Ovarian torsion
Ovarian cysts increase the risk for ovarian torsion; cysts which are larger than 4 cm are associated with approximately 17% risk. The torsion can cause obstruction of blood flow and lead to infarction. Types
Functional
Functional cysts form as a normal part of the menstrual cycle. There are several types of functional cysts:

Follicular cyst, the most common type of ovarian cyst. In menstruating women, a follicle containing the ovum, an unfertilized egg, will rupture during ovulation. If this does not occur, a follicular cyst of more than 2.5 cm diameter may result. Corpus luteum cysts appear after ovulation. The corpus luteum is the remnant of the follicle after the ovum has moved to the fallopian tubes. This normally degrades within 5 to 9 days. A corpus luteum that is more than 3 cm is defined as cystic. Theca lutein cysts occur within the thecal layer of cells surrounding developing oocytes. Under the influence of excessive hCG, thecal cells may proliferate and become cystic. This is usually on both ovaries. Non-functional
Non-functional cysts may include the following:
An ovary with many cysts, which may be found in normal women, or within the setting of polycystic ovary syndrome
Cysts caused by endometriosis, known as chocolate cysts
Hemorrhagic ovarian cyst
Dermoid cyst
Ovarian serous cystadenoma
Ovarian mucinous cystadenoma
Paraovarian cyst
Cystic adenofibroma
Borderline tumoral cysts

Diagnosis
Ovarian cysts are usually diagnosed by ultrasound, CT scan, or MRI, and correlated with clinical presentation and endocrinologic tests as appropriate.
An ovarian cyst is a fluid-filled sac within the ovary. Often they cause no symptoms. Occasionally they may produce bloating, lower abdominal pain, or lower back pain. The majority of cysts are harmless. If the cyst either breaks open or causes twisting of the ovary, it may cause severe pain. This may result in vomiting or feeling faint, and even cause head aches. Most ovarian cysts are related to ovulation, being either follicular cysts or corpus luteum cysts. Other types include cysts due to endometriosis, dermoid cysts, and cystadenomas. Many small cysts occur in both ovaries in polycystic ovary syndrome (PCOS). Pelvic inflammatory disease may also result in cysts. Rarely, cysts may be a form of ovarian cancer. Diagnosis is undertaken by pelvic examination with an ultrasound or other testing used to gather further details.Often, cysts are simply observed over time. If they cause pain, medications such as paracetamol (acetaminophen) or ibuprofen may be used. Hormonal birth control may be used to prevent further cysts in those who are frequently affected. However, evidence does not support birth control as a treatment of current cysts. If they do not go away after several months, get larger, look unusual, or cause pain, they may be removed by surgery.Most women of reproductive age develop small cysts each month. Large cysts that cause problems occur in about 8% of women before menopause. Ovarian cysts are present in about 16% of women after menopause and if present are more likely to be cancer.