Fibroids are solid growths made of muscle tissue that develop in or on the uterus, while cysts are fluid-filled sacs that typically form on the ovaries. Despite causing some overlapping symptoms like pelvic pain and bloating, they differ in what they’re made of, where they grow, how they behave, and how they’re treated. Up to 70% to 80% of people with a uterus in the U.S. develop fibroids by age 50, making them extremely common. Ovarian cysts are similarly widespread, with most forming naturally during the menstrual cycle.
What Each One Actually Is
A fibroid (also called a leiomyoma) is a non-cancerous tumor that grows from the smooth muscle layer of the uterus. Think of it as an overgrowth of the uterine wall itself. Fibroids are solid and dense, with a composition of muscle and connective tissue. They can grow inside the uterine cavity, within the muscular wall, or on the outer surface of the uterus. Some remain as small as a seed, while others grow large enough to visibly enlarge the abdomen.
An ovarian cyst, by contrast, is a fluid-filled sac that develops in or on an ovary. The most common type, called a functional cyst, forms as part of the normal egg-release process each month and usually resolves on its own within a few cycles. Other types include endometriomas (filled with old blood, linked to endometriosis) and dermoid cysts (which can contain hair, fat, or other tissue). Most cysts range from about half an inch to four inches, though in rare cases they exceed 10 centimeters and are classified as “giant” ovarian cysts.
How Symptoms Differ
Both fibroids and cysts can cause pelvic pain, abdominal bloating, and urinary pressure. But two symptoms help distinguish them. Heavy menstrual bleeding, sometimes severe enough to cause anemia, points toward fibroids. One-sided abdominal pain points toward an ovarian cyst, since cysts typically affect one ovary at a time.
Fibroids tend to cause symptoms that build gradually as the growths enlarge: increasingly heavy or prolonged periods, a feeling of fullness or pressure in the lower abdomen, frequent urination from the fibroid pressing on the bladder, and rectal pressure. Many fibroids cause no symptoms at all and are discovered incidentally during imaging for something else.
Cysts often produce a sharp or dull ache on one side, pain during sex, breast tenderness, and lower back or thigh pain. A cyst can also cause a more dramatic emergency. If it ruptures or causes the ovary to twist around its blood supply (a condition called ovarian torsion), the result is sudden, intense abdominal pain, sometimes with fever, vomiting, dizziness, or rapid breathing. This requires immediate medical attention.
The Role of Hormones
Both conditions are influenced by reproductive hormones, but through different mechanisms. Estrogen promotes fibroid growth by increasing the number of hormone receptors in fibroid tissue, which in turn makes the tissue more responsive to progesterone. A study from the Women’s Health Across the Nation (SWAN) project found that women with higher levels of both estrogen and testosterone had a 52% increased risk of developing their first fibroid. This hormonal connection explains why fibroids tend to shrink after menopause, when estrogen levels drop significantly.
Functional ovarian cysts are a direct product of the hormonal cycle. Each month, rising hormones stimulate a follicle to grow and release an egg. A cyst forms when the follicle doesn’t release the egg as expected, or when it seals off and fills with fluid after ovulation. Hormonal contraceptives that suppress ovulation can reduce the formation of new functional cysts, though they don’t shrink existing ones.
How They’re Diagnosed
Ultrasound is the first-line imaging tool for both. On an ultrasound, fibroids appear as solid masses with a dense, textured pattern, while cysts show up as dark, fluid-filled pockets with thin walls. The distinction is usually straightforward, but it can get tricky. Fibroids sometimes undergo a process called cystic degeneration, where parts of the tissue break down and fill with fluid, making them look more like cysts on a screen.
When ultrasound results are unclear, an MRI can pinpoint where a pelvic mass originates. MRI detects fibroids with 88% to 93% sensitivity. Radiologists look for a telltale “bridging vessel sign,” which shows blood vessels running from the uterus directly into the mass, confirming it grew from uterine tissue rather than the ovary. If a mass can be clearly separated from a normal-looking ovary on imaging, it’s unlikely to be a cyst.
Cancer Risk
Fibroids are non-cancerous. The risk of a fibroid actually being a rare uterine cancer called leiomyosarcoma is very low, and the two conditions are generally considered to develop independently rather than one transforming into the other.
Most ovarian cysts are also benign, especially functional cysts in women of reproductive age. The concern increases with certain features: cysts that are large, have solid components mixed with fluid, contain irregular internal structures, or persist through multiple cycles. These characteristics prompt further evaluation to rule out ovarian cancer. Post-menopausal women who develop new ovarian cysts warrant closer monitoring since functional cysts shouldn’t form once ovulation has stopped.
Treatment Approaches
Many fibroids and cysts need no treatment at all. Small, symptom-free fibroids are typically monitored over time. When fibroids do cause problems, treatment options range from hormonal therapies that manage symptoms and slow growth, to procedures that destroy or remove fibroid tissue. A myomectomy surgically removes fibroids while preserving the uterus, which matters for people planning future pregnancies. Less invasive options include MRI-guided focused ultrasound, which uses high-energy sound waves to destroy fibroid tissue without any incisions, and radiofrequency ablation, which applies targeted heat to shrink fibroids. Hysterectomy (removing the uterus entirely) remains a definitive option for severe cases when future fertility isn’t a concern.
Functional ovarian cysts usually resolve within one to three menstrual cycles without intervention. Your doctor may recommend a follow-up ultrasound to confirm the cyst has gone. Cysts that are large, persistent, or causing significant pain may be removed surgically through a cystectomy, a procedure that takes out the cyst while preserving the ovary. In rare cases where the ovary is damaged beyond repair or cancer is suspected, the entire ovary may need to be removed.
Effects on Fertility and Pregnancy
Fibroids can interfere with fertility depending on where they’re located. Those growing inside the uterine cavity or distorting its shape are most likely to affect implantation of a fertilized egg. Fibroids have been linked to both difficulty conceiving and recurrent pregnancy loss. Removing problematic fibroids before attempting pregnancy can improve outcomes, though the surgical approach matters. The thickness of the fibroid’s capsule near the inner uterine lining affects how well the muscle heals afterward, which has implications for future pregnancies.
Most ovarian cysts don’t affect fertility. Functional cysts are a sign that ovulation is occurring normally. However, endometriomas associated with endometriosis can damage ovarian tissue and reduce egg quality over time. Large cysts of any type may also interfere with ovulation if they prevent normal follicle development. Surgical removal of cysts, while sometimes necessary, carries a small risk of reducing the ovary’s egg reserve, so the decision involves weighing symptom relief against reproductive goals.