The nature and consequences of childbirth pain.

For most women, childbirth is associated with very severe pain often exceeding all expectations. Some childbirth education groups and popular texts on the subject, however, seem disposed to encourage unrealistic expectations: claiming that labour is other than painful and that pharmacological analgesia is both unnecessary and harmful. All too often, those who promote such views witness women in labour only occasionally and are rarely responsible for patient care. Pain associated with uterine contractions should be distinguished from that associated with delivery: for there are important differences in the clinical characteristics, neural pathways and physiological responses. In the first stage of labour pain is largely visceral in origin, whereas during the transitional and second stages somatic pain becomes more pronounced. As described in this review, it is now well established that uterine contraction pain evokes a generalised neuroendocrinal stress response producing widespread physiological effects during the first stage of labour. They include increased oxygen consumption, hyperventilation and respiratory alkalosis; increased cardiac output, systemic peripheral resistance and blood pressure; delayed gastric emptying; impaired uterine contractility and diminished uterine perfusion; and metabolic acidaemia. While other factors (such as anxiety, starvation and physical exertion) are also partly responsible for inducing some of these effects, pain appears to be the most potent source because they are all obtunded by effective epidural analgesia.