Careful examination of the placenta passed after a foal’s birth will reveal a soft, putty-like formation, about 14cm x 1.5cm. In folklore, this was often called a ‘moonstone’ and was thought to bring good luck, possibly because it was not always seen. Puzzled owners of foaling mares may be interested to know that what they are looking at is closer to a kidney stone than a gemstone.
A hippomane is a concentration of foetal kidney waste that have built up in the pregnant mare’s womb. They vary in colour and size. Looking a little like liver, they are layered when cut. Their make up consists of high concentrations of nitrogen, calcium, phosphorous, sodium, potassium and magnesium. As well as minerals from the foetal urine, they also contains cell debris from the foetal membranes and it is these that provide a nucleus for the calculus to form. It is found in the allantoic fluid at about day 85 and can later be found in the placenta.
It is always found in the allantois (a sac-like structure in the placenta that collects liquid waste from the embryo). Fragments can also be found in the urachus (a canal within the umbilicial cord that drains the foetus’s urinary bladder). Sometimes, additional smaller hippomanes are found floating in the fluid or attached to the membranes. Dr James Rooney has offered the view that the hippomane develops as an out-pouching of the allantois which later breaks away and floats free.
