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Objective—To determine the incidence, ultrasonographic characteristics, and risk factors associated with embryonic development characterized by formation of an embryonic vesicle without an embryo in mares. Design—Prevalence survey. Animals—159 pregnant mares. Procedure—From 1994 to 1998, mares between 11 and 40 days after ovulation with normal and abnormal embryonic development were examined ultrasonographically, and characteristics of each conceptus were recorded. Results—The incidence of abnormal embryonic development in mares characterized by formation of an embryonic vesicle without an embryo was 7/159 (4.4%) during the 5 breeding seasons. Age and breed of mare or type of semen used did not differ for mares with normal and abnormal embryonic development. The percentage of mares in which the conceptus was undersized during ≥ 1 examination was significantly higher for mares with abnormal conceptuses (5/7), compared with mares with normal conceptuses (2/147; 1.4%). The percentage of examinations during which the conceptus was undersized was significantly higher for abnormal conceptuses (12/27; 44.4%), compared with normal conceptuses (4/448; 0.9%). Conclusion and Clinical Relevance—To diagnose an embryonic vesicle without an embryo, mares should be examined by use of transrectal ultrasonography on day 25 after ovulation. When an embryo cannot be identified at that time, mares should be reexamined at intervals of 1 to 3 days until day 30. Because undersized conceptuses are more likely to be abnormal, development of undersized conceptuses should be monitored closely. (J Am Vet Med Assoc 2000;217:58–63)