He has practised infertility treatments for over 35 years and published more than 150 peer-reviewed papers. His main interest is the safety of infertility treatments, including single embryo transfer in fresh and frozen cycles.

Study question Is the treatment outcome compromised after superovulation for fresh IVF/ICSI in comparison to frozen cycle with spontaneous ovulation and luteal support with progesterone? Summary answer Live birth rate (LBR) is dependent on embryo quality both in the fresh and frozen cycles with no sign of harmful effect of the superovulation. What is known already Freeze-all strategy has been advocated in recent years based on the assumption that luteal phase after superovulation is not optimal for embryo implantation. The effects of variable hormonal treatments, given in association with ART, on the endometrium, are still largely unknown. Therefore, more data is needed in order to optimize the treatment policies. Study design, size, duration This is an observational retrospective single-center cohort study. Data were collected from Oulu University Hospital’s ART-database including a total of 5647 single embryo transfer cycles from years 1995–2020. Patients stimulated with long agonist protocol for IVF/ICSI and day 2–3 transfer were included. Frozen embryo transfer was performed in a natural cycle with an ovulation test used for timing of transfer. Luteal support with progesterone was given for two weeks in all cycles. Participants/materials, setting, methods There were 3053 IVF/ICSI fresh cycles (2237 top and 816 N-top) and 2594 frozen cycles (806 top and 1788 N-top). The main outcome measure was LBR compared between fresh and frozen cycles when either a top or a N-top embryo was transferred. As a secondary outcome, clinical pregnancy rate was investigated. Data on the age and body mass index (BMI) of the patients was available. Student’s T-test was used to compare continuous variables. Main results and the role of chance The groups did not differ regarding the age and BMI of the patients. After the transfer of a top quality embryo the clinical pregnancy rate (35.4 vs. 30.8%; p < 0.05) and LBR (29.4 vs. 25.5%; p not significant) was slightly higher in the fresh cycle. After the transfer of a N-top quality embryo the clinical pregnancy rate (19.9 vs. 19.4%) and LBR (15.9 vs. 15.4%) were similar both in the fresh and frozen cycles. Limitations, reasons for caution This study only represents cleavage stage embryo transfers, and all FETs were performed in a natural cycle. In a retrospective study there may also be residual confounding that could not be excluded. Wider implications of the findings: This study provides further evidence that treatment outcome regarding LBR is not affected by superovulation therapy. Hence, the use of freeze-all strategy is warranted only in cases with a risk of ovarian hyperstimulation syndrome. Trial registration number Not applicable