Equine fetal membranes are normally expelled within 3 hr after parturition, but expulsion may be delayed for 8–12 hr or even longer without signs of illness. The cause of retention of fetal membranes often is not known, but the condition is associated with infection, abortion, short or prolonged gestation, uterine atony, and dystocia. Mares that have retained their fetal membranes appear to be at increased risk of recurrence of the condition, and Friesian mares are particularly predisposed. Retention of just a portion of the fetal membranes entirely within the uterus (usually at the tip of the previously nongravid uterine horn) is less conspicuous but equally likely to result in complications. For this reason, the chorionic surface of the expelled membranes should be examined carefully to ensure they have been completely expelled.
Retention of fetal membranes may mediate development of metritis or even peritonitis. Laminitis is a potential sequela. For these reasons, it is common practice to administer oxytocin (5 U, IM, every 2–3 hr) beginning 3–4 hr after parturition if the membranes have not yet been expelled. Calcium supplementation may be beneficial. Manual removal of retained membranes carries the risk of uterine damage or prolapse and is not recommended beyond gentle tugging to displace already loosened membranes. In cases of prolonged retention of fetal membranes, antimicrobials should be administered prophylactically, along with other therapeutic strategies aimed at preventing laminitis (see Laminitis in Horses). Mares that have recovered from retention of fetal membranes do not generally have lower fertility.