Transfer of Frozen Embryos in Natural Cycle: Evaluation of Impact of Spontaneous Versus HCG-triggered Ovulation on Pregnancy Rate

Inclusion Criteria: * The patient must have given their free and informed consent and signed the consent form * The patient must be a member or beneficiary of a health insurance plan * The patient is aged between 18 and 40 * Patient has regular cycles and classified as 'easy' for transfer of a frozen embryo at blastocyste stage (vitrification day 5) Exclusion Criteria: * The subject is participating in an interventional study, or is in a period of exclusion determined by a previous study * The subject refuses to sign the consent * It is impossible to give the subject informed information * The patient is under safeguard of justice or state guardianship * Patient using donated oocytes * Necessity of pre-implantation diagnosis * Patients with stage 3 or 4 endometriosis or adenomyosis