Letrozole in Preventing Recurrence of Endometrioma Following Laparoscopic Ovarian Cystectomy

Inclusion Criteria: * Women aged 18-45 years old * Scheduled to have laparoscopic ovarian cystectomy * Unilateral or bilateral endometrioma on pre-operative ultrasound and confirmed by histology Exclusion Criteria: * Use of long-acting hormonal therapy in the 3 months before inclusion in the study * Histological report of laparoscopic ovarian cystectomy showed atypical endometrioma * Complex surgery including resection of deep infiltrating disease, bowel resection or hysterectomy * Incomplete excision of endometrioma/ incision or drainage rather than ovarian cystectomy * Suspicion of malignancy * Contraindications to combined oral contraceptive pills, including: uncontrolled hypertension (systolic \> 160mmHg or diastolic \> 100mmHg), diabetes with retinopathy/ nephropathy/ neuropathy, current or past history of venous thromboembolism, ischemic heart disease, history of cerebrovascular accident, migraine with aura, severe liver disease, oestrogen-sensitive cancers, undiagnosed abnormal uterine bleeding, smokers (\> 15 cigarettes/day) aged 35 years or above, or having body mass index \>=35 kg/m2 * Planning to get pregnant in the coming 1 year * Refusal to join the study