The human egg develops inside the ovary, specifically within tiny fluid-filled structures called follicles embedded in the ovary’s outer layer (the cortex). Each follicle houses a single egg cell surrounded by supportive cells that nourish it as it matures. From its earliest dormant state to the moment it’s released during ovulation, the entire development process takes place within the ovary before the egg begins a journey through the fallopian tube toward the uterus.
How Eggs Form Before Birth
Egg development begins surprisingly early. During fetal development, a female embryo’s ovaries produce immature egg cells at a rapid pace between the second and seventh month of gestation, peaking at roughly 7 million. After that seventh month, the number drops sharply. By birth, only about 1 to 2 million remain. By puberty, the count falls further to 300,000 to 500,000. By age 37, approximately 25,000 are left, and by age 51, around 1,000.
No new eggs are created after birth. Every egg you’ll ever ovulate was already present in your ovaries before you were born, sitting in a dormant state called a primordial follicle until hormonal signals wake it up decades later.
Inside the Ovary: Where Follicles Grow
The ovary has two main layers. The outer layer, the cortex, contains several hundred thousand follicles. The inner layer, the medulla, is mainly a network of blood vessels that supplies the cortex with nutrients and oxygen. All follicle development happens in the cortex.
Each follicle is a self-contained unit: one egg cell at the center, surrounded by a layer of granulosa cells that feed and protect it. A basement membrane separates each follicle from the surrounding tissue, creating a kind of capsule. As the follicle matures, additional layers of cells form around it, and eventually a fluid-filled cavity develops inside, giving the follicle room to expand.
Stages of Follicle Development
An egg doesn’t go from dormant to mature overnight. It progresses through several distinct stages, each with visible structural changes.
- Primordial follicle: The smallest and most abundant type. The egg sits surrounded by a single thin layer of flat cells. Most follicles stay in this dormant stage for years or even decades.
- Primary follicle: The granulosa cells shift from flat to cube-shaped and begin multiplying. A protective shell called the zona pellucida forms between the egg and the surrounding cells. At this stage, the follicle starts expressing receptors that can respond to hormones from the brain.
- Secondary follicle: The granulosa cells multiply into several layers, and the surrounding tissue organizes into specialized outer sheaths called thecal layers. These layers produce hormones that support further growth.
- Graafian (mature) follicle: One follicle is selected as the “dominant” follicle each cycle. It develops a large fluid-filled cavity and grows rapidly. This is the follicle that will release its egg during ovulation.
The transition from primordial to primary follicle happens without hormonal input, but the later stages depend on two key hormones released by the pituitary gland in your brain. The first stimulates follicle growth and maturation during the first half of your menstrual cycle. The second triggers a surge that causes the dominant follicle to rupture and release its egg. That release is ovulation.
How the Egg Leaves the Ovary
When the dominant follicle ruptures, the egg is expelled onto the surface of the ovary. It doesn’t fall into the fallopian tube on its own. Instead, finger-like projections at the end of each fallopian tube, called fimbriae, actively reach toward the ovary and catch the egg. These projections are lined with tiny hair-like structures (cilia) that beat rapidly in the direction of the uterus, sweeping the egg into the tube.
This catching mechanism is remarkably precise. Each month, in response to ovulation, the fimbriae extend closer to the ovary’s surface so they can easily capture the newly released egg.
The Egg’s Journey Through the Fallopian Tube
Once inside the fallopian tube, the egg doesn’t move quickly. Transport through the tube takes about 30 hours, driven by the wave-like beating of cilia and gentle muscular contractions in the tube wall. The egg then pauses at a junction point in the tube for roughly another 30 hours. This is where fertilization typically happens, in the wider middle section of the tube called the ampulla.
If sperm are present and one successfully penetrates the egg, the fertilized egg begins dividing immediately and descends toward the uterus. If no sperm are present, the unfertilized egg continues into the uterus and is absorbed back into the body.
What Happens After Fertilization
A fertilized egg spends about five to six days traveling from the fallopian tube to the uterus, dividing rapidly along the way. By the time it arrives, it has become a cluster of cells called a blastocyst. This blastocyst then sheds its outer shell and begins burrowing into the thickened lining of the uterus.
Implantation typically occurs about 9 days after ovulation, though the window ranges from 6 to 12 days. The uterine lining is only receptive to implantation for a very brief period, roughly 2 days, during each cycle. If the timing aligns, the blastocyst embeds itself and a pregnancy begins. If it doesn’t, the lining sheds during menstruation.
Why Egg Supply Matters Over Time
Because the ovaries never produce new eggs after birth, the supply steadily declines with age. The drop isn’t gradual and even. Going from 1 to 2 million eggs at birth to 300,000 at puberty is a massive reduction that happens during childhood, entirely in the background. The decline continues through your 20s and 30s, accelerating around the mid-to-late 30s. At age 37, roughly 25,000 eggs remain. Along with the drop in quantity, the proportion of eggs with chromosomal abnormalities increases, which is why fertility and the chance of a healthy pregnancy both decline with age.
Of the hundreds of thousands of eggs present at puberty, only about 400 to 500 will ever fully mature and ovulate during a person’s reproductive years. The rest gradually break down and are reabsorbed through a natural process called atresia.