Jalil Abshenas - Department of Clinical Sciences, Faculty of Veterinary Medicine, Shahid Bahonar University of Kerman, Kerman, Iran.
Homayoon Babaei - Department of Clinical Sciences, Faculty of Veterinary Medicine, Shahid Bahonar University of Kerman, Kerman, Iran.
Iman Mahdavi - Graduated Student of Veterinary Medicine, Shahid Bahonar University of Kerman, Kerman, Iran.
Mojtaba Alimolaei - Razi Vaccine and Serum Research Institute, Kerman Branch, Iran.
Ali Shafipour - Graduated Student of Veterinary Medicine, Shahid Bahonar University of Kerman, Kerman, Iran.

Objective- The aim of this study was to determine the first possible time for pregnancy recognition and several features of developing embryo between days ۸ to ۴۴ of pregnancy.Design- Experimental studyAnimals- Six healthy registered Caspian maresProcedures- Daily ultrasound examinations were conducted from days ۸ to ۴۴ (ovulation = day ۰) of pregnancy to monitor the conceptus in Caspian mares (No. of pregnancy= ۹).Results- Embryonic vesicle was observed for the first time on day ۹.۶۷ ± ۰.۳۳ (mean ± SEM, range ۹-۱۱ days) in ۷.۱۷ ± ۰.۴۸ mm diameter. The vesicle was fixed in distal part of uterine horn on day ۱۶.۴۴ ± ۰.۲۴ (age range ۱۵-۱۷ days). The vesicle was spherical from days ۹ to ۱۶ (mean ± SEM growth rate, ۲.۱۶ ± ۰.۱۴ mm/day), and non spherical from days ۱۶ to ۱۹ with reduced growth rate from day ۱۸-۲۷ (۰.۶۰ ± ۰.۱۲ mm/day) and then averagely grew at a  rate of ۱.۲۷ ± ۰.۱۵ mm/day until day ۴۴ of pregnancy. Embryo proper and heartbeat were first detected on days ۲۰ ± ۰.۳۷ and ۲۳.۱۱ ± ۰.۳۵, respectively. Formation of allantoic sac at the same time of decreasing of yolk sac was initiated on day ۲۳.۵۵ ± ۰.۴۴ and it was completed on day ۳۵.۳۳ ± ۰.۵۵.Conclusion and Clinical Relevance- The dates of occurrence of morphologic features of developing Caspian mare embryo were similar to the previous reports in mares and jennies and also ultrasound measurement of embryo and embryonic vesicle is an accurate tool and simple way for estimating the age of pregnancy in Caspian mares.

Transrectal ultrasonography, embryo, Embryonic vesicle, Caspian mare