Objective- The aim of this study was to determine the first possible time for pregnancy recognition and several features of developing embryo between days 8 to 44 of pregnancy.
Design- Experimental study
Animals- Six healthy registered Caspian mares
Procedures- Daily ultrasound examinations were conducted from days 8 to 44 (ovulation = day 0) of pregnancy to monitor the conceptus in Caspian mares (No. of pregnancy= 9).
Results- Embryonic vesicle was observed for the first time on day 9.67 ± 0.33 (mean ± SEM, range 9-11 days) in 7.17 ± 0.48 mm diameter. The vesicle was fixed in distal part of uterine horn on day 16.44 ± 0.24 (age range 15-17 days). The vesicle was spherical from days 9 to 16 (mean ± SEM growth rate, 2.16 ± 0.14 mm/day), and non spherical from days 16 to 19 with reduced growth rate from day 18-27 (0.60 ± 0.12 mm/day) and then averagely grew at a  rate of 1.27 ± 0.15 mm/day until day 44 of pregnancy. Embryo proper and heartbeat were first detected on days 20 ± 0.37 and 23.11 ± 0.35, respectively. Formation of allantoic sac at the same time of decreasing of yolk sac was initiated on day 23.55 ± 0.44 and it was completed on day 35.33 ± 0.55.
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.