In a normal birth the mare will have the foal and then in about one to three hours, sometimes even in less time, she will expel the afterbirth or placenta from the birth canal. The placenta is the sac or membrane that surrounds the fetus while it is in the uterus. There are many myths around the afterbirth and placenta, but there are serious health concerns if some or all of the placenta is not expelled from the reproductive tract.

In most cases where the placenta is retained longer than two to three hours it is important to call a vet immediately. Retained placentas may be related to bacterial infections or other health conditions that can lead to laminitis, metritis and scepticemia that in turn can lead to death of the mare if not treated. In most cases part of the afterbirth will be protruding from the vulva, which will let the owner know that there is a problem. In more dangerous and difficult to diagnose cases the placenta will not be visible and if the foaling was done outside in the pasture the owner may not even realize the afterbirth was not expelled. In cases where the afterbirth is retained a discharge from the vulva will be noticeable within a day or two that will be thick and very foul smelling. The mare will not eat and will be highly despondent and may shift her weight if laminitis has developed. Checking to ensure that all the afterbirth is expelled is important, especially if the mare has a past history or retaining part or all of the placenta.

The exact reasons why some mares have problems with retained placentas are not known. It may be due to an infection such as endometriosis or equine herpes virus or even due to injury while carrying the foal. Some researchers believe that hormonal imbalances during pregnancy play a major role in retention of the afterbirth. Oxytocin, the hormone responsible for producing the uterine contractions that birth the foal and push out the afterbirth are typically very low in mares that have problems with retained placenta. This low level of hormone may weaken or prevent the uterus from contracting enough to completely expel the placenta.

Treatment of retained placenta usually involves an injection of oxytocin to try to stimulate the uterine contractions so the mare can expel the placenta naturally. If the horse does not pass the placenta within six hours of treatment with oxytocin, either by intramuscular shot or IV, then manual removal may be required. This should be done very carefully to prevent possibly causing a prolapse if the placenta is pulled too hard and still attached to the uterus. Careful monitoring of the mare will be required with manual removal or incomplete natural expulsion as infections and serious laminitis is likely to develop as the retained parts of the placenta cause further complications.

Mares that have had problems with retained placenta should not be bred in the foal heat or first heat after the birth of the foal as they are likely to have greater chances of complications with this pregnancy.