Ovarian cyst - Causes of ovarian cysts

Ovarian cysts often develop for no apparent reason in women who have monthly periods.

There are a number of different types of ovarian cyst, which are categorised as either:

• functional cysts – these are harmless cysts that form as part of the menstrual cycle, and are the most common type
• pathological cysts – tumours in the ovaries that can either be benign (harmless) or malignant (cancerous)

Functional cysts

There are two types of functional ovarian cyst:

• follicular cysts
• luteal cysts

These are described below.

Follicular cysts

Follicular cysts are the most common type of ovarian cysts.

Each month, a woman's ovaries release an egg which travels down the fallopian tubes into the womb (uterus), where it can be fertilised by a man's sperm.

Each egg forms inside the ovary in a tiny structure known as a follicle. The follicle contains fluid that protects the egg as it grows and it bursts when the egg is released.

However, sometimes a follicle does not release an egg, or it does not shed its fluid and shrink after the egg is released. If this happens, the follicle can get bigger as it swells with fluid. The fluid-filled follicle becomes a follicular ovarian cyst.

In most cases, only one follicular cyst develops at a time. It will often disappear without treatment after a few weeks.

Luteal cysts

Luteal cysts are less common than follicular cysts. They develop when the tissue left behind after an egg has been released (corpus luteum) fills with blood.

Luteal cysts usually disappear on their own after a few months, but they can sometimes rupture (burst), causing internal bleeding and sudden pain.

Pathological cysts

A dermoid cyst is the most common type of pathological cyst in women under 40 years of age. In women over 40, a cystadenoma is the most common type.

Dermoid cysts

Dermoid cysts develop from the cells used to create eggs. As eggs have the ability to create any type of cells, dermoid cysts can consist of a wide range of different types of human tissue, including blood, fat, bone and hair.

Dermoid cysts have the potential to grow very large. They can sometimes grow up to 15cm (6 inches) in diameter. They are not usually cancerous but often need to be surgically removed.

Cystadenomas

Cystadenomas develop from cells that cover the outer part of the ovary. There are two main types:

• serous cystadenomas
• mucinous cystadenomas

Serous cystadenomas do not usually grow very large but they can cause symptoms if they rupture.

In contrast, mucinous cystadenomas can grow very large (up to 30cm, or 12 inches), filling up the inside of the abdomen and placing pressure on other organs such as the bladder and bowel. This can cause symptoms such as indigestion and a frequent need to urinate.

Larger mucinous cystadenomas carry the risk of rupturing or blocking the blood supply to the ovaries, which is known as torsion. As with dermoid cysts, mucinous cystadenomas are rarely cancerous. 

Conditions that cause ovarian cysts

In some cases, ovarian cysts are caused by certain conditions, such as endometriosis or polycystic ovary syndrome (PCOS).

Endometriosis occurs when pieces of the tissue that line the womb (endometrium) are found outside the womb in the fallopian tubes, ovaries, bladder, bowel, vagina or rectum. Blood-filled cysts can sometimes form in this tissue.

PCOS is a condition that causes lots of small, harmless cysts to develop on your ovaries. The cysts are caused by a problem with the balance of hormones that are produced by the ovaries.