The purpose of the study is to evaluated whether there is a difference in pregnancy rate when transferring frozen embryos between patietns having spontaneously ovulated versus those triggered to voulate with HCG.

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