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STUDY QUESTION Is there a difference in clinical pregnancy rates (CPRs) in good prognosis patients after single embryo transfer (SET) on Day 5, in case of stable culture at 36.6°C or 37.1°C? SUMMARY ANSWER CPR (with heartbeat at 7 weeks) after blastocyst transfer do not differ after culturing at 36.6°C or 37.1°C. WHAT IS KNOWN ALREADY Since the beginning of IVF, embryo culture has been performed at 37.0°C; however, the optimal culture temperature remains unknown. Changes in incubator types have led to significant improvements in temperature control. Stable temperature control, i.e. with temperature differences of max. 0.1°C between chambers, is possible in some incubators. A previous prospective pilot study showed that embryo development on Day 5/6 was not affected when embryos were cultured at a stable temperature of 36.6°C or 37.1°C, but culture at 37.1°C resulted in an increased CPR when compared to culture at 36.6°C (74.2% vs 46.4%). STUDY DESIGN, SIZE, DURATION A prospective randomized controlled trial was performed in a tertiary fertility centre between February 2017 and November 26, 2022. A sample size of 89/89 patients with fresh single embryo transfer (SET) was required to achieve 80% power to detect a difference of 0.22 between group proportions (0.43–0.65) at a significance level of 0.05 using a two-sided z -test with continuity correction. PARTICIPANTS/MATERIALS, SETTING, METHODS Patients were recruited on the day of oocyte retrieval based on inclusion criteria with final randomization after denudation once six mature oocytes were present. The primary endpoint was CPR (heartbeat at 7 weeks); secondary endpoints were fertilization rate, blastocyst development, biochemical pregnancy rate, live birth rate (LBR), and cumulative live birth rate (CLBR). MAIN RESULTS AND THE ROLE OF CHANCE A total of 304 patients were eligible for the study; of these 268 signed the consent, 234 (intention-to-treat) were randomized and 181 (per-protocol) received a SET on Day 5: 90 received culture at 36.6°C and 91 at 37.1°C. Patients were on average 32.4 ± 3.5 versus 32.5 ± 4.2 years old, respectively. No differences were observed in embryological outcomes per cycle between culture at 36.6°C versus 37.1°C: 12.0 ± 3.8 vs 12.1 ± 3.8 COCs retrieved (P  = 0.88), 10.0 ± 3.1 versus 9.9 ± 2.9 mature oocytes inseminated (P  = 0.68), with a maturation rate of 84.2% (901/1083) versus 83.5% (898/1104) (P  = 0.87); and 8.0 ± 3.1 versus 7.9 ± 2.7 normally fertilized oocytes with a fertilization rate of 79.7% (720/901) vs 80.5% (718/898) (P  = 0.96), respectively. On average 1.5 ± 1.7 versus 1.4 ± 1.9 (P  = 0.25) and 1.1 ± 1.1 versus 0.9 ± 1.0 (P  = 0.45) supernumerary blastocysts were vitrified on Day 5 and Day 6, respectively. The utilization rate per fertilized oocyte was 46.1% vs 41.5% (P  = 0.14). A SET was performed for 181 patients, leading to a biochemical pregnancy rate of 72.2% (65/90) versus 62.7% (57/91) (P  = 0.17), respectively. The CPR per fresh transfer cycle was 51.1% (46/90) versus 48.4% (44/91) [OR (95% CI) 1.11 (0.59–2.08), P  = 0.710]. To date, a CLBR of 73.3% (66/90) versus 67.0% (61/91) (P  = 0.354) has been observed, respectively. In each group, seven patients without live birth have remaining blastocysts frozen. The CPR for the intention-to-treat groups were 38.3% vs 38.6% [OR (95% CI) 0.98 (0.56–1.73), P  = 0.967], respectively, for culture at 36.6°C versus 37.1°C. LIMITATIONS, REASONS FOR CAUTION Only selected patients with expected good prognosis were eligible for the study. WIDER IMPLICATIONS OF THE FINDINGS Embryos tend to tolerate small changes in temperature deviations during culture to the blastocyst stage, as demonstrated by their similar implantation potential at two slightly different temperatures. STUDY FUNDING/COMPETING INTEREST(S) There is no funding or conflicts of interest to declare. TRIAL REGISTRATION NUMBER NCT03548532. TRIAL REGISTRATION DATE 23 October 2017 DATE OF FIRST PATIENT'S ENROLMENT 10 November 2017 [ABSTRACT FROM AUTHOR]