On the day of oocyte retrieval, participants will be randomized to stable culture at 37.1°C or 36.6°C with the ultimate aim of finding a difference in clincial pregnancies.

A previous study in our centre analysed stable embryo culture at 36.6°C and 37.1°C, on sibling oocytes. The primary aim was to analyse possible differneces in embryo development (utilization rate) up to day 5/6. In this study, the investigators found a high difference in clinical pregnancy rate between both groups (46.4% versus 77.4%). Since this study was not powered to find differences in clincial pregnancy rates between both groups, this RCT was especially powered as a superiority trial to find differences in clincial pregnancy rates between both groups. On the day of oocyte retrieval, patients will be allocated to culture at one of both temperatures.

Inclusion Criteria:

• Day 5 transfer
• Single embryo transfer
• Only ejaculated sperm (fresh or frozen, autologous or heterologous)
• BMI <35
• Age <40 years
• Cycle rank <3 for the current child
• Last ultrasound: 8 follicles of at least 12 mm
• At least 6 mature oocytes

Exclusion Criteria:

• IVF
• IVF versus ICSI
• Failed fertilization in previous cycle
• no previous cycle without embryo transfer
• No use of Ca ionophores for embryo quality or fertilization problems

For statistical Analysis

within 6 months after study completion

Only relevant information will be given to the statistician (who is also a clinical doctor in our centre)