Labour and Analgesia

Introduction

Labour is a challenging time for mothers and their babies. The success of labour and the ability of the fetus to negotiate a journey through the maternal pelvis depends upon the successful interaction of three variables – the ‘power’ (contractions), ‘passen­ger’ (fetus) and ‘pelvis’ (bony pelvis and pelvic soft tissues). For women, this is often a life-changing event, with the experience that a woman has at this time potentially having both short- and long-term physical and emotional effects. For most women, this is a positive, joyous life event, but the challenges faced during labour can potentially result in adverse outcomes for mother and baby. Midwives and obstetricians are intimately involved in the care of women in normal labour, in trying to ensure normality, in recognising at an early stage when obstetric intervention may be required in the maternal or fetal interest, and in ensuring that the woman’s choices and opinions are respected. The majority of women in the United Kingdom will have a normal delivery in pregnancy after 37 weeks’ gestation after a healthy uncomplicated pregnancy, with almost two-thirds of women going into labour spontaneously. Of these women, 40% will be in their first pregnancy. Good communication with women is essential at this time to help them feel supported and in control, with the aim of making birth a positive experience for all concerned.

Evolution and Human Labour

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 to the survival of any species and, in most, the interval between conception and parturition varies little. This is not the case in human pregnancy, in which delivery can occur many weeks before or after the expected due date.

Labour in humans is surprisingly hazardous. Evolution ought to have favoured those mothers who deliver without problems. Yet, for those without access to good medical care, the lifetime risk of dying from labour and postnatal complications may be as high as 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 this time the head was 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 to 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 birth. 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.

This process requires efficient uterine activity and is aided by ‘moulding’ of the fetal head. Moulding is possible because the individual skull bones are unfused – therefore, they can move or even override each other to form the most efficient shape for delivery. The pelvic ligaments, particularly the cartilaginous joint of the symphysis pubis, relax antenatally under the influence of relaxin to maximise the pelvic diameters. Successful delivery also requires the fetus to enter the pelvis in the appropriate position. When these criteria are not met, problems may occur, which are discussed further in Chapter 34. The difficulty with human delivery is related to the balance between our need to run (and, therefore, have a narrow pelvis) and our need to think (and, therefore, have a big head).

Primigravid Compared With Multigravid Labour

There is a considerable difference between the labour of a primigravida (a woman having her first labour) and that of a parous woman who has had a previous vaginal birth ( Table 31.1 ). A successful vaginal birth first time around usually leads to subsequent births being relatively uneventful. Conversely, a caesarean section or other complications in a first labour can lead to subsequent obstetric problems.

The Uterus During Pregnancy

The uterus is a thick-walled hollow organ, normally located entirely within the lesser pelvis ( Table 31.2 ) in the non-pregnant state. The smooth muscle fibres interdigitate to form a single functional muscle that increases markedly during pregnancy, mainly by hypertrophy (an increase in size of cells) and to a lesser extent by hyperplasia (an increase in the number of smooth muscle cells).

From early pregnancy onwards, the uterus contracts intermittently; 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 are thought to begin at a ‘pacemaker point’ close to the junction of the uterus and the fallopian tube (although this has never been confirmed anatomically) and spread from this point downwards. The intensity of contractions is maximal at the fundus (where the muscle is thickest), intermediate at the mid-zone and least at the lower segment.

The Initiation of Labour

In humans, 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. Some of the information we have about this process is derived from animal studies in which our understanding is more complete. In some mammals, changing levels of estrogen and progesterone regulate the timing of onset of labour. In other animals – for example, sheep – there is some evidence that the fetal adrenal secretion of corticosteroids is the trigger. In humans, however, the process appears to be more complicated, involving the coordinated inhibition and activation of a variety of factors. Our limited understanding of this process means that any proposed mechanism is hypothetical and open to constant change as our knowledge advances.

There is increasing evidence that human parturition involves activation of inflammatory pathways finally leading to cervical ripening, the onset of uterine activity, and membrane rupture. This process appears to be controlled in some way by a complex interaction of various mediators. Therefore, the onset of labour appears to occur when there is the coordinated ‘release’ of the uterus and cervix from the inhibitory effects of various pro-pregnancy factors and the simultaneous activation of various pro-labour factors ( Box 31.1 ). How these factors interact and why labour occurs at a specific time (whether pre-term or at term) is unclear and still the subject of much ongoing research.

Pro-Pregnancy Factors

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. In addition, progesterone seems to have an anti-inflammatory role. It also decreases cytokine production and the influx of immune cells into the myometrium and cervix occurring in normal labour. Its role in maintenance of uterine quiescence during pregnancy is illustrated by the fact that the progesterone antagonist mifepristone increases myometrial contractility and has been successfully used to induce labour. Progesterone withdrawal has been shown in animal studies in sheep and goats to trigger labour. However, human parturition is not associated with a fall in serum progesterone levels, suggesting instead that there may be a ‘functional’ progesterone withdrawal, perhaps caused by changes in the progesterone receptor. This may lead, in turn, to a decrease in the transcription of genes which cause uterine relaxation and an increase in the transcription of genes which increase the sensitivity of the myometrium to uterotonic agents and also cause activation of inflammatory pathways within the cervix and myometrium.

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. Nitric oxide may be involved in the process of cervical ripening, which involves remodelling of the extracellular matrix and collagen. Catecholamines act directly on the myometrial cell membrane to alter contractility and beta-sympathomimetics have been used as tocolytics to suppress pre-term labour. The specific roles of catecholamines in physiological terms and the role of the hormone relaxin are unclear, although they may indirectly cause uterine muscle relaxation by stimulating prostacyclin production.

Pro-Labour Factors

Oxytocin, a nonapeptide from the posterior pituitary, is a potent stimulator of uterine contractility. However, circulating levels do not change as term approaches. The rise in oxytocin receptor levels explains the increase in sensitivity of the uterus to circulating levels of oxytocin as pregnancy advances. It is unlikely that the onset of labour in humans is triggered by oxytocin release. However, it is clear that oxytocin release during labour increases the frequency and force of uterine contractions.

Changes in oxytocin receptor concentration are mediated in part by activation of the fetal hypothalamic–pituitary axis with release of fetal adrenocorticotrophic hormone. Prostaglandin levels also increase prior to the onset of labour. These are synthesised from arachidonic acid by cyclo-oxygenase (COX); 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. There is some evidence that the increased levels may be mediated by maternal corticotrophin-releasing hormone secretion. Inflammatory cells are recruited into the fetal membranes, uterus and cervix at the onset of labour, perhaps as a result of distension of the uterus and/or endocrine hormone signaling from the fetus itself. A number of cytokines are produced – such as interleukin (IL)-8, tumour necrosis factor (TNF)-alpha, IL-6, and IL-1β) – which, in turn, set up activation of pro-inflammatory transcription factors. This inflammatory response is thought to contribute to cervical ripening and membrane rupture via an increase in collagenase activity. It may also contribute to an increase in uterine activity by inhibition of progesterone and activation of contractile genes (COX-2, oxytocin receptor).

The Mechanism of Normal Labour and Birth (See Also Chapter 1)

The mechanism of labour involves effacement and then dilatation of the cervix, followed by expulsion of the fetus by uterine contractions. The lower part of the uterus is anchored to the pelvis by the transverse cervical (or cardinal) ligaments and uterosacral ligaments, allowing the shortening uterine muscle to drive the fetus downwards ( Box 31.2 ).

Descent and birth of the baby

As the cervix dilates and the uterus contracts, the fetus needs to descend into the pelvis. The widest two points of the fetus are the head in the anteroposterior plane ( Fig. 31.1 ) and the shoulders laterally, from one shoulder tip to the other (bisacromial diameter). The head rotates from a lateral position at the pelvic brim to the anteroposterior position at the outlet ( Figs 31.2 – 31.4 ). This rotation has the advantage that by the time the head is delivering through the outlet, the shoulders will be entering the inlet in the transverse position, maximising the chance of successful birth.

The position of the head as it traverses the canal is described according to the position of the occiput in relation to the mother’s pelvis. The head usually enters the pelvic brim in either the right or left occipitotransverse position ( Fig. 31.5A ). The contracting uterus above causes the head to flex so that the smallest diameter is presented for birth.

As the head descends it reaches the V-shaped pelvic floor at the level of the ischial spines (see Fig. 31.5B – D ). The shape of the pelvic floor encourages the fundamentally important head rotation. Fig. 31.6 illustrates the tendency for the longest part of the head to fit into the lowest part of the V-shaped gutter, achievable only by a 90-degree rotation to either the occipitoanterior (OA) or occipitoposterior position. In most cases, the head rotates anteriorly. The consequences of posterior rotation are discussed on page 415. The head, now OA, descends beyond the ischial spines and extends, distending the vulva until it is eventually delivered (see Fig. 31.5E , F ).

Meanwhile, at the pelvic inlet, the shoulders are now presenting in the transverse position. They, too, descend to the pelvic floor and rotate to the anteroposterior position in the V of the pelvic floor (see Fig. 31.5G ). By this time, the head has been completely delivered and it is free to rotate back to the transverse position along with the shoulders. The anterior shoulder can then be delivered by downward traction of the head so that the lateral traction on the fetal trunk allows the shoulder to be freed from under the pubic arch (see Fig. 31.5G ). The posterior shoulder is delivered with upward lateral traction and the rest of the baby usually follows without difficulty (see Fig. 31.5H ).

The third stage of labour is from birth of the baby until delivery of the placenta. The uterus contracts, shearing the placenta from the uterine wall. This separation is often indicated by a small rush of dark blood and a ‘lengthening’ of the cord. The placenta can then be delivered by gentle cord traction (see Fig. 31.5I ). However, caution is required to avoid uterine inversion.