Yolk sac tumors, formerly called endodermal sinus tumors, make up approximately 10–20% of ovarian germ cell malignancies, and have the worst prognosis of all ovarian germ cell tumors. They occur both before menarche (in one-third of cases) and after menarche (the remaining two-thirds of cases). Half of the people with yolk sac tumors are diagnosed in stage I. Typically, they are unilateral until metastasis, which occurs within the peritoneal cavity and via the bloodstream to the lungs. Yolk sac tumors grow quickly and recur easily, and are not easily treatable once they have recurred. Stage I yolk sac tumors are highly treatable, with a 5-year disease-free survival rate of 93%, but stage II-IV tumors are less treatable, with survival rates of 64–91%.