Mega allantoic vesicles of the equine placenta

An ultrasound examination of a pregnant 12‐year‐old Palomino mare's abdomen revealed multiple, variably sized, round, polypoid, cystic structures within the allantois. These cystic areas had clear hypoechoic centres and were surrounded by thin, irregular hyperechoic regions. The foaling was unremarkable. Grossly, the allantois was multifocally expanded by variably sized, 1–15 cm diameter, balloon‐like allantoic vesicles containing clear watery or gelatinous fluid. Histological examination demonstrated marked separation of the allantoic surface from the chorionic surface by oedema fluid or gelatinous stroma. No other lesions were observed in the placenta. Similar lesions were observed in 2 other mares. No clinical complications were observed with the foals, fillies and mares after parturition. This is the first report of such large allantoic vesicles in the equine placenta.