Following consent, participants will be randomised to one of the two trial interventions (endometrial biopsy, or ultrasound scan only = no intervention) by a member of the research team. The procedures will be scheduled for luteinising hormone (LH) day +7 to +9 of the menstrual cycle directly preceding commencement of down-regulation medications. A urine pregnancy test will be performed before the studied procedure. Just prior to the biopsy and directly following it, a transvaginal ultrasound scan will be performed to assess the length of the uterine cavity and endometrial parameters, including blood flow characteristics using power Doppler imaging. Endometrial sample will be obtained using standard sampling procedure by appropriately trained clinician.