Physiology and management of normal labour

Labour is more difficult in humans than in most other mammals. Our ancestors, the Australopithecines, adopted the upright posture about five million years ago. Natural selection produced a smaller pelvis, which more efficiently transmits forces from the hind legs to the spine. About 1.5 million years ago brain size began to increase (probably associated with improved social integration and later with the language instinct), with the result that the head of the human fetus at term now takes up most of the available space in the mother's pelvis.

It was probably only because of the development of rotation of the head during labour some 300 000 years ago that the system of human birth works at all. The fetal head usually engages in the occipito-transverse position and rotates to occipito-anterior as it passes through the pelvis, allowing the shoulders to engage in the pelvic brim in the transverse position. Once the head is born, the shoulders rotate into the anterior-posterior position, which facilitates their delivery.

The normal uterus is spontaneously contractile, and it is largely the progesterone secreted from the placenta that suppresses activity of the uterus during pregnancy, keeping the fetus within the uterus. In addition, the cervix remains firm and non-compliant. At term, changes occur in the cervix that make it softer, and uterine contractions become more frequent and regular. The precise mechanisms of these changes remain obscure. Changes in the ratio of oestrogen to progesterone, fetal steroid secretion, and changes in the tension of …