[Introduction of the antagonist: on a determined day or depending on follicular growth?]

Introduction of the antagonist: on a determined day or depending on follicular growth? GnRH antagonists prevent LH surge in IVF cycles. GnRH antagonist can be administrated at a fixed day (day 6) or according to mean follicle size and estradiol levels (flexible regimen). Less monitoring is required for IVF cycles with the fixed regimen. For the flexible regimen, GnRH antagonist should be initiated on the day when the leading follicle reaches 14 mm diameter. Flexible regimen should decrease total antagonist and gonadotropin doses. Similar pregnancy rates are observed between the fixed and the flexible GnRH antagonist regimens.