Approx. 9% of all calvings result in multiple births.

The number of fraternal (dizygotic) twins in cattle is higher than the number of identical (monozygotic) twins. Fraternal twins occur when two oocytes are fertilized, whereas identical twins emerge when a zygote (a fertilized oocyte) splits into two.

In cattle the phenomenon of freemartins may occur in fraternal twins. The female calf of a pair of fraternal twins will often be sterile.

At the embryonic stage the blood vessels may be connected between the two embryos, and the blood of the two calves is mixed. To which extent the blood is mixed and which one of the embryo’s blood will be dominant, is not known. As a consequence, you cannot perform a DNA analysis on blood from twins because you cannot be sure from which animal the blood originates. The genetic analyses, however, can be done on tissue, hair, or semen. So, it is imperative that twins are registered as such.

Sometimes an animal not born a twin may look like a twin when the analysis is made. The reason might be that one of two foetuses was lost during pregnancy. Therefore, the laboratory may wish to have a new sample of either hair, tissue or semen.

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