Pain in the first stage of labour results from dilatation of the cervix and lower uterine segment and from distension of the body of the uterus by uterine contractions and is transmitted by afferent nerve fibers (accompanied by sympathetic fibers) of the 10th, 11th and 12th thoracic and 1st lumbar nerves. The descending head causes pressure on the lumbosacral plexus responsible for pain felt in the back , thighs and legs