Why some primates have even tougher births than humans

Scientists have long thought humans have a uniquely difficult birth compared with other primates. And it's true that, from an evolutionary perspective, we face an obstetrical dilemma. As we evolved to walk on two feet, our pelvis changed shape and size, and our birth canal became smaller and oddly twisted. Meanwhile, the evolution of our enormous brain meant that a large-headed baby needed to fit through it.