Multiple Conceptuses (Twins)

Multiple Conceptuses (Twins)
Multiple ovulations tend to be more common in barren and maiden mares than in lactating mares. Twins in the mare are almost always associated with double ovulations.. The frequency of double ovulations depends on the breed and type of mare. Unfortunately, double ovulations are particularly common in Thoroughbred mares and are far less common in pony mares. Identical twins are very rare in the horse. Double ovulations occur in 8-25% of oestrous cycles Twins are highly undesirable in the mare. Let’s examine why this is so: Most twin pregnancies will terminate: either by early foetal resorption, late term abortions or the birth of small growth retarded foals.


. Placental competition leads to insufficiency and usually results in abortion. In addition, mare fertility is usually reduced in the following breeding season. In the past, twins accounted for 10-30% of abortions but this figure is thought to have decreased very significantly with better veterinary management. The mare's placenta is structurally simple (diffuse and epitheliochorial), and requires occupation of most of the endometrial surface to provide adequate nourishment to the foal

Twin pregnancies pose a problem, because two foetuses are trying to develop with a placental attachment area designed for one (where the membranes of the two pregnancies meet, there is no placenta). In early pregnancy there appears to be a mechanism for causing death of the smaller of twins in some cases; this reduces the scale of later problems. If twins persist as pregnancy advances, the nutritional requirement of the foetuses increases, foetal growth is limited by placental attachment area and there are

three common outcomes are One foetus becomes larger than the other. The smaller, emaciated foetus dies and usually both are aborted at 8-9 months of gestation. This is the most common outcome (80% of cases). The second is the foetuses are similar in size, and the placentas each occupy half of the uterus. The foetuses go to term and two small weak sickly foals are delivered. These may die or have to be destroyed.

In third, the size difference between the foetuses is large and the smaller foetus dies early in the pregnancy and is mummified. The larger twin is normally born alive and is able to survive. This happens because the placenta of the living twin expands its territory and encloses the mummified embryo. The evidence of twinning may never be suspected. The incidence of multiple ovulations is approximately 20-25% in the Thoroughbred mare.