Birth is a remarkable and explosive event that occurs usually between 35 and 39 weeks after conception (37-41 after the mother's last menstrual period). It is typified by painful contractions of the uterus (womb), progressive dilation of the cervix (the lowermost part of the uterus), and descent of the presenting part of the baby through the mother's birth canal (uterus and vagina). There are three stages of labour. The first lasts until full dilation of the cervix, the second until birth of the baby, and the third ends with delivery of the placenta.
The processes that initiate normal physiological labour in humans are poorly understood. The sheep has provided the most useful experimental animal model for the study of the fetus and has provided many important insights into human pregnancy. In the sheep, labour is initiated by the fetus, which starts the process by producing a surge of corticosteroid hormones from its adrenal glands. This results in a change in the balance of steroid sex hormones (oestrogen and progesterone) in the ewe which, in turn, encourages the production of prostaglandins in the membranes attached to the placenta. Prostaglandins (so called because they were first discovered in prostate glands in men) stimulate contractions of the uterus, thus initiating labour. It is an attractive concept that the baby should orchestrate the timing of labour and birth when it has reached sufficient maturity to be ready for life outside the uterus. The process is not, however, so clear-cut in humans, although prostaglandins do seem to be important. In many women, changes take place over days or weeks before labour proper starts, including some opening of the cervix and increasing Braxton Hicks contractions (non-painful, non-regular ‘hardening’ of the uterus), which are named after the nineteenth-century London obstetrician who first described them.
The first sign of labour is usually that uterine contractions become increasingly painful or frequent, although labour may sometimes start with rupture of the placental membranes and release of the amniotic fluid that surrounds the baby throughout pregnancy. In later labour, the contractions occur every two or three minutes. With each contraction, the blood supply to the placenta is cut off, stopping the supply of oxygen to the baby. Well-grown babies have more than sufficient reserves to deal with this potential stress, but poorly-grown babies with low metabolic reserves may become rapidly and seriously distressed. It is, therefore, important to monitor the heart rate of the baby during labour to check for distress. This may be done by listening through the fetal stethoscope (named after the French obstetrician, Adolphe Pinard), or by electronic methods — although the simplest and oldest method was to place the ear directly on the abdomen.
As the contractions pull against the cervix, already softened by the effect of prostaglandins, it dilates. Once it is completely open, the presenting part of the baby is propelled downwards by the contractions, deeper into the mother's pelvis. In 96% of babies at term (after 37 weeks), the presenting part is the head; in 3% it is the rump (breech). As the head reaches the funnel-shaped muscles of her pelvic floor, the mother feels an urge to push, and her efforts add to the impact of the contractions. The head rotates and emerges through the lower end of the vagina, to be followed by the rest of the baby.
Sometimes it proves necessary to deliver the baby by Caesarean section during labour, notably because of signs of distress in the baby or a failure to make progress — either because the baby's head is too large for the mother's pelvis, or because the uterine contractions prove inadequate. When such problems occur during the second stage of labour, birth can be hastened by application of obstetric forceps or the vacuum extractor. Forceps were first constructed and used by the Chamberlens, a Huguenot family who fled to England during the sixteenth century. By discreet use of the instruments under sheets they kept their secret hidden for decades. The first practical vacuum extractor was made by the nineteenth-century Scottish obstetrician, James Young Simpson; further refinements occurred during the twentieth century.
Simpson's other contribution to the care of women in labour was of still greater significance. At a famous dinner party in Edinburgh in 1847 he proved to himself (and to his accommodating guests) the pain-relieving power of chloroform. His advocacy of chloroform as an analgesic during labour met vigorous opposition, backed by citation of the Bible, for God had reportedly said to Eve, after she ate the forbidden fruit:
I will greatly multiply your pain in childbirth. In pain you shall bring forth children (Genesis 2: 16).Public opinion in Britain was changed by Queen Victoria's use of chloroform during one of her many labours; the acceptance of pain-relieving measures as legitimate was thereby ensured. Labour is a painful experience for most women and options that are available nowadays include opiate drugs, inhalation of nitrous oxide gas, transcutaneous nerve stimulation (TENS), and epidural anaesthesia. A hot bath helps with many types of pain and has become popular during labour; indeed some women now like to give birth in water. However, access to any such facilities is denied to very many women world-wide, many of whom do not have the availability of even very basic clinical care. The consequences are dramatically and tragically illustrated in the developing world where, in rural areas far from clinical facilities, women may labour for days. In these communities, adolescent girls are especially at risk of obstructed labour because the pelvis is both incompletely grown and also often stunted because of chronic undernutrition. Prolonged obstructed labour typically results in death of the baby, and often in death of the mother from infection or, if she survives, in vesico-vaginal fistula — a channel between bladder and vagina that causes chronic incontinence of urine, social ostracism, and great misery. The ability to transform the lives of these young women by successful surgery has been demonstrated by the famous fistula hospital in Addis Ababa, Ethiopia. However, prevention is, as always, better than cure, and the ‘partogram’, a graphical representation of the progress of labour developed by Professor Hugh Philpott in Zimbabwe, gives early warning of obstructed labour and is now widely used throughout the world. Women who have had babies previously suffer a different consequence of neglected obstructed labour: rupture of the uterus, with severe internal haemorrhage and high risk of death.
The challenge for obstetricians and midwives providing care for women in labour in more affluent settings is to ensure safety for mother and baby whilst avoiding meddlesome and unnecessary clinical interventions and allowing the women free choice of possible options which would include, for example, her favoured position at birth, or delivery at home. High rates of Caesarean section, induction of labour, and continuous electronic fetal heart monitoring suggest, to some, that the balance is not yet right.
— Jim Neilson
See also birth; development and growth: infancy; pregnancy.
