Transuterine migration of the fetus in the mare between day 42 and parturition.

The research observes the movement of a fetus within a mare’s uterus between the 42nd day of pregnancy and parturition, noting that a significant portion witness transuterine migration. It is also highlighted that mares with a fetus in the same horn for two consecutive seasons had a higher rate of abortion.

Study Objective and Approach

• The primary goal of this research was to investigate the location of the fetus within mare’s uterine horns at various stages of pregnancy and deduce patterns or factors influencing this.
• The study involved a comprehensive four-year examination of a total of 2187 Thoroughbred and Standardbred mares.

Key Findings

• In maiden mares, there was an almost equal distribution of pregnancies between the left and right uterine horns (44% and 56% respectively).
• No transuterine migration was observed in maiden mares after their first foal, as examined in a group of 139 mares.
• A significantly higher portion of lactating mares (62%) had pregnancies consecutively switching between the uterine horns. This means that the fetus implanted and grew to term on one side of the uterus in one breeding season, and then the next pregnancy took place in the opposite uterine horn in the following season.
• However, there was a group of mares (19%) for whom the pregnancies, across all three examination times, consistently took place in one uterine horn. The reason for this remained unclear.
• Transuterine migration was observed in 18% of the mares. That is, for these mares, the fetus was found to have moved from its location at the 42-day examination to a different location within the uterus at a later stage of pregnancy.
• Of these 18% mares, half had the placental cord attached at the entrance of the other, non-pregnant horn. This suggests that at some point after the 42nd day, the fetus migrated from one horn to the other.
• The study also found a significant correlation between transuterine migration and rates of abortion. Mares that had a fetus in the same uterine horn for two consecutive seasons were found to have a higher rate of abortion compared to those with migrations, a crucial finding indicating potential complications due to lack of migration.

Conclusion

• The study sheds light on the phenomenon of transuterine migration in mares, establishing it as a common occurrence.
• Further research would be required to understand the cause of non-migration in certain cases and its potential connection to higher rates of abortion.
• Such insights can be valuable in managing and improving equine reproduction and overall mare health.