In a typical menstrual cycle, your body releases exactly one egg. Out of the roughly one to two million eggs you’re born with, only about 400 will ever be released during your reproductive years, spaced out one per cycle from puberty through menopause.
That single egg, though, is the survivor of a much larger monthly competition. Understanding how your body selects it, when more than one can slip through, and how fertility treatments change the math can help you make sense of everything from natural conception odds to IVF expectations.
How Your Body Picks the One
Each cycle, your body doesn’t just prepare a single egg and call it a day. Between days six and 14 of your cycle, a hormone called FSH (follicle-stimulating hormone) triggers a group of follicles in one ovary to start maturing. Each follicle contains an immature egg, and several begin developing at once.
By around day 10 to 14, one follicle pulls ahead. This “dominant” follicle grows faster, responds more strongly to hormonal signals, and essentially shuts down the others. The remaining follicles stop developing and are reabsorbed by the body. Then, around day 14, a surge of a second hormone (LH, or luteinizing hormone) causes that dominant follicle to rupture, releasing its single mature egg into the fallopian tube. That’s ovulation.
The follicles that don’t make it aren’t recycled for later use. They’re gone for good, which is one reason egg count declines steadily with age even though only one egg is released per cycle. Your body recruits and loses far more eggs each month than the one it actually ovulates.
When More Than One Egg Is Released
Sometimes, two or more eggs are released in the same cycle. This is called hyperovulation, and it’s the biological basis for fraternal twins. Unlike identical twins (which come from one fertilized egg splitting), fraternal twins happen when two separate eggs are each fertilized by a different sperm.
Several factors make hyperovulation more likely. Genetics play a clear role: if fraternal twins run in your family, you may carry a predisposition to release multiple eggs. Age is another factor. People nearing the edges of their childbearing years, both teenagers and those in their late 30s and 40s, are more likely to hyperovulate. In teenagers, the hormonal feedback system that normally limits ovulation to one egg is still maturing. In older individuals, rising FSH levels (the body’s attempt to compensate for declining egg quality) can stimulate more than one follicle to full maturity.
Hyperovulation isn’t something you can feel or detect at home. It doesn’t cause distinct symptoms compared to regular ovulation, and standard ovulation predictor kits won’t tell you whether one or two eggs were released.
How PCOS Changes the Picture
Polycystic ovary syndrome (PCOS) creates what looks like a contradiction: the ovaries contain far more follicles than usual, yet ovulation often doesn’t happen at all. Under current diagnostic guidelines, one sign of PCOS is having 20 or more follicles visible on one ovary via ultrasound, or ovaries larger than 10 cubic centimeters in volume.
Those extra follicles don’t mean extra eggs are being released. In PCOS, the hormonal signals that normally allow one dominant follicle to emerge are disrupted. Multiple follicles start developing but none fully matures, so ovulation becomes irregular or stops entirely. This is why PCOS is one of the most common causes of difficulty getting pregnant, despite the ovaries appearing “full” of potential eggs on imaging.
Egg Numbers During IVF and Egg Freezing
Fertility treatments deliberately override the one-egg-per-cycle default. During an IVF or egg freezing cycle, injectable hormones stimulate the ovaries to mature many follicles at once instead of letting just one dominate. The goal is to retrieve 10 to 15 mature eggs in a single cycle, giving the best balance between success rates and minimizing the risk of overstimulation.
The number of eggs retrieved depends heavily on age. On average:
- Under 35: about 16 eggs per retrieval
- 35 to 37: about 13 eggs
- 38 to 40: about 11 eggs
- 41 to 42: about 9 eggs
- Over 42: about 7 eggs
People with PCOS tend to produce more eggs during stimulated cycles, averaging around 23 eggs per retrieval compared to about 17 for those without PCOS. That higher yield sounds like an advantage, but more eggs doesn’t always mean more viable embryos. Egg quality matters as much as quantity, and a large number of retrieved eggs can also increase the risk of ovarian hyperstimulation syndrome, a painful and occasionally serious side effect of the medications.
Not every retrieved egg will be mature, and not every mature egg will fertilize successfully. Of the eggs collected, typically about 70 to 80 percent are mature enough to attempt fertilization, and of those, roughly 50 to 80 percent will fertilize normally. The numbers narrow at each step, which is why clinics aim for that 10-to-15 range as a starting point.
Why Egg Count Declines With Age
Your lifetime supply of eggs is set before birth. By puberty, you have roughly 300,000 to 400,000 remaining. Each cycle, the batch of follicles recruited but not selected for ovulation is permanently lost. By your mid-30s, the pace of this decline accelerates, and by your early 40s, both the number and quality of remaining eggs drop significantly.
This is why IVF retrieval numbers fall with age, and why natural fertility declines well before menopause. Menopause itself, which occurs around age 51 on average, marks the point when the egg supply is essentially depleted. But fertility typically becomes quite limited five to ten years before that. The eggs that remain in your late 30s and 40s are also more likely to carry chromosomal abnormalities, which affects both the chance of conception and the risk of miscarriage.