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Abstract Study question Does transitioning to a different oil overlay for embryo culture affect success rates? Summary answer When moving from a light mineral oil to a 100% paraffin oil, there was a significant reduction in blastulation rate. What is known already Oil overlay is a necessary component of embryo culture. It limits evaporative loss of the embryo culture medium, preventing the concentration of salts, and promoting homeostasis by maintaining pH and osmolality in the culture environment. Classification for embryo culture oils include “paraffin” and “mineral”, though the Chemical Abstracts Service (CAS) and the European Pharmacopeia provide that the identification number between several versions of “paraffin” and “mineral” oil for embryo culture is identical, thus indicating that they are the same chemical composition. Despite this, there remains debate regarding the optimal oil for embryo culture. Study design, size, duration This study is a retrospective review and analysis of laboratory and clinical outcome data obtained over a 2-year period in order to compare the resulting blastocyst utilization rates between different embryo culture oil. Clinical pregnancy and miscarriage data is also included. A total of 1,476 oocyte retrieval cycles occurred during the review period, of which, 364 cycles included a fresh embryo transfer. From these retrievals an additional 623 subsequent FET cycles were performed. Participants/materials, setting, methods Autologous cycles at a university-based IVF laboratory between January 2022 and December 2023 were included. Through 2022, a light mineral oil (Irvine Scientific) was used in the laboratory, n = 731. In 2023 the lab moved to 100% paraffin oil, n = 742. During the period of analysis, the only notable change in the laboratory was the oil used for overlay. Culture media, incubators (non-humidified), and embryo culture conditions remained constant. Chi-square and t-test were used for analysis. Main results and the role of chance In 2022, the average patient age was 37.0 (SD 4.2) and in 2023 the average patient age was 36.3 (SD 4.1) (p < 0.001). When evaluating the average number of blastocysts available to transfer or vitrify, there was a statistically significant drop in the blastocyst utilization rate of 7.2% from 2022 to 2023 (60.1% to 52.8%, respectively) (p < 0.001). There were no significant differences in either positive beta-hCG (57.5% vs 52.4%, p-value 0.327) or clinical pregnancy rates (48.0% vs 42.2%, p-value 0.259) from the fresh transfers. Biochemical (9.5% vs 9.7%, p-value 0.940) and miscarriage rates (18.6% vs 15.4%, p-value 0.584) were also not different in the fresh transfers. We analyzed fresh and frozen blastocyst transfers cumulatively of which all were cultured during the study period. A total of 246 patients had at least two transfers. There was no statistical differences between 2022 and 2023 for positive beta-hCG (77.3% vs 75.4%, p-value 0.490), clinical pregnancy rate (71.4% vs 69.8%, p-value 0.612), or miscarriage rate (10.4% and 7.6%, p-value 0.153). Limitations, reasons for caution Oil overlay in embryo culture is only one of many factors that can play a role in patients’ outcomes. Though this retrospective analysis does show statistical significance in blastulation rate, an observational prospective study could be beneficial in supporting these findings. Wider implications of the findings We found a difference in blast utilization rates when different oil was used. Transitioning between oil overlays is often less scrutinized than embryo culture medium changes. This study highlights that all changes should be monitored for continued patient outcome success. Trial registration number No