Evidence for the Presence of a Viable Bovine Embryo During Days 18 to 28 After Insemination

This narrative review asks how accurately available tests identify the presence of a viable bovine embryo during days 18–28 after insemination and what evidence is required to improve their interpretation. We compare each method’s biological target, sampling window, diagnostic performance, reference examination, and validation. Luteal progesterone and color Doppler ultrasonography provide information on corpus luteum function and can support identification of probable non-pregnancy, but persistent luteal activity does not confirm an embryo. Pregnancy-associated glycoproteins provide trophoblast-associated evidence near the later part of this interval. Cervical interferon-tau and maternal interferon-stimulated genes offer earlier conceptus-associated signals, although sampling, specificity, threshold transfer, and independent validation remain limitations. Circulating nucleic acids and multi-omics signatures remain exploratory. None of these indirect signals independently demonstrates embryonic cardiac activity. B-mode ultrasonography provides direct evidence of a living embryo when cardiac activity is reliably visualized, although early performance depends on examination timing and technical conditions. A correct diagnosis at sampling is not invalidated by subsequent pregnancy loss; studies using a later reference examination must distinguish later pregnancy classification from contemporaneous diagnosis. The clearest scientific role of earlier assessment is to investigate embryo establishment and loss using appropriately validated endpoints. Routine commercial benefit cannot be inferred from earlier detection alone, particularly when breeding dates are uncertain or additional handling is required. Priorities are to establish the relationship between early signals and embryo viability, validate prespecified thresholds in independent populations, and evaluate decision value only where a specific unmet need has been demonstrated.