In the literature, the risk of moderate to severe OHSS is 3 to 6% and reaches 31% in high risk populations 9 days after oocyte triggering with hCG. Many studies report no or a markedly decreased risk of OHSS after triggering ovulation with a GnRH agonist. However, criteria to define OHSS are rarely explained and OHSS itself is not thoroughly asserted. It is well known that OHSS is associated with hypercoagulability. However, no study after triggering with a GnRH agonist assessed haemostasis in these high-risk patients with high circulating estradiol levels.

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.

Seventy-five patients will be required for this study.

Inclusion Criteria:

• estradiol level ≥ 3000 pg/mL and/or more than 20 follicles ≥ 11mm on the day of triggering and antagonist protocol

Exclusion Criteria:

• agonist protocol no health assurance estradiol level < 3000 pg/mL and less than 20 follicles ≥ 11mm on the day of triggering