Comparing the Use of Dienogest and Combined Oral Contraceptive Pills (Microgynon) to Reduce the Risk of Recurrence of Endometriotic Cyst After Conservative Surgery

Inclusion Criteria: * Good general health * Older than the age of legal consent (i.e. 18 years old) * Sonographic diagnosis of ovarian endometrioma with diameter at least 4cm on 2 separate scans at least 6 weeks apart * No contraindication to use of progesterone or combined oral contraceptive pills * Not attempting to conceive either at the time of study entry or for at least 2 years after surgery * Willing and able to participate after the study has been explained Exclusion Criteria: * Operative findings not suggestive of endometriotic cyst * Contraindications to progestogens or oral contraceptive pills * Unwillingness to tolerate menstrual irregularity * Planning pregnancy within 2 years of study * Cannot understand English, Cantonese or Putonghua