Most ovarian cysts are fluid-filled sacs or pockets within or on the surface of an ovary. They occur as the result of normal ovarian functions, including ovulation (the release of an egg, called ovum, from the ovary) and hormone production. Because these cysts are produced from normal ovarian function, they are called functional cysts. 

These fluid filled sacs are commonly referred to as simple cysts, and are almost always benign. Cysts that are solid or contain both solids and fluids (known as complex cysts) might not always be benign and require further investigation.

The ovaries are two organs about the size and shape of an almond located on each side of a woman’s uterus. Eggs develop and mature in the ovaries and are released in monthly cycles during a woman’s childbearing years (from puberty to menopause). Each month, a woman’s ovary grows tiny cysts (follicles) that hold the eggs. The follicles nourish eggs with fluids. When an egg is mature, the sac breaks open so the egg can travel through the fallopian tube for fertilization.

The sac is now called a corpus luteum cyst and persists to secrete hormones that support an early pregnancy. If a pregnancy does not occur, the cyst dissolves and stops producing hormones. It is this drop in hormone levels that brings on menstruation.

The most common type of ovarian cysts occurs when the sac does not break open and continues to grow (failure of ovulation) or when the sac does not dissolve after fertilization does not occur. Functional cysts, which form as a normal part of ovulation, usually resolve on their own. Abnormal cysts are less common and usually benign, but in rare cases they may be cancerous.

Many women develop cysts at some time during their lives. Most are harmless and cause little or no discomfort. Oftentimes, women may be unaware of their presence. Cysts usually disappear without treatment within a few months. However, they can produce serious symptoms, particularly if they rupture (or burst).

Women who ovulate regularly tend to develop more ovarian cysts, since functional cysts are by far the most common. Therefore, cysts are less common in women who do not ovulate regularly, such as menopausal women or those taking birth control pills (which work, in part, by preventing ovulation).

Ovarian cysts can also develop during pregnancy and may cause painful cramping. More than one egg each month can be produced in the ovary, but usually only one egg will develop to maturity. The remaining egg may result in an ovarian cyst. Additionally, some cysts can fail to dissolve and persist during pregnancy causing complications.

It is important to note that ovarian cysts are different from a condition called polycystic ovarian syndrome (PCOS). In PCOS, multiple small follicles grow and form a very characteristic appearance on ultrasound (called polycystic ovaries). Most women with PCOS fail to ovulate, leading to a hormonal imbalance that can cause multiple symptoms, such as male-pattern hair growth (hirsutism), oily skin, menstrual irregularities and difficulty getting pregnant (infertility). However, not all women with PCOS develop ovarian cysts and not all ovarian cysts are a sign of PCOS.