The ovaries have two main jobs: producing eggs for reproduction and releasing hormones that influence nearly every system in your body. These two small glands, each about the size of a large grape, manage fertility, drive the menstrual cycle, and produce hormones that protect your bones, heart, and brain well beyond their reproductive role.
Where the Ovaries Are and What They Look Like
The ovaries sit on either side of the uterus in the lower abdomen, held in place by ligaments. They don’t actually touch the uterus. They’re oval-shaped, firm, and range in color from light gray to white. The average ovary is about 4 centimeters long, though before menopause they can be as large as a kiwi (around 6 centimeters). After menopause, they shrink to roughly the size of a kidney bean.
Each ovary has three layers. The outer layer acts as a protective shell. The middle layer is made of connective tissue packed with follicles, which are tiny sacs that hold immature eggs. The innermost layer contains blood vessels that supply the ovary with nutrients and oxygen.
How the Ovaries Produce and Release Eggs
You’re born with all the eggs you’ll ever have. At birth, your ovaries contain about 2 million immature eggs. By puberty, that number drops to around 400,000. By age 37, roughly 25,000 remain, and by menopause (around age 51), only about 1,000 are left, none of them fertile.
Each month, your brain sends signals that kick-start egg development. The pituitary gland, a pea-sized structure at the base of the brain, releases two key hormones: follicle-stimulating hormone (FSH) and luteinizing hormone (LH). FSH prompts several follicles in the ovary to start growing, but usually only one will fully mature. That process takes longer than most people realize. Complete follicle maturation requires at least 85 days, with several months needed just for a new follicle to reach the earliest visible stage.
When the dominant follicle reaches maturity, rising estrogen levels trigger a sudden surge of LH from the pituitary. That LH surge is the direct trigger for ovulation. It causes the follicle to rupture and release the mature egg into the fallopian tube, where it can be fertilized by sperm. This is the moment the entire cycle builds toward.
Hormones the Ovaries Produce
The ovaries are a major part of the hormone system, producing several hormones that affect far more than just reproduction.
Estrogen
Estrogen is the ovaries’ signature hormone. The most potent form, estradiol, is responsible for maturing and maintaining the reproductive system. During the menstrual cycle, rising estradiol levels trigger egg maturation and thicken the uterine lining so a fertilized egg can implant. But estrogen also supports bone density, protects blood vessels, influences cholesterol levels, and affects mood and skin health. A second form, estrone, is produced by the ovaries along with fat tissue and the adrenal glands, and plays a supporting role in sexual development.
Progesterone
After ovulation, the empty follicle transforms into a temporary hormone-producing structure called the corpus luteum. This structure releases progesterone, which further thickens and stabilizes the uterine lining. Progesterone also calms the uterine muscles to prevent contractions that could expel a fertilized egg. If pregnancy doesn’t occur, the corpus luteum breaks down after about two weeks, progesterone drops, and the uterine lining sheds as a period.
Other Hormones
The ovaries also produce small amounts of testosterone, which contributes to sex drive, energy, and muscle maintenance. They release relaxin, a hormone that, during the menstrual cycle, helps relax the uterine wall in preparation for possible pregnancy. If pregnancy occurs, relaxin levels continue rising through the first trimester, supporting implantation and placenta growth, and eventually helping loosen pelvic ligaments during labor. The ovaries also produce anti-Müllerian hormone (AMH), which helps regulate the early development of follicles. Doctors often measure AMH levels as an indicator of how many eggs remain.
What Happens During the Menstrual Cycle
The menstrual cycle has two ovarian phases, divided by ovulation in the middle.
During the follicular phase (roughly the first half of the cycle), FSH from the brain stimulates follicles to grow on the surface of the ovaries. As the dominant follicle matures, it releases increasing amounts of estrogen. That estrogen thickens the uterine lining and, when it reaches a critical concentration, sensitizes the pituitary to release the LH surge that triggers ovulation.
During the luteal phase (the second half), the corpus luteum takes over, pumping out progesterone to maintain the uterine lining. If a fertilized egg implants, the embryo produces a hormone called hCG, which keeps the corpus luteum alive and producing progesterone for about 12 weeks. After that, the placenta takes over hormone production and the corpus luteum is no longer needed. If no pregnancy occurs, the corpus luteum degrades, hormone levels fall, and the cycle resets with menstruation.
How Ovarian Hormones Affect the Rest of Your Body
The hormones the ovaries produce reach virtually every organ. Estrogen helps maintain bone density by slowing the rate at which bone is broken down. It also has protective effects on blood vessels, helping keep them flexible and supporting healthy cholesterol balance. This is why the loss of ovarian hormones at menopause has consequences that extend well beyond fertility.
Research consistently shows that the earlier menopause occurs, the greater the risk to bone and heart health. Women who experience premature ovarian insufficiency or early menopause face a higher risk of osteoporosis, fractures, and cardiovascular disease because of a prolonged period without the protective effects of estrogen. Even in women who reach menopause at the typical age, the decline in ovarian hormones is closely linked to accelerating bone loss and changes in heart disease risk.
How the Ovaries Change Over Time
The ovaries are not static organs. They change dramatically across a lifetime. In childhood, they’re small and inactive. At puberty, rising signals from the brain activate them, and they begin cycling through follicle development, ovulation, and hormone production each month. During the reproductive years, the ovaries are the body’s primary source of estrogen and progesterone.
As egg supply dwindles in the late 30s and 40s, the ovaries begin producing less estrogen. Cycles may become irregular. FSH levels rise as the brain works harder to stimulate follicles that are increasingly scarce. This transition, called perimenopause, can last several years before menstruation stops entirely. After menopause, the ovaries still produce small amounts of hormones, particularly testosterone, but their dominant role in estrogen production ends. They physically shrink, sometimes to as small as 2 centimeters, a fraction of their peak size.