Clinical review

Physiology and management of normal labour

Philip Steer,  Caroline Flint. 

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.

View larger version (30K): [in this window] [in a new window]   Longitudinal view of fetal head in the pelvis showing how little room there is

View larger version (39K): [in this window] [in a new window]   Horizontal view of engagement in left occipito-anterior position

View larger version (39K): [in this window] [in a new window]   Rotation of fetal head as it descends through the pelvis. The maximum diameter of the head matches that of the pelvis at each level (maximum diameters are indicated by an arrow)

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 the uterine wall as the fetus grows probably all play a part. Evidence is increasing that the long term interests of the fetus are best served by it being large at birth. However, this represents a problem for the mother, as some women experience long term pelvic damage from delivering large babies. This conflict between the interests of the baby and the mother is probably the reason that the duration of pregnancy is so variable. The mother gives birth more easily if the baby is premature, but the baby survives best if born at term and larger.

View larger version (11K): [in this window] [in a new window]   Factors implicated in the onset of labour

	Stages of labour

Top Stages of labour Use of drugs Support for mother

Labour can be divided into three stages, which are unequal in length. The fundamental change underlying the process of the first stage is progressive dilatation of the cervix. This gives rise to the familiar symptoms and signs of labour. The cervix is richly supplied with nerve endings, and as it starts to dilate, this gives rise to the characteristic pain of labour. In addition, the plug of viscous mucus that has protected against the ingress of bacteria during pregnancy often emerges as a show. The dilatation of the cervix reduces the support for the fetal amniotic membranes, which bulge through the cervix, and often the rupture of these membranes can be the initiating phenomenon of active labour.

Under optimal circumstances regular uterine contractions are prompted by the development of contacts between cells considered to be sites of low resistance. These gap junctions are sites of low electrical resistance which allow the passage of depolarisation waves from one muscle cell to another across the uterus. Ideally the process coincides with the ripening of the cervix. If the contractions start or the membranes rupture before the cervix is properly ripe, the process is stimulated by the release of prostaglandins from the membranes and the uterine decidua. Then labour has to pass through a latent phase during which the cervix dilates only very slowly. This can be very demoralising for the mother and increases the risk of infection during labour.

View larger version (18K): [in this window] [in a new window]   Dilatation of the cervix: (a) cervix not taken up or dilated in late pregnancy; (b) cervix 1 cm dilated; (c) cervix 2-3 cm dilated with a bag of membranes bulging; (d) cervix 5 cm dilated with the membranes ruptured and amniotic fluid escaping

Monitoring labour
Progress in the latent phase of labour is assessed with the Bishop score. The cervix should change at a minimum of one Bishop score point an hour if labour is to end within a reasonable time (only 20% of women move more slowly than this). A score of 11 indicates the onset of the active phase of labour, during which the average rate of cervical dilatation in women in their first labour is 1 cm/h. In parous women the cervix dilates fasteron average 1.6 cm/h.

View larger version (13K): [in this window] [in a new window]   Latent and active phases of labour in a primiparous woman

View larger version (68K): [in this window] [in a new window]   Partogram: the broken lines show expected progress of cervical dilatation in multiparous (left) and primiparous (right) women

View larger version (39K): [in this window] [in a new window]   Essential changes between intrauterine circulation and extrauterine circulation (the bypasses that close at or soon after birth are marked with parallel lines)

Oxygen supply
The oxygen supply is reduced during labour because contractions interfere with the flow of oxygenated maternal blood to the placenta. However, the fetus normally adapts well to this. The fetal circulation is unaffected by contractions (as the fetus is enclosed within the uterus), unless there is cord entanglement with compression. The normal oxygen tension in the fetal blood before labour is about 4 kPa. During labour it falls to about 3 kPa. However, redistribution of the flow within the fetus to protect the vital organssuch as the heart and brainmeans that a healthy fetus copes well with this stress.

Delivery of placenta
Once the baby is born, the uterus continues to contract strongly and can now retract, decreasing markedly in size. This shears off the placenta from the uterine wall. If the placenta is allowed to be delivered with normal contractions (sometimes called "physiological management"), this can take up to an hour. Use of an oxytocic drug speeds this process fourfold and reduces average blood loss by about 50%. A recent study published in the Lancet showed that in a large randomised controlled trial only 6.8% of women receiving active (drug) management had significant bleeding, compared with 16.5% receiving physiological management.

View larger version (15K): [in this window] [in a new window]   Meta-analysis of effects of prophylactic oxytocic drugs in blood loss during third stage of labour

	Use of drugs

Top Stages of labour Use of drugs Support for mother

Oxytocic drugs should be given with the birth of the anterior shoulder. The use of antenatal ultrasound screening has virtually eliminated the possibility of giving the oxytocic before the birth of an undiagnosed second twin. Syntocinon is the most used oxytocic known to be effective; the addition of ergometrine may reduce blood loss even further but can cause serious hypertension in susceptible womenfor example, those with pre-eclampsia. Because of the speed with which the uterus retracts after stimulation with an oxytocic, the placenta should be removed by controlled cord traction as soon as it is perceived to have separated from the uterine wall.

View larger version (22K): [in this window] [in a new window]   Pressure recording of a uterine contraction in the later first stage of labour (note differentials of pressure rise with objective palpation of contractions and pain felt by the mother)

	Support for mother

Top Stages of labour Use of drugs Support for mother

As most labours are spontaneous and end with a normal delivery, the main purpose of the birth attendant (usually a midwife) is to provide support for the mother and her partner in labour and to monitor the process for abnormality. The birth attendant therefore needs to understand both the physical processes and the emotional needs of the mother.

It is difficult for a nulliparous woman to understand the sensations (including sometimes severe pain) that she will experience during childbirth until they actually occur. The birth attendant therefore needs to interpret the woman's sensations for herfor example, explaining that fears that "the baby is stuck" and "my perineum is going to split apart" are normal and do not necessarily indicate an abnormality.

Some women need a quietly supportive approach; others need boisterous encouragement (especially while pushing in the second stage). Sensitivity and experience are needed to match the type of support with the needs of the mother.

View larger version (83K): [in this window] [in a new window]   Delivery in a standing position

Delivery positions
Because women in labour are in pain they often feel the need to move around a great deal. It is therefore helpful to provide an environment in which the woman can vary her position at willfor example, soft mats and bean bags on the floor, cushions, or birth pools. The woman should be encouraged to deliver in whichever position she feels comfortable, providing that this does not affect the fetus significantly. Some women like standing or squatting, but the most commonly used position is lying propped up on a bed; the only position that should routinely be proscribed is the supine position (that is, with the woman flat on her back). This often causes caval compression, restricting venous return from the legs, and can result in the supine hypotension syndrome, leading to fetal and maternal hypoxia. If the supine position has to be usedfor example, for vacuum or forceps deliverya wedge should be placed under the mother's buttocks and lower back to tilt the uterus away from the inferior vena cava.

View larger version (91K): [in this window] [in a new window]   Delivery in hands and knees position

Other roles of birth attendant
The value of physical guidance of the fetus by the attendant at a normal birth is controversial. The HOOP ("hands on or poised") study, in which control of the emergence of the head and shoulders is being compared with entirely spontaneous birth is still under way.

View larger version (28K): [in this window] [in a new window]   Positions in labour used by women delivering in hospital or at home

	Acknowledgments

The diagram showing essential changes between intrauterine and extrauterine circulation is adapted from Chamberlain (Lecture notesobstetrics. 7th ed. Oxford: Blackwell Science, 1997). The line drawing showing the results of meta-analysis of effects of prophylactic oxytocic drugs is adapted from Prendeville W, Elbourne D (Cochrane Pregnancy and Childbirth Database (1995, issue 2)). The graph showing positions in labour used by women is adapted from Home Births (National Birthday Trust survey, 1994).

Philip Steer is professor of obstetrics and consultant obstetrician at the Imperial College School of Medicine, Chelsea and Westminster Hospital, London; Caroline Flint is honorary professor at Thames Valley University and an independent midwife in London.

The ABC of Labour Care is edited by Geoffrey Chamberlain, emeritus professor of obstetrics and gynaecology at the Singleton Hospital, Swansea. It will be published as a book in the summer.