Pain in labor, like the other kind of pain, start at transduction, transmission and modulation. The original gate-control theory, the gate-control theory Mark II and the neuromatrix theory of pain are proposed to explain the complexity of pain perception and pain relief methods such as internal and external factors. Labor pain is originated from the uterine contraction and cervical dilatation and stretching of the neighbor-pelvic organs during the mechanism of labor. The pain is referred to the dermatomes supplied by the same spinal cord segments which receive input from the uterus, cervix and the neighbor-pelvic organs.