The Effect of Vitrification Versus Slow Freezing on Day 3: a Randomised Controlled Trial.

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.