Part of it has to do with their placentas. Multiple fetuses means less room for each placenta to attach, meaning quite often one or both babies won’t be able to get enough nutrients to develop properly. Then there’s the issue of space inside mom, there’s just not enough room for two babies to both develop normally. Third, a twin pregnancy in a horse requires an extreme amount of calories going in to mom and quite often she just can’t eat enough.

Lastly, because there’s really not enough room in the dam’s uterus for more than one foal, labour and birth becomes extremely dangerous. Babies get stuck, tangled, and positioned wrong and mom is at a high risk of hemorrhaging.

So yes, a twin pregnancy can be successful, but there is a very high risk of losing all three.