The ovaries serve two essential functions: producing eggs for reproduction and releasing hormones that influence nearly every system in your body. These two small, almond-shaped organs sit on either side of the uterus in the pelvic cavity, each roughly 3.5 centimeters long and 2 centimeters wide. Despite their size, they drive the menstrual cycle, support pregnancy, and produce hormones that protect your bones, heart, and brain throughout your reproductive years.
Egg Production and Maturation
The ovaries are the body’s egg bank, and the supply is set before birth. A female fetus has roughly 6 to 7 million immature egg cells by the 20th week of pregnancy. That number drops steadily: by birth, about 1 to 2 million remain. By puberty, the count is down to 300,000 to 400,000. By age 40, approximately 25,000 are left. The ovaries never produce new eggs after birth. Every egg you’ll ever ovulate was already present when you were born.
Each egg is housed inside a structure called a follicle, which is essentially a protective shell of nourishing cells. Follicle development happens in stages. In the early phase, follicles grow and mature on their own without hormonal signals from the brain. Later, they become responsive to two key hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH), both sent from the pituitary gland in the brain. FSH prompts a group of follicles to start growing each cycle, while one “dominant” follicle eventually outpaces the rest and becomes the one that releases its egg.
How Ovulation Works
Ovulation is the moment the dominant follicle ruptures and releases a mature egg into the fallopian tube, where it can be fertilized. This happens roughly once per cycle, triggered by a sharp surge in LH. That surge activates enzymes that weaken the outer wall of the follicle, allowing the egg to pass through.
After the egg is released, the leftover follicle tissue transforms into a temporary hormone-producing structure called the corpus luteum. The corpus luteum pumps out progesterone and some estrogen for roughly two weeks, preparing the uterine lining for a potential pregnancy. If fertilization doesn’t occur, the corpus luteum breaks down, hormone levels fall, and menstruation begins.
Hormones the Ovaries Produce
Hormone production is arguably just as important as egg production. The ovaries are the body’s primary source of estrogen and progesterone, and they also release smaller amounts of other signaling molecules that fine-tune the reproductive system.
Estrogen is produced mainly by the growing follicles during the first half of the menstrual cycle. It thickens the uterine lining, supports vaginal and breast tissue, and plays protective roles in the bones, heart, and brain. Rising estrogen levels also act as a signal back to the brain, eventually triggering the LH surge that causes ovulation.
Progesterone comes primarily from the corpus luteum after ovulation. Its main job is to stabilize and mature the uterine lining so a fertilized egg can implant. If pregnancy occurs, progesterone levels stay elevated to maintain the lining. If not, the drop in progesterone triggers your period.
Inhibin is produced by the granulosa cells inside follicles and by the corpus luteum. It acts as a brake on FSH, helping ensure that only one dominant follicle matures each cycle rather than many. This feedback loop keeps the system tightly regulated.
The ovaries also produce small amounts of androgens, which serve as raw material for estrogen production and contribute to libido and energy levels.
The Brain-Ovary Communication Loop
The ovaries don’t operate independently. They’re part of a feedback loop with the hypothalamus and pituitary gland in the brain. The hypothalamus releases a signaling hormone in rhythmic pulses, which tells the pituitary to produce FSH and LH. Those two hormones travel through the bloodstream to the ovaries and direct follicle growth, ovulation, and hormone output.
The ovaries then send signals back. Rising estrogen tells the brain to reduce FSH so fewer follicles keep growing. Inhibin reinforces that signal. When estrogen reaches a critical threshold, it flips from suppressing LH to triggering the LH surge that causes ovulation. After ovulation, progesterone from the corpus luteum signals the brain to hold off on starting a new cycle. This constant back-and-forth is what produces the predictable rhythm of the menstrual cycle.
Timing of the Menstrual Cycle
A study of more than 600,000 menstrual cycles found that the average follicular phase (from the start of your period to ovulation) lasts about 16.9 days, though it can range anywhere from 10 to 30 days. The luteal phase (from ovulation to the start of your next period) averages 12.4 days, not the 14 days often cited in textbooks. In shorter cycles, both phases compress; in longer cycles, it’s mainly the follicular phase that stretches out. The luteal phase stays relatively consistent regardless of total cycle length.
Effects Beyond Reproduction
Ovarian hormones reach far beyond the reproductive tract. Estrogen in particular plays a protective role in two major areas: bone density and cardiovascular health.
Estrogen helps maintain bone mineral density by slowing the natural breakdown of bone tissue. When estrogen levels drop after menopause, bone loss accelerates, which is why postmenopausal women are more susceptible to osteoporosis than men of the same age. Research on long-term hormone therapy in postmenopausal women has shown significant increases in spinal bone density when estrogen levels are restored, with benefits persisting even at lower doses.
The cardiovascular system also benefits from ovarian estrogen. Premenopausal women develop heart disease at lower rates than men of the same age, a gap that narrows after menopause. Estrogen helps maintain favorable cholesterol profiles by reducing LDL (“bad” cholesterol) and supporting HDL (“good” cholesterol). A 20-year follow-up study of postmenopausal women on hormone therapy found significant reductions in LDL, lower diastolic blood pressure, and decreased cardiovascular risk overall. These findings point to the cardioprotective role that ovarian estrogen plays during the reproductive years.
Estrogen also influences skin elasticity, mood regulation, sexual function, and sleep quality. Many of the symptoms associated with menopause, including hot flashes, sleep disruption, mood changes, and decreased libido, trace directly back to falling ovarian estrogen levels.
How Ovarian Function Changes With Age
The ovaries don’t shut down overnight. The transition to menopause, called perimenopause, typically spans several years and involves increasingly erratic hormone patterns. FSH levels start rising as the remaining follicle pool shrinks. Inhibin B drops, which removes a key brake on FSH. Progesterone output from the corpus luteum declines as ovulation becomes less reliable.
During this transition, cycles can swing between unusually high estrogen levels (when multiple follicles respond to elevated FSH) and very low estrogen levels (when ovulation fails entirely). This hormonal unpredictability is what drives the irregular periods, hot flashes, and mood shifts many people experience in their 40s.
Post-menopause is defined as 12 consecutive months without a period. At this point, the follicle supply is essentially depleted. FSH and LH levels remain high because the brain keeps signaling ovaries that can no longer respond. Estrogen and progesterone drop to very low levels. Interestingly, androgen production from the ovaries declines more gradually, since it comes from a different cell type (stromal cells) rather than from follicles. But overall androgen levels still trend downward, largely because the adrenal glands, which are the other major source, also produce less with age.