Early Pregnancy Diagnosis in Mares Before Day 18 of Gestation: Emerging Molecular Biomarkers and Diagnostic Strategies — A Review

Abstract

Early pregnancy detection before Day 18 post-ovulation is essential for improving reproductive efficiency and reducing economic losses in equine breeding programs. Although transrectal ultrasonography and hormonal assays remain reliable diagnostic tools, their sensitivity during the earliest stages of gestation is limited. In this review article, recent peer-reviewed studies were systematically examined to evaluate emerging molecular and metabolic biomarkers associated with early pregnancy in mares. Particular attention was given to circulating microRNAs, lipidomic and metabolomic profiles, proteomic indicators, and extracellular vesicle–associated molecules. These biomarkers reflect early embryo–maternal communication and physiological adaptations occurring shortly after conception. Comparative analysis indicates that while each biomarker category provides valuable biological insight, none alone achieves optimal diagnostic accuracy before Day 18. Furthermore, many molecular approaches require specialized laboratory infrastructure, trained personnel, and increased financial investment, which may limit their immediate applicability in field conditions. Time-consuming sample processing and the lack of standardized analytical protocols also represent significant practical challenges. Current evidence suggests that integrated multi-biomarker strategies combining molecular and metabolic indicators may enhance diagnostic sensitivity and specificity compared with single-marker approaches. However, large-scale validation studies and the development of cost-effective, rapid, and standardized diagnostic platforms are necessary before routine clinical implementation. Advancing accessible early detection tools may substantially improve reproductive management and help reduce early embryonic loss in mares.