Study design, size, duration: In a French academic reproductive medicine centre, a systematic prospective observational follow-up of all patients triggered by GnRH agonist for excessive follicular response will be conducted. Participants/materials, setting, methods: All patients undergoing antagonist protocol and at high risk of OHSS (estradiol level ≥ 3000 pg/mL and/or more than 20 follicles ≥ 11mm on the day of triggering) will be triggered by GnRH agonist. No luteal phase support and a "freeze-all" strategy will be performed. On the day of oocyte retrieval (T0), at 48h (T1) and at day 7 (T2), OHSS and hypercoagulability will be systematically assessed. Haemostasis data will be compared to the initial status of each patient.