Anogenital distance as a cross‑species biomarker of fetal androgen action: Origins, translation, and implications for reproductive toxicology.

Anogenital distance (AGD) is a dose-responsive biomarker of disrupted fetal androgen signalling. Because AGD remains relatively stable after birth, its measurement provides a retrospective indicator of fetal androgen action and has proven valuable in toxicity research, particularly for identifying chemical exposures with anti-androgenic properties. Although initially described as an anatomical trait for sexing neonatal animals, rodent toxicity studies in the 1990s established AGD as an androgen-sensitive endpoint following in utero exposure. Importantly, reduced AGD was consistently associated with a broader spectrum of male reproductive abnormalities, including cryptorchidism and hypospadias, collectively reflecting impaired masculinization during development. The identification of a distinct masculinization programming window (MPW) subsequently provided a mechanistic framework for interpreting AGD variation. Human studies have since confirmed that AGD captures aspects of prenatal endocrine disruption, including phthalate exposure, supporting its use as a translational biomarker linking experimental models, epidemiology, and regulatory toxicology. This review traces key developments in the conceptual evolution of AGD, from a sexually dimorphic anatomical trait used for visual sex determination to a formalized biomarker applied in animal and human studies of endocrine disruption.