Uterine artery doppler indices in mares carrying horse and donkey pregnancies

ABSTRACT: Interspecific pregnancies, in which mares carry horse-donkey hybrid fetuses (mule pregnancies), present physiological challenges distinct from typical horse pregnancies, especially regarding placental development and uterine hemodynamics. While doppler ultrasonography is well-established for assessing uterine artery blood flow in horses, its application in interspecific pregnancies remains limited, creating a gap in reproductive management for mule production programs. This study compared pulsatility (PI) and resistivity (RI) indices of uterine arteries in mares carrying horse (GS, n = 13) or donkey (GD, n = 13) pregnancies throughout gestation, while evaluating effects of maternal age. Using transrectal Doppler ultrasonography (SonoScape S6®, 3-7MHz), bilateral uterine artery measurements were performed every 15 days until 150 days and Monthly thereafter until parturition. Results revealed distinct hemodynamic patterns: GD pregnancies showed delayed peak PI (66-80 days vs GS: 20-35 days) and earlier stabilization (186-215 days vs GS: 306-335 days), suggesting altered placental maturation. A significant age effect was observed, with older mares exhibiting higher RI in the right uterine artery (r = 0.107, P < 0.05). While GS mares showed bilateral PI reduction, GD mares displayed unilateral (left artery) changes, indicating asymmetric adaptation. These findings provided the first reference values for uterine artery indices in interspecific equine pregnancies, demonstrating that standard horse pregnancy parameters cannot be directly applied to donkey pregnancies. The study established Doppler ultrasonography as a valuable tool for monitoring hybrid pregnancies and highlights the need for species-specific management approaches in equine reproduction, particularly for aging mares in mule breeding programs. Future research should investigate molecular mechanisms underlying these hemodynamic differences and explore targeted interventions to optimize pregnancy outcomes.