Veterinary research communications2026; 50(6); 527; doi: 10.1007/s11259-026-11472-1

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

Campos, DG
·
Campos, IS
·
Silva, PHH
·
Bringel, B
·
Douglas, RH
·
Pinna, AE
·
Jacob, JCF
·
Portela, VM
·
Amaral, CDS
·
Antoniazzi, AQ

Abstract: 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.

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Publication Date: 2026-08-25 PubMed ID: 42640365PubMed Central: 12322513DOI: 10.1007/s11259-026-11472-1Google Scholar: Lookup

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MeSH
Conflict of Interest
References
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Journal Article

Summary

This research summary has been generated with artificial intelligence and may contain errors and omissions. Refer to the original study to confirm details provided. Submit correction.

Plain Language Overview

This study investigated how pregnancies from different species of equines (horse mares bred with either horse stallions or donkeys) differ in their hormone levels, formation of hormone-producing structures, and blood flow in the uterus during early pregnancy.
It found that interspecies pregnancies (mare x donkey) showed distinct hormonal and uterine blood flow patterns compared to same-species pregnancies (mare x stallion), which helps explain differences in pregnancy maintenance and loss.

Detailed Explanation

Background and Objectives

Interspecific equine pregnancies occur when a mare is bred with a donkey, producing a mule or hinnies.
These pregnancies are known for differing endocrine (hormonal) patterns and have higher risks of pregnancy loss compared to standard horse-to-horse pregnancies.
The specific interactions among hormones, the formation and activity of supplementary corpora lutea (SCL, structures on the ovary that produce progesterone), and blood flow parameters in the uterus related to the origin of the conceptus (offspring) are not fully understood.
The study aimed to:

Evaluate how conceptus origin (stallion-derived vs. donkey-derived) influences hormonal profiles, SCL formation, and uterine blood flow.
Analyze how these variables relate to each other during early pregnancy (Days 49 to 98).

Methodology

Twenty mares were initially included; however, data from 14 mares were analyzed due to losses or incomplete monitoring.
Pregnancy groups:

6 mares with stallion-derived pregnancies (horse x horse)
8 mares with donkey-derived pregnancies (horse x donkey)

Measurements taken at gestational Days 49, 63, 84, and 98 included:

Serum progesterone (P4) concentrations
Equine chorionic gonadotropin (eCG) concentrations, a hormone related to supporting pregnancy
Counting the number of supplementary corpora lutea (SCL), indicative of luteal activity
Uterine artery Doppler indices: pulsatility index (PI) and resistivity index (RI), which measure blood flow resistance and pulsatility in uterine arteries

Key Findings

Donkey-derived pregnancies showed significantly:

Lower progesterone (P4) and equine chorionic gonadotropin (eCG) levels compared to stallion-derived pregnancies
Reduced formation of supplementary corpora lutea (SCL)
Lower uterine artery pulsatility index (PI) and resistivity index (RI), indicating different uterine blood flow patterns

Stallion-derived pregnancies remained relatively stable for hormone concentrations and uterine blood flow indices throughout the study period.
Donkey-derived pregnancies showed a progressive decline in both P4 and eCG, alongside decreases in uterine PI and RI over time.
Statistically significant positive correlations were observed:

Between progesterone (P4) levels and pulsatility index (PI) (correlation coefficient r = 0.846, p < 0.001)
Between equine chorionic gonadotropin (eCG) and pulsatility index (PI) (r = 0.733, p = 0.015)

All pregnancies, despite these differences, were maintained to term.

Interpretation and Significance

The data support the conclusion that the origin of the conceptus (horse vs donkey sperm) influences endocrine activity, luteal function, and uterine blood flow during early pregnancy in mares.
Lower progesterone and eCG along with fewer supplementary corpora lutea in donkey-derived pregnancies suggest altered luteal support, which may explain some increased pregnancy risks observed previously in interspecific pregnancies.
Lower PI and RI values indicate reduced resistance in uterine arterial blood flow, which may be an adaptive or compensatory vascular response to lower hormone concentrations.
The strong correlations between hormone levels and blood flow indices highlight an integrated relationship between endocrine support and uterine vascular adaptations during pregnancy.
Understanding these differences is important for managing interspecific equine pregnancies and improving pregnancy outcomes.

Cite This Article

APA

Campos DG, Campos IS, Silva PHH, Bringel B, Douglas RH, Pinna AE, Jacob JCF, Portela VM, Amaral CDS, Antoniazzi AQ. (2026). Endocrine and uterine hemodynamic differences between intra- and interspecific equine pregnancies during early gestation. Vet Res Commun, 50(6), 527. https://doi.org/10.1007/s11259-026-11472-1

Publication

Veterinary research communications

ISSN: 1573-7446

NlmUniqueID: 8100520

Country: Switzerland

Language: English

Volume: 50

Issue: 6

PII: 527

Researcher Affiliations

Campos, Diego Guedes

Laboratory of Reproductive Biotechnology (BIOREP), Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil. diegoguedescampos@hotmail.com.

Campos, Isabela Syllos

Department of Veterinary Pathology and Diagnostic Imaging, Fluminense Federal University (UFF), Niterói, RJ, Brazil.

Silva, Paulo Henrique Hümmelgen

Laboratory of Reproductive Biotechnology (BIOREP), Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil.

Bringel, Beatriz

BET Reproductive Laboratory, Lexington, KY, USA.

Douglas, Robert H

BET Reproductive Laboratory, Lexington, KY, USA.

Pinna, Aline Emerim

Department of Veterinary Pathology and Diagnostic Imaging, Fluminense Federal University (UFF), Niterói, RJ, Brazil.

Jacob, Julio Cesar Ferraz

Department of Animal Reproduction and Evaluation, Institute of Animal Science (IZ), Federal Rural University of Rio de Janeiro (UFRRJ), Seropédica, RJ, Brazil.

Portela, Valério Marques

Laboratory of Reproductive Biotechnology (BIOREP), Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil.

Amaral, Carolina Dos Santos

Laboratory of Reproductive Biotechnology (BIOREP), Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil. carolina.amaral@ufsm.br.

Antoniazzi, Alfredo Quites

Laboratory of Reproductive Biotechnology (BIOREP), Federal University of Santa Maria (UFSM), Santa Maria, RS, Brazil.

MeSH Terms

Animals
Female
Pregnancy
Horses / physiology
Uterus / blood supply
Uterus / physiology
Progesterone / blood
Pregnancy, Animal / physiology
Hemodynamics
Equidae / physiology
Chorionic Gonadotropin / blood
Corpus Luteum / physiology

Conflict of Interest Statement

Declarations. Ethics approval and animal welfare: All procedures involving animals were approved by the Animal Ethics Committee of the Universidade Federal Fluminense (CEUA/UFF; protocol no. 001/19) and were conducted in accordance with institutional and national guidelines for animal care and use. Consent to participate: Not applicable. Consent to publish: Not applicable. Competing interests: The authors declare no competing interests.

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