1. The Japanese GTD registration system

The registration areas included 14 out of 47 prefectures in 1974 and gradually increased to 22 prefectures in 1992. Each year the prefectural registration center in each prefecture asks all hospitals and clinics that have obstetrics departments and/or gynecology departments to report new GTD cases. The data of a new GTD case at diagnosis is reported from a hospital or clinic where the initial treatment was provided. The prefectural registration center gathers the data of all new GTD cases in the prefecture and reports to the Japan Society of Obstetrics and Gynecology (JSOG) every year. The collected data was only for diagnoses before 1999. The following data has been collected since 1999; age, history of pregnancies, antecedent pregnancy and the date of termination, diagnosis of GTN, the date of diagnosis, pathological examination, the hCG level when GTN was diagnosed, sites of GTN, the Japanese GTN score, and International Federation of Gynecology and Obstetrics (FIGO) stage and score.