Labour is defined as the onset of regular uterine activity associated with effacement and dilatation of the cervix and descent of the presenting part through the cervix. The control and timing of delivery is crucial in the survival of any species and, in most, the interval between conception and parturition varies little. This is not the case in human pregnancy, where delivery can occur many weeks before or after the expected date.

Labour in humans is surprisingly hazardous. Evolution ought to have favoured those mothers who deliver without problems, and yet, for those without access to good medical care, the lifetime risk of dying from labour and postnatal complications may be ≥ 10%.

Apes are able to give birth with little problem. Their pelvises are relatively large, the fetal head is relatively small and the fetus is born facing anteriorly. When the Australopithecines adopted an upright posture around 4 million years ago, the pelvic shape became narrower in the anteroposterior plane to allow more efficient weight transfer from the trunk to the femurs. As the fetal head was still relatively small, the Australopithecines were also able to deliver without much problem, although the head was this time in the transverse position.

With further evolution 1.5 million years ago, to Homo erectus and then Homo sapiens, the volume of the brain increased from around 500 mL to 1000–2000 mL. This increased the chance of the head being bigger than the pelvis (cephalopelvic disproportion) and to deliver successfully, it became necessary for the head to rotate during delivery. The head entered the pelvic brim in the transverse position, as the inlet is widest in the transverse plane, but rotated at the pelvic floor to the anteroposterior plane, which is the widest diameter of the pelvic outlet.

From early pregnancy onwards, the uterus contracts intermittently, and the frequency and amplitude of these contractions increase as labour approaches. These ‘Braxton Hicks’ contractions are irregular, low frequency and high amplitude in character and are only occasionally painful. They normally begin at a ‘pacemaker point’ close to the junction of the uterus and the fallopian tube and spread from this point downwards. Intensity is maximal at the fundus (where the muscle is thickest), intermediate at the mid-zone and least at the lower segment.

The mechanisms controlling the onset of labour are different for different mammals. In some mammals, it is the changing levels of oestrogen and progesterone which regulate the timing of onset. In other animals, for example sheep, there is some evidence that the fetal adrenal secretion of corticosteroids is the responsible trigger. In humans, however, despite major advances in molecular biology and the science of reproduction, there is still much that is not known about the physiological processes involved in the initiation of labour. Any proposed mechanism must therefore be considered to be a hypothesis.

Progesterone is derived from the corpus luteum for the first 8 weeks or so of pregnancy and thereafter from the placenta. It has the direct effect of decreasing uterine oxytocin receptor sensitivity and therefore promotes uterine smooth muscle relaxation. That it plays a significant pro-pregnancy role is illustrated by the fact that the progesterone antagonist mifepristone increases myometrial contractility, and has been successfully used to induce labour. Nitric oxide, a highly reactive free radical, is also a pro-pregnancy factor. Some studies have observed a fall in uterine nitric oxide synthetase activity as pregnancy advances, but these findings are not confirmed in other studies. Catecholamines act directly on the myometrial cell membrane to alter contractility and beta-sympathomimetics are used as tocolytics to suppress preterm labour. The specific roles for catecholamines in physiological terms and the role of the hormone relaxin are unclear.

Oxytocin, a nonapeptide from the posterior pituitary, is a potent stimulator of uterine contractility. Circulating levels, however, do not change as term approaches. Oestrogen levels do increase, and oestrogens increase oxytocin receptor expression within the uterus. This gradual rise may be mediated in part by fetal adrenocorticotrophic hormone (ACTH). Prostaglandin levels also increase prior to the onset of labour. These are synthesized from arachidonic acid by cyclo-oxygenase (COX), and COX-2 enzyme expression in the fetal membranes has been observed to double by the time labour begins. Prostaglandins promote cervical ripening and stimulate uterine contractility both directly and by upregulation of oxytocin receptors, and there is some evidence that the increased levels may be mediated by maternal corticotrophin-releasing hormone (CRH) secretion.