Singleton freemartinism in cattle: A structured narrative review and historical case reappraisal

Freemartinism classically affects a female calf gestated with a male co-twin. A female may be born as an apparent singleton when a male co-conceptus is lost after placental vascular communication has permitted cellular exchange and exposure to testicular products. This structured narrative review and historical case reappraisal evaluates evidence for singleton freemartinism, clarifies diagnostic boundaries and reassesses a 1995 case following multiple-embryo transfer. PubMed, Scopus and FAO AGRIS were searched through 14 September 2026, supplemented by citation searching. Evidence supports the existence of singleton freemartinism but does not establish its prevalence. An apparent singleton birth does not demonstrate that gestation was singleton throughout. In an appropriate developmental context, persistent XX/XY blood chimerism, the coexistence of XX and XY blood-cell populations, is evidence that fetal cell exchange occurred. Detection of a Y-linked sequence establishes only that target DNA is present in the specimen. Neither finding alone demonstrates male-type reproductive-tract development or pathological freemartinism. Diagnosis therefore requires integration of gestational history, characterized chromosome-based or molecular findings, reproductive anatomy, functional findings and histopathology. Archival sections show ovarian follicles and testis-like tubular structures in the same gonad. Interpretation is limited by the unknown sex of the lost embryo, incomplete gonadal examination, lack of cell-level localization and cell-type identification, and unknown fertility. Twinning, multiple-embryo transfer and conceptus loss may increase opportunities for the condition to arise, but evidence demonstrates neither increased prevalence nor a quantitative contribution to reproductive failure. Prospective multicenter studies linking gestational records with molecular, anatomical and functional outcomes are required.