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Study Question: Can a modified specific gravity technique be used to distinguish viable from nonviable embryos?, Summary Answer: Preliminary data suggests a modified specific gravity technique can be used to determine embryo viability and potential for future development., What Is Known Already: Single embryo transfer (SET) is fast becoming the standard of practice. However, there is currently no reliable method to ensure development of the embryo transferred., Study Design, Size, Duration: A preliminary, animal-based in vitro study of specific gravity as a predictor of embryo development using a mouse model., Participants/materials, Setting, Methods: After a brief study to demonstrate embryo recovery, experiments were conducted to assess the ability of the specific gravity system (SGS) to distinguish between viable and nonviable embryos. In the first study, 1-cell mouse embryos were exposed to the SGS with or without previous exposure to an extreme heat source (60°C); measurements were repeated daily for 5 days. In the second experiment, larger pools of 1-cell embryos were either placed directly in culture or passed through the SGS and then placed in culture and monitored for 4 days., Main Results and the Role of Chance: In the first experiment, viable embryos demonstrated a predictable pattern of descent time over the first 48 h of development (similar to previous experience with the SGS), while embryos that were heat killed demonstrated significantly altered drop patterns (P < 0.001); first descending faster. In the second experiment, average descent times were different for embryos that stalled early versus those that developed to blastocyst (P < 0.001). Interestingly, more embryos dropped through the SGS developed to blastocyst than the culture control (P < 0.01)., Limitations, Reasons for Caution: As this is a preliminary report of the SGS technology determining viability, a larger embryo population will be needed. Further, the current in vitro study will need to be followed by fecundity studies prior to application to a human population., Wider Implications of the Findings: If proven, the SGS would provide a noninvasive means of assessing embryos prior to transfer after assisted reproductive technologies procedures, thereby improving fecundity and allowing more reliable SET., Study Funding/competing Interests: The authors gratefully acknowledge the funding support of the U.S. Jersey Association, the Laura W. Bush Institute for Women's Health and a Howard Hughes Medical Institute grant through the Undergraduate Science Education Program to Texas Tech University. None of the authors have any conflict of interest regarding this work., Trial Registration Number: none., (© The Author 2015. Published by Oxford University Press on behalf of the European Society of Human Reproduction and Embryology. All rights reserved. For Permissions, please email: journals.permissions@oup.com.)