Abstract:Foetal adnexa are derived from the blastocyst, they rapidly grow during the first stage of the pregnancy and are finally expelled after foaling. Congenital anomalies, as well as acquired (infectious and non-infectious) diseases of the foetal adnexa can result in reproductive disorders. As a consequence, the comprehensive and systematic examination of the foetal adnexa is widely regarded as a very useful procedure for diagnostic purposes. In this respect, it is important to correctly identify and to interpret the main features of the equine foetal membranes. We describe herein a case of yolk sac remnant, recently observed in a 10-year-old Quarter Horse mare. After parturition, foetal adnexa were routinely collected and carefully examined. At this point, a large, round-shaped mass of about 9 cm in diameter was observed, connected to the umbilical cord through a fibrous and pedunculated stalk. Such mass was very hard, it showed a bony appearance and contained reddish-to-brown serous fluid. Foetal membranes were otherwise normal. On the basis of what above, the diagnosis of ossified remnant of the yolk sac was made. The yolk sac is a temporary annex and ensures the early exchange between the uterine environment and the embryo, thanks to its dense vessel network. In horses, the transition from the yolk sac to the allantoic sac occurs between the 21st and 40th days of gestation, and the yolk sac obliteration is complete at the 3rd-4th month of pregnancy. However, the yolk sac can persist as a mass which vaguely resembles a skull and is often misdiagnosed as a twin foetus that failed to develop. In horses, the persistence of a yolk sac remnant is considered an uncommon, idiopathic condition of unknown origin. It most commonly represents an incidental finding, which usually has any clinical relevance for the foal and do not affect the future breeding career of the mare.