body.
So while your ovaries activate multiple follicles each month, they almost always release just one mature egg. This happens around day 14 of a typical 28-day cycle, though the exact timing varies from person to person and even cycle to cycle.
How the Body Picks the Winner
Two hormones produced by the pituitary gland, a small structure at the base of the brain, drive the entire process. The first, follicle-stimulating hormone (FSH), does exactly what its name suggests: it triggers the growth of follicles and prepares eggs for release. The second, luteinizing hormone (LH), surges midcycle to trigger the actual release of the mature egg from its follicle. These two hormones work together through a feedback loop. As the dominant follicle grows, it produces rising levels of estrogen, which eventually signals the brain to release a burst of LH. That surge is what causes ovulation.
This feedback system is tightly regulated, which is why most cycles produce exactly one egg. When the system is disrupted, the result can be either no egg at all or, less commonly, more than one.
When More Than One Egg Is Released
Releasing two or more eggs in a single cycle is called hyperovulation, and it’s the reason fraternal twins exist. About 20% of people who menstruate have the capacity to hyperovulate, though it doesn’t necessarily happen every cycle.
Several factors make hyperovulation more likely:
- Genetics: The tendency often runs in families, which is why fraternal twins sometimes cluster in certain family lines.
- Age: Women near the extremes of their childbearing years are more prone to it. In teenagers, the hormonal feedback system is still maturing and may allow extra FSH to slip through. In women approaching their late 30s and 40s, a shrinking egg supply can trigger higher FSH levels, stimulating more than one follicle to reach maturity.
- Stopping hormonal birth control: After discontinuing the pill or other hormonal contraception, your body may temporarily release extra FSH while readjusting, which can trigger hyperovulation.
When No Egg Is Released
Not every cycle produces an egg. A cycle without ovulation, called an anovulatory cycle, happens when the hormonal signals don’t fire in the right sequence. You may still get a period (or something that looks like one), which is why anovulation often goes unnoticed.
Common causes include significant weight changes in either direction, very intense exercise routines, thyroid disorders, and polycystic ovary syndrome (PCOS). Certain medications, including some anti-epileptic and antipsychotic drugs, can also suppress ovulation. In many cases, treating the underlying condition or adjusting lifestyle factors restores normal ovulation.
Your Lifetime Egg Supply
Unlike sperm, which are produced continuously, a woman’s eggs are a finite resource established before birth. Most girls are born with about 2 million immature eggs. By puberty, that number has already dropped to around 400,000, not because they were used, but because the body naturally reabsorbs them over time. Of those 400,000, only about 400 to 500 will ever fully mature and be released during ovulation across an entire reproductive life. By the time menopause arrives, typically around age 51, roughly 1,000 immature eggs remain, but they are no longer fertile.
This steady decline in both egg quantity and quality is the main reason fertility decreases with age, even though ovulation may still be occurring regularly.
How Fertility Treatment Changes the Numbers
During IVF, injectable hormones override the body’s natural selection process and push multiple follicles to mature simultaneously instead of just one. The number of eggs retrieved in a single cycle varies widely, from as few as 2 to more than 40, mostly depending on age and ovarian reserve.
Average egg retrieval counts by age give a sense of the range:
- Ages 20 to 30: 15 to 20 eggs
- Ages 31 to 35: 12 to 18 eggs
- Ages 36 to 40: 8 to 12 eggs
- Age 41 and older: 5 to 10 eggs
Research shows that retrieving between 10 and 20 mature eggs typically offers the best chance of producing at least one viable embryo. Women with PCOS, who have an unusually high number of egg-containing follicles, sometimes produce more than 40 eggs in a single stimulated cycle. These numbers highlight just how many follicles are available each month that normally would have been discarded by the body’s natural selection process.