The true incidence of ovarian torsion is unknown. Ovarian torsion is described as the 5th leading cause of gynecologic emergencies and represents about 15% of surgically treated adnexal masses.  An adnexal mass, typically with a diameter of 5 cm or greater, is the greatest risk factor for torsion although in one case series the mass size associated with torsion ranged from 1 cm to 30 cm with an average size of 9.5 cm. In premenarchal females, a case series reported over 50% of girls with torsion had normal anatomy (i.e. no mass). [4,5,6,7,8,9] Ovarian torsion can also occur in the pregnant female, with the greatest incidence between 10 and 17 weeks although possible in all three trimesters. In pregnancy the masses most likely to torse were 6 to 8 cm, with larger masses less likely to develop torsion. [7,8,9,10]
Another important population to consider the diagnosis of ovarian torsion is the patient undergoing reproductive and infertility evaluations and using ovulation induction agents. These patients are at risk for ovarian hyperstimulation syndrome, and in one case series, 8% of patients developed torsion from physiologic cysts. 
Other risk factors include polycystic ovarian syndrome and tubal ligation, possibly secondary to adhesions. As with any condition or disease process those with prior history are at risk for repeat occurrence. In patients with prior ovarian torsion, 11% were likely to recur, and of those, women with normal anatomy are at greater risk. [5,6]
The classic presentation of ovarian torsion includes acute onset of moderate to severe pain with nausea and vomiting in a patient with a prior history of an adnexal mass. Nausea typically occurs at the onset of pain. Fever and chills can occur but are more likely to occur with premenarchal females since their course is typically slower at onset with later presentation to the ED for care. They are more likely to present with fever and diffuse pain, rather than focused right or left lower quadrant pain.