Mares have a twin pregnancy rate of between 3 and 30% depending on the breed of the horse.
A commonly accepted rate in Thoroughbred mares in Australia is 10 – 15%. Mares that are allowed to carry twin pregnancies are likely to suffer complications as a result. They frequently abort twins or if they give birth to live twins the mares are more likely to suffer dystocia (foaling difficulties), retained foetal membranes and decreased live foaling rates in the following season. Twin foals suffer a higher rate of stillbirth, and those born alive are usually smaller, weaker, and more susceptible to infection with slower development than singleton foals. Due to these factors, it is commonly accepted practice to identify twins by ultrasonography and manually ablate one by rectal palpation and manipulation during the mobility phase of the pregnancy (up to 16 days). This procedure has a high success rate but occasionally can result in the loss of both embryos.
There is some evidence reported in the veterinary scientific literature to indicate that this manual ablation procedure is associated with a lower live foaling rate when performed on older mares compared to the general population of mares; however it is generally accepted that the benefits of the procedure outweigh the potential disadvantages. SEH veterinarians are aware of the risks of this procedure and act to manage these risks as far as is practically possible.
For a variety of reasons, including asynchronous double ovulations, mobility of the embryonic vesicles, the presence of cysts in the uterus and limitations of the facilities and technology, it is not always possible to detect twins or multiple pregnancies. SEH veterinarians are aware of the possibility of twin conceptions and they take all reasonable steps to fully examine the reproductive tract of the mare for twins at each examination. While the risk of twins or multiple pregnancies not being detected is small and is decreased by repeated reproductive examinations, horses have the capacity for variation in their anatomy and physiology. As a result, it is not possible to give guarantees that a particular mare is not carrying twins.