If you mare does pass the placenta, lay it out so your vet can be sure both horns are present in their entirety. | Photo: Courtesy Dr. Alana King

During pregnancy the equine placenta and uterus are tightly associated through highly vascularized (filled with blood vessels) infolded structures called microvilli, which allow nutrient and waste exchange between the mare and foal. Once the foal is born, the placenta is no longer needed and the mare must deliver all of it. The umbilical cord breaks, and blood vessels within the placenta lose pressure and collapse, causing the microvilli to recede from the uterine wall. Concurrent rhythmic uterine contractions help expel the placenta.

Most mares pass these fetal membranes shortly after foaling. If it hasn’t happened within three hours postpartum, it’s considered a retained placenta. This retention may be complete, involving the entire placental surface area, or partial, which usually includes the tips of one or both placental horns.

Foaling complications, abortion, and infection all increase the mare’s risk of retaining a placenta, but the condition can also occur in apparently normal foalings. Draft mares, particularly Friesians, are especially susceptible