Twin births of horses, where both foals are born alive and survive, are extremely rare. Usually, twin pregnancy leads to the miscarriage of one or both embryos because of insufficient supply of nutrients etc. The placenta of the mare is only designed to provide one embryo per pregnancy. In most cases, one of the embryos is reabsorbed by the body in the first weeks of gestation. If the embryo dies in the later stage, it is either rejected or mummified. Sometimes both embryos are rejected.
However, if both foals are carried to full term, one of the foals is often born dead or very small and weak, so that it often dies after a few hours. Often both do not survive the first day. Furthermore, it can happen that the mother expels one of the foals and does not let it drink. Often, the mare does not have enough milk for two foals.
Both twin foals surviving without any help from humans – especially in the blizzard – and growing up healthy, as in the story of Hreppur and Stjóri, is a special stroke of luck and extremely remarkable.
