How are teacup dogs bred?

Teacup dogs originated in the US, first bred from the two smallest puppies in a litter, producing smaller dogs. The gene is now stable and numbers greater, so close inbreeding is no longer needed. Through repeated refinement, 5-7 breeds exist, common ones being Teacup Teddy, Teacup Pomeranian and Teacup Chihuahua. Because of their tiny size, breeding is difficult with few pups per litter and delivery needs assistance, so teacup dogs cost far more than most breeds.