Multiple pregnancies are an increasing problem in breeding management. They are usually multizygotic (originating from several ova, i.e., the result of multiple ovulations, thus giving nonidentical offspring). Monozygotic twins (originating from a single fertilized ovum that splits to produce identical foals) are very rare, with the first case in England being confirmed as that of two foals born in 2002 to a mare owned by Her Majesty the Queen.
Studies suggest that the vast majority of multiple equine pregnancies are multizygotic twins (94.8%), only 5.2% of multiple pregnancies are triplets, and the occurence of quadruplets is extremely rare.
Our university records indicate that up to 30% of estrous cycles in Thoroughbred mares result in multiple ovulations. Arabian and Warmblood mares also have high incidences of multiple ovulations. However, in a population of native ponies in the United Kingdom, the incidence is less than 1%.
Apart from breed there are other factors–including nutrition, age, and time of year–that seem to affect the incidence of multiple ovulations. Mares that are well-fed and in good body condition have the highest incidence of multiple ovulations, especially if they are over 15 years of age. Work we have carried out demonstrates a clear increase in multiple ovulation rates as mares get older (15% of 3- to 5-year-old mares have multiple ovulations versus 35.1% of 18- to 22-year-old mares). Mares also have a higher incidence of multiple ovulations during the middle of the breeding season (June/July/August in the Northern Hemisphere or December/January/February in the Southern Hemisphere).
Since multiple pregnancies in mares are almost always multizygotic, multiple ovulation is normally a good indicator that there is a high risk of multiple pregnancies, as in theory there is an equal chance of all ovulations be