1. Effect of Circadian rhythm of temperature on embryo culture Ronny Janssens, Neelke De Munck, Greta Verheyen. Centre for Reproductive Medicine, UZ Brussel, Laarbeeklaan 101, B-1090 Brussels, Belgium Introduction/Aim Since the onset of IVF, constant incubator settings matching the core body temperature (CBT, 37°C) are used, but the optimal culture temperature is yet undefined. The circadian rhythm (CR) of the core body temperature is a well-documented physiological phenomenon with a maximum temperature in the early evening and a minimum in the early morning with a maximum range of 0.9°C. Recent (technical) improvements to desktop incubators allow a more accurate and stable temperature regulation (temperature gradient of 0.1°C between chambers) and also includes the simulation of a circadian temperature rhythm. The purpose of this prospective study was to evaluate whether the use of a circadian rhythm improves embryo development. Methods Before the study, each individual chamber of two incubators were calibrated and adjusted to 37.0°C (37.01°C, ± 0.04°C). Sibling oocytes from 50 patients were cultured until day 5/6 at 37.0 °C (CBT) in our standard incubator (G-185, K-Systems, Denmark, Figure 1) or in a prototype incubator (G-210, K-Systems, Denmark) with a circadian temperature rhythm (CR) with a range of 36.6°C (1 to 6 a.m.) to 37.5°C (11 a.m. to 9 p.m. Figure 2). Contact: Ronny Janssens ronny.janssens@uzbrussel.be ronny.janssens@uzbrussel.be Results A total of 674 mature oocytes were randomly distributed for culture at 37.0°C (CBT) or exposed to a CR. No significant difference (p=0.113) in fertilization rate was observed (84.0% vs. 78.9%). On day 3 of development, the number of excellent and good quality embryos did not differ between both groups (78.8% vs. 76.7%; p=0.608). Though no significant difference was obtained on day 5 of development (59.2% vs. 52.3%; p=0.146), a significant difference in the utilization rate per mature oocyte (42.1% vs 32.6%: p=0.014) was observed. Conclusion This is the first report of human embryos cultured in a circadian temperature rhythm. Although fertilization and embryo development were similar between both groups, the utilization rate was lower in the CR cultured embryos. The selection of the CR settings in the current study do not seem to improve embryo development. Further studies are needed to determine the optimal temperature or CR range for human embryos. It is known that temperature gradients exist in female reproductive tissues (Hunter et al. RBM Online 24, 2012) and that ovarian temperature is cooler than rectal temperature (Hunter et al., Hum Reprod 12, 1997). Maybe the minimum and/or maximum temperature chosen was too high or too low but it is also possible that real in-vivo temperature conditions are less variable than the circadian rhythm variations. In a next experiment we will try to define an optimal culture temperature for human embryo culture. CBTCRp No. of Cycles50 No. of MII674 337 Fertilized (%)283 (84.0)266 (78.9)0.113 Day 3 Embryo Quality Excellent (%)153 (54.1)131 (49.2) 0.608 Good(%)70 (24.7)73 (27.4) Moderate (%)36 (12.7)39 (14.7) Poor (%)24 (8.5)23 (8.6) Day 5 Evaluation No. of Embryos to Day 5250241 Excellent Quality (%)51 (20.4)39 (16.2) 0.146 Good Quality (%)97 (38.8)87 (36.1) Moderate Quality (%)41 (16.4)52 (21.6) Poor Quality (%)61 (24.4)63 (26.1) Utilization Transferred on Day 53126 Cryopreserved on Day 57747 Cryopreserved on Day 63437 Per MII (%)142/337 (42.1)110/337 (32.6)0.014 Figure 1 Figure 2