Endocrine and uterine hemodynamic differences between intra- and interspecific equine pregnancies during early gestation

Interspecific equine pregnancies (mare x donkey) are associated with altered endocrine patterns and increased pregnancy loss; however, the relationship between hormonal dynamics, luteal activity, and uterine hemodynamics in relation to conceptus origin remains incompletely understood. This study aimed to evaluate the influence of conceptus origin on endocrine profiles, supplementary corpora lutea (SCL) formation, and uterine hemodynamics, and to investigate the relationships among these variables. Twenty mares were initially included; however, six were excluded due to pregnancy loss or incomplete follow-up, resulting in a final sample of 14 mares (six stallion-derived and eight donkey-derived pregnancies). Serum progesterone (P4), equine chorionic gonadotropin (eCG), SCL number, and uterine artery Doppler indices, including pulsatility index (PI) and resistivity index (RI), were evaluated at Days 49, 63, 84, and 98 of gestation. Donkey-derived pregnancies showed significantly lower P4 and eCG concentrations, reduced SCL formation, and lower uterine PI and RI compared to stallion-derived pregnancies (p < 0.05). While stallion-derived pregnancies remained relatively stable over time, donkey-derived pregnancies showed progressive declines in P4 and eCG concentrations, accompanied by reductions in uterine PI and RI. Positive correlations were observed between P4 and PI (r = 0.846; p < 0.001) and between eCG and PI (r = 0.733; p = 0.015). All pregnancies were maintained to term. These findings indicate that conceptus origin is associated with distinct endocrine and uterine hemodynamic patterns, supporting integrated hormonal-vascular adaptations in equine pregnancy.