Randomized controlled trial (RCT) testing the hypothesis that vitrification is superior to slow freezing.

Inclusion Criteria:

• Patients (with female age < 40 years)
• Patients undergoing a 1st oocyte retrieval for an IVF/Intracytoplasmatic Sperm Injection (ICSI) treatment
• Patients planned for an embryo transfer on day 3.
• Patients with supernumerary embryos of sufficient quality for freezing/vitrifying on day 3.
• Fresh personal (own oocytes)
• One inclusion in the study per patient.

Exclusion Criteria:

• Patients undergoing a 2nd or higher ranked oocyte retrieval after a fresh cycle with failed pregnancy.
• Female age ≥ 40 year
• Patients with Preimplantation Genetic Diagnosis (PGD)
• Patients with donor oocytes
• Patients with positive serology whose embryos may not be frozen.
• Patients planned for a transfer on day 2 or day 5.
• Patients receiving assisted hatching in the embryo transfer in the frozen cycles.
• Patients who have been included in the study before.