‘When the child is grown big and the mother cannot continue to provide him with enough nourishment, he becomes agitated, breaks through the membranes, and incontinently passes out into the external world free from any bonds’
Hippocrates, On Generation, 4th century BC
‘The stimulus for labour may originate in certain states of vital development or physical expansion of the fundus, corpus or cervix uteri and in altered conditions of the fetus, liquor amnii or placenta and the loosening or decadence of the membranes….’
James Young Simpson
Lectures on Midwifery, 1860
The 20th century, across most of which ‘Munro Kerr’ has stretched, witnessed the most spectacular growth and advance of medical science and with it a steady improvement in our understanding of the birth process. A hundred years ago the obstetrician’s art depended mainly on the insights brought by the giants of 18th century obstetrics, notably William Smellie (1697–1763) and William Hunter (1718–1783), both incidentally born within 20 miles of Munro Kerr’s birthplace. Smellie, who became acknowledged as ‘The Master of British Midwifery’, was the consummate man-midwife and teacher. His monumental Treatise on the Theory and Practice of Midwifery (1752), based on his extensive clinical experience, described and defined the birth process as never before and formed the basis for the clinical conduct of labour. His definition of the mechanisms of labour shed light on the convoluted journey through the birth canal which the fetus is required to follow. His Sett of Anatomical Tables with Explanations and an Abridgement of the Practice of Midwifery (1754) amplified these fundamental principles. This atlas, for which Smellie employed the Dutch artist Jan van Rymsdyk, was only surpassed 20 years later when Hunter, employing the same artist, published his spectacular Anatomy of the Human Gravid Uterus (1774).
When Munro Kerr was preparing the original Operative Midwifery in 1908 there had been little further progress. The relevant anatomy was fairly well understood but the physiology of the myometrium and cervix, and the biochemistry, endocrinology and pharmacology of human labour were almost entirely unknown. At the start of the new millennium the young obstetrician may consider that those mysteries have almost all been solved following a century of discoveries which saw the emergence of oxytocin, oestrogen, progesterone, prostaglandins and many other hitherto unknown substances. But it would be surprising indeed if the close of the 21st century does not reveal an even more complex picture.
Current Understanding
As a starting point for the wide range of clinical issues addressed within this textbook, a brief review follows of some of the key elements of basic medical science pertaining to human labour and delivery as currently understood. This, by necessity, will be superficial and selective. For more detailed and comprehensive accounts the reader is referred to current textbooks of reproductive physiology, anatomy, biochemistry and endocrinology.
Myometrial Function
The myometrium is the engine which drives human labour, during which it displays a highly sophisticated and co-ordinated set of forces. The simple objective of these is to efface and dilate the cervix and drive the fetus through the birth canal. In contrast to other smooth muscle systems, the myometrium displays three unique properties which are crucial for its function:
- 1.
It must remain quiescent for the greater part of human pregnancy, suppressing its natural instinct to contract until called upon to do so at the appointed time.
- 2.
During labour it must display a pattern which affords adequate periods of relaxation between contractions without which placental blood flow and fetal oxygenation would be compromised.
- 3.
It possesses the capacity for retraction , vital to prevent exsanguination after delivery but also essential during labour. Retraction is a unique property of uterine muscle whereby a shorter length of the muscle fibre is maintained, without the consumption of energy, even after the contraction that produced the decrease in length has passed. As the cervix is effaced and pulled around the fetal presenting part, an inability of the myometrial fibres in the uterine corpus to retract, in essence to steadily reduce their relaxed lengths, would mean that the tension on the cervix could not be maintained.
At its most basic, human labour may be regarded as an interaction between the corpus and the cervix ( Fig 1-1 ). For the maintenance of pregnancy the corpus must be quiescent and the cervix closed and uneffaced. In labour the corpus contracts and the cervix yields. A useful analogy may be to compare this process to the experience of putting on, for the first time, a roll-neck pullover. Just as with the fetus, the head must be flexed to present its smallest diameters to the cervix, or neck of the pullover, which is effaced round the presenting part and ultimately dilated as a result of traction applied by the arms, which are in this connection analogous to the myometrial fibres.
Although it has been conventional to acknowledge a ‘lower uterine segment’ arising from the uterine isthmus (between the non-pregnant corpus and cervix), in practice it may be more helpful simply to see the boundary between corpus and cervix as the ‘fibromuscular junction’ which marks the change from a mostly muscular corpus to a predominantly fibrous cervix. Obstetric purists may argue that the concept of a ‘lower segment’ is helpful in the definition of placenta praevia and in directing the site of contemporary caesarean sections but, those issues apart, it is of little relevance and it is a difficult concept to define either anatomically or physiologically.
At its simplest, contraction of the myometrial cell requires actin and myosin to combine in the contractile filament actomyosin ( Fig 1-2 ). This reaction is catalyzed by the enzyme myosin light-chain kinase, which is heavily calcium dependent. Calcium in turn relies for its availability on oxytocin and prostaglandin F2α, which transport it into the cell and also free it from intracellular stores. On the other hand this reaction is inhibited by progesterone, cyclic AMP and β-adrenergic agents. A particular insight into how the myometrial effort is co-ordinated into a concerted function came from the recognition of the essential requirement for gap junctions (biochemically characterized as Connexin-43) to be formed between individual myometrial cells, allowing cell-to-cell transmission of electrical impulses and ions. Thus, the corpus can display a wave of contractility propagated across its cell population which becomes a functional syncytium rather than a disorganized rabble of individual muscle fibres.
