A functional ovarian cyst is a fluid-filled sac that forms on an ovary as a normal part of the menstrual cycle. Unlike other types of ovarian cysts, functional cysts develop from the structures your ovaries already produce every month. Most are small, cause no symptoms, and disappear on their own within two to three menstrual cycles.
The Two Types of Functional Cysts
Functional cysts come in two varieties, each tied to a different stage of your cycle.
A follicular cyst forms in the first half of your cycle. Normally, an egg grows inside a small sac called a follicle, which ruptures to release the egg at ovulation. Sometimes the follicle never ruptures. Instead of releasing its egg, it keeps growing and fills with fluid, becoming a cyst. On ultrasound, follicular cysts appear as smooth, thin-walled sacs filled with clear fluid.
A corpus luteum cyst forms after ovulation. Once the egg leaves the follicle, the empty sac normally shrinks into a small structure that produces hormones to support a potential pregnancy, then breaks down. Sometimes this structure seals itself shut and fills with fluid instead of dissolving. The result is a corpus luteum cyst, which can sometimes contain blood (called a hemorrhagic cyst).
Why They Form
Functional cysts happen because the normal hormonal signals that guide ovulation don’t fire quite right. A follicular cyst can result from too much of the hormone that stimulates egg growth, or from a missing surge of the hormone that triggers the follicle to open. Corpus luteum cysts form when the post-ovulation structure simply doesn’t break down on schedule. These aren’t signs of disease. They’re minor hiccups in a process your body runs every month, and they’re common enough that many people have one without ever knowing.
Symptoms to Recognize
Most functional cysts are completely silent. They show up incidentally on imaging done for another reason and never cause a single symptom. Larger cysts, however, can produce noticeable effects:
- Pelvic pain: A dull ache or sharp pain on one side, below your bellybutton. It may come and go.
- Pressure or fullness: A heavy sensation in your lower abdomen.
- Bloating: Your belly may feel swollen or tight.
- Menstrual changes: Your period may arrive late or feel different than usual, particularly with corpus luteum cysts that produce hormones.
Pain during sex or with certain movements is also possible when a cyst is large enough to shift the ovary’s position.
How They’re Diagnosed
A pelvic ultrasound is the standard tool. Functional cysts have a distinctive, reassuring appearance: they’re single-chambered sacs with thin, smooth walls and clear fluid inside. There are no solid parts, no internal dividers, no irregular features. This clean look is what separates them from more complex cysts that need closer evaluation. A corpus luteum cyst may show some internal echoes from blood, which gives it a slightly more complicated appearance, but an experienced sonographer can usually identify it.
Your doctor may also check blood work in certain situations, particularly if you’re postmenopausal or the cyst doesn’t look straightforward on imaging.
How They Differ From Other Ovarian Conditions
The word “functional” is key. It means the cyst arose from the ovary’s normal function, not from abnormal cell growth. This distinguishes functional cysts from several other conditions. Endometriomas (sometimes called “chocolate cysts”) form from endometrial tissue growing on the ovary. Dermoid cysts contain tissue like hair or teeth from embryonic cells. Cystadenomas grow from the ovary’s surface. These are all structural abnormalities, not byproducts of your cycle.
Polycystic ovary syndrome (PCOS) is another common source of confusion. In PCOS, the ovaries contain many small follicles (often 12 or more) that haven’t matured, giving the ovary a characteristic “string of pearls” look on ultrasound. A functional cyst is typically a single, larger sac rather than a cluster of tiny ones, and it resolves on its own rather than persisting cycle after cycle.
When a Cyst Becomes a Problem
Functional cysts rarely cause complications, but two situations deserve attention. A cyst can rupture, releasing its fluid into the pelvic cavity. This often causes sudden, sharp pain on one side that fades over hours to days. Most ruptured cysts resolve without treatment, though a hemorrhagic cyst that ruptures can occasionally cause enough internal bleeding to need medical attention. Sudden severe pain, dizziness, or feeling faint after sharp pelvic pain warrants an emergency visit.
The other concern is ovarian torsion, where a cyst makes the ovary heavy enough to twist on its blood supply. This causes intense, sudden pain often accompanied by nausea or vomiting. Cysts larger than 5 centimeters have been associated with torsion risk, though the relationship between size and torsion isn’t as straightforward as it might seem. Some studies have found no significant link between cyst size and the likelihood of twisting. Torsion is uncommon but requires prompt treatment to save the ovary.
What Happens Next: Monitoring and Resolution
The standard approach for a functional cyst is watchful waiting. In premenopausal women, simple cysts smaller than 5 centimeters typically resolve within two to three menstrual cycles without any intervention. Your doctor may schedule a follow-up ultrasound in 6 to 12 weeks to confirm the cyst has shrunk or disappeared.
For simple cysts between 5 and 7 centimeters, imaging guidelines recommend yearly ultrasound follow-up. These cysts are still almost certainly benign, but their size warrants periodic checks. In postmenopausal women, even smaller cysts (over 1 centimeter) are tracked with yearly ultrasound, since functional cysts are less expected after menstruation stops. If a hemorrhagic cyst is larger than 5 centimeters, a follow-up ultrasound at 6 to 12 weeks is recommended to confirm it’s resolving.
Birth Control Pills Don’t Speed Recovery
Because early birth control pills were linked to fewer functional cysts forming in the first place, many doctors began prescribing them to treat existing cysts. This practice persists, but the evidence doesn’t support it. A Cochrane review of multiple trials found that combined oral contraceptives did not help functional cysts resolve any faster than doing nothing. In none of the trials examined did oral contraceptives make cysts go away sooner.
Hormonal birth control may help prevent new functional cysts from forming by suppressing ovulation, which can be useful if you get recurrent cysts. But for a cyst that already exists, the best approach is simply giving it time. Two to three cycles of patience is more effective than any medication.