In mares, the fetal portion of the placenta, or fetal membranes, are normally expelled within 3 hours after birth. Although some mares may retain the fetal membranes longer without suffering ill effects, many mares with retained membranes become toxic and may even die. The cause of placental retention is not known. If none of the fetal membranes have been expelled, the condition will be obvious by the membranes hanging from the mare's vulva. However, retention of only a small portion of the placenta within the uterus may not be noticed and will result in serious complications.

Once passed, the fetal membranes should always be carefully examined to ensure that they have been completely expelled. The fetal membranes from the side (horn) of the uterus that contained the foal will be thicker, and the other, nonfetal side (horn) will be thin and puckered. It is important that both tips be present. The puckered tip, or nonfetal horn, is most commonly retained.

If the entire fetal membranes have not been passed by 3 hours after the foal is born, you should contact your veterinarian so that oxytocin can be administered. The oxytocin will cause uterine contraction and expulsion of the fetal membranes. If the membranes have still not passed by 8 hours after the foal's birth, your veterinarian will administer antibiotics and anti-inflammatory drugs. The placenta should never be manually pulled out, as this may cause parts of the placenta to be retained or cause damage to the lining of the uterus.

A retained placenta should always be treated promptly. Failure to treat this condition can lead to serious consequences for the mare, including endotoxemia, founder (laminitis), and death.

Last full review/revision July 2011 by Robert O. Gilbert, BVSc, MMedVet, DACT, MRCVS; Fabio Del Piero, DVM, DACVP, PhD; R. J. Erskine, DVM, PhD; Paul Nicoletti, DVM, MS; Jerome C. Nietfield, DVM, PhD, DACVP; Donald Peter, DVM, MS, DACT; Patricia L. Sertich, MS, VMD, DACT; Katrin Hinrichs, DVM, PhD, DACT; Brad E. Seguin, DVM, MS, PhD DACT