CHAPTER 4 NORMAL LABOR AND DELIVERY
1. The Passage and the Passenger
Labor is the process of giving birth. It involves physiological changes in the genital organs resulting in the expulsion of the fetus through the vagina. The uterine contractions and cervical changes permit the passage of the fetus and other products of conception from the uterus through the birth canal. The process of labor involves four components commonly called the“Four Ps.” These four Ps are:
（1）Powers—the involuntary uterine contractions to push the foetus out.
（2）Passenger—must be of a size and shape to negotiate through the varying dimensions of the birth canal.
（3）Passage—must be of an adequate size and proper configuration to allow descent, rotation and expulsion of the foetus.
（4）Maternal Psychological response—sometimes affects the progress of labour.
All these components must be well coordinated for labour to progress normally thereby resulting in a successful outcome.
2. True Pelvis
This part of the pelvis forms a bony canal through which the foetus must pass during labour, hence it is of great significance. The true pelvis is shallow anteriorly, formed by the symphysis pubis （4-5 cm）and deep posteriorly, formed by the sacrum and coccyx （10 cm）. It is divided into three parts - inlet, cavity and outlet.
2.1 Inlet
The inlet is narrowest antero-posteriorly and widest from side to side, i.e. in the transverse diameter. Therefore, the foetal head enters the pelvic inlet with its longest diameter （antero-posterior）in the widest part of the pelvis （transverse diameter）.
From front to back, there are three diameters worth mentioning:
（1）Antero-posterior diameter （also known as the true conjugate, anatomical conjugate or conjugate vera）（11 cm）. It is the distance from the sacral promontory to the upper border of the pubic symphisis.
（2）Obstetrical conjugate: （10 cm）It is the distance between the mid point of the sacral promontory to the most prominent part on the posterior surface of the pubic bone. In a normal pelvis, it is not easy to feel the sacral promontory or at best it can be felt with difficulty.
（3）Diagonal conjugate: It is the distance between the lower borders of the symphysis pubis to the midpoint on the sacral promontory. It measures 12 cm.
Transverse Diameter
It is the distance between the two farthest points on the pelvic brim over the iliopectineal lines. It measures 13 cm. The diameter usually lies slightly closer to the sacral promontory and divides the brim into an anterior and a posterior segment.
MID Pelvis
It is a space bounded above by the plane of the greatest pelvic dimensions and below by a plane called the mid pelvic plane. It roughly corresponds to the lower half space of the cavity of pelvis.
Antero-posterior diameter （11.5 cm）. It is measured from the lower border of the symphysis pubis to the junction of S4 and S5 or the tip of S5 （11 cm）, whichever is applicable.
Bi-spinous or Transverse Diameter （10.5 cm）
This is the distance between two ischial spines.
2.2 Outlet
From an obstetric point of view, it is better to regard the pelvic outlet as a constricted lower portion and not merely its lower bony limits. Therefore, the pelvic outlet is described as an obstetrical outlet and an anatomical outlet.
Axis
It is represented by a line-joining the centre of the plane of least pelvic dimensions with the sacral promontory. Its direction is almost vertical.
Diameters
（1）Transverse or bispinous diameter （10.5 cm）: It is measured between the two ischial spines.
（2）Antero-posterior diameter （11 cm）: It extends from the lower border of the symphysis pubis to the tip of the sacrum.
Pelvic Axis
Anatomical If the axes of inlet, cavity and outlet are joined together, they form a uniform curve, which traverses the centre of the canal of the bony pelvis with its convexity snugly fitting the concavity of sacrum. This is also called “the curve of Carus” and is directed at first downwards and backwards （axis of inlet）, then gradually more and more forwards until it reaches the axis of the outlet.
2.3 The Passenger
Maternal pelvis must be of an adequate size and a proper configuration to allow the passage of the foetus. However, even in such a pelvis, the birth may be difficult if the foetus is too large or is in an abnormal position. For a successful out-come, the foetal skull, shoulders, trunk and buttocks have to pass through the maternal pelvis. From an obstetric viewpoint, the foetal head is the most important part of the foetus. If it can pass through the pelvis safely, there is usually no difficulty in delivering the rest of the body, although occasionally the shoulders may cause some trouble.
Foetal Head
It is comprised of the vault, the face and the base. Four bones- the sphenoid, the ethmoid and the two temporal bones form the base of the skull. These bones are closely knit; hence the base is rigid and incompressible. On the other hand, the vault is made up of thin, pliable, flat bones, which are compressible to some extent. These bones are the two frontal bones in front, the occipital bone at the back and the two parietal bones in between. These bones at birth are not closely knit but separated by membranous interspaces called sutures and the intersection of these sutures are known as fontanelles.
Fontanelles
A wide gap at the intersection of the sutures is called a fontanelle. Six fontanelles exist in the skull at term but only two of these are of obstetrical significance. These are:
Anterior fontanelle or Bregma: Felt as a “soft spot” just above the forehead of a newborn; is large and diamond shaped. It is formed by the frontal suture anteriorly, the sagittal suture posteriorly and the two coronal sutures on either side. Its average measurement is around 3 cm antero-posteriorly and 2 cm transversely. It ossifies by 18 months and failure to do so by 24 months is pathological.
Posterior fontanella or lambda: This triangular fontanella is formed by the junction of three suture lines—sagittal suture anteriorly and lambdoid sutures on either side. It measures around 1.2×1.2 cm. It ossifies by term but may be membranous in preterm babies. This fontanelle also helps to denote the position of the head in relation to the maternal pelvis.
Diameters/Circumferences of Skull
The shape and diameter of the circumference of the foetal skull varies with the degree of flexion and hence the presentation. A normal true pelvis permits the foetal skull, in various vertex and face presentations, to pass through but not a foetal head in brow presentation （diameter 13 cm）.
As the vault is compressible, these diameters can be re duced in length to some extent if the need arises, to allow the passage of the foetal head through the maternal pelvis.
Presenting Parts of Foetal Skull
The skull is arbitrarily divided into several regions of obstetric importance. These are:
·Vertex: This is a quadrangular area bounded anteriorly by bregma and coronal sutures, posteriorly by lambda and lambdoid sutures and laterally by arbitrary lines passing through the parietal eminences. Foetal head lies in flexion during this presentation.
·Face: This is an area bounded by the root of the nose along with supraorbital ridges and the junction of the chin or floor of mouth with the neck. The foetal head is fully extended during this presentation.
·Brow: This is an area of forehead from the root of nose and supraorbital ridges to the bregma and coronal sutures. The foetal head lies midway between full flexion and a full extension in this presentation.
Other areas on the foetal skull generally described are:
·Sinciput: Area in front of anterior fontanella corresponding to forehead
·Occiput: area limited to occipital bone
·Mentum: refers to the chin of the foetus
·Parietal eminences: as the name indicates, are eminences of parietal bones on either side
·Sub-occiput: It is the junction of the foetal neck and occiput, also called the nape of the neck
·Sub-mentum: It is the junction between neck and chin.
3. Mechanism of Labour
3.1 Definition of Labour
Labour is the process by which a foetus is delivered by the vaginal route. The series of changes in position and attitude that the presenting part has to make, during its passage through the maternal pelvis and pelvic floor during the course of labour, constitute the mechanism of labour. Before discussing the mechanism of labour, knowledge of certain terms is essential.
The mechanism of labour is dictated by the pelvic dimensions and configuration （both bony and soft parts）, the size of the passenger and strength of the contractions. It is customary to describe these movements as the movements of the head but in reality the head is only the index, while the trunk also participates in and probably also initiates some movements. These movements are:
（1）Engagement
（2）Descent
（3）Flexion
（4）Internal rotation
（5）Restitution
（6）External rotation
There is a mechanism for every presentation and position, while delivering vaginally. The commonest positions are left or right occipito-anterior: thus, these will be described here. The attitude is one of flexion and, therefore, the denominator is the occiput.
Engagement
Engagement is the mechanism by which the greatest transverse diameter of the foetal head—bi-parietal diameter （BPD）（9.4 cm）is at or has passed the pelvic inlet （brim）. This provides a clear indication that the pelvic inlet is large enough to accommodate the widest portion of the foetal head and thus is of adequate size. For an average foetal head, the linear distance between the occiput and the plane of the BPD is less than the distance between the pelvic inlet and the ischial spines. So when the occiput is at ischial spines the BPD has usually passed the pelvic inlet and the vertex is therefore engaged.
Descent
This is a continuous process throughout the first and second stage of labour. In a grand multigravida the foetal head may be engaged at the onset of labour, so only a slight descent may occur during the first stage of labour. While in a primigravida descent starts with the engagement before the first stage of labour. Factors resulting in descent are:
（1）Pressure of the amniotic fluid.
（2）Direct pressure of the uterine fundus with contractions.
（3）Bearing down efforts.
（4）Extension and straightening of the foetal ovoid. Flexion
The head is already flexed to an extent at the time of engagement and further flexion occurs during the first stage of labour. The descending head meets the resistance from the cervix, the walls of pelvis, pelvic floor and thus the chin is brought into a more intimate contact with the chest. The shortest antero-posterior diameter suboccipito-bregmatic （9.5 cm）is thus substituted for the longer suboccipito-frontal/occipito-frontal diameter. When this flexion is deficient, the diameter of engagement is longer and difficulties attending the passage of head are consequently greater.
Internal Rotation
This is defined as turning of the head in such a manner that the occiput gradually moves anteriorly towards the symphysis pubis. This carries the long diameter of the head into the antero-posterior diameter, i.e. the longest diameter of the pelvic outlet from the previous occipitoanterior, occipito lateral positions （in occipito anterior the head rotates by one-eighth of a circle and in occipito transverse the head rotates through quarter of a circle forward）. Internal rotation brings the occiput forwards under the pubic arch. With this there is a twist in the neck of the foetus. The twist in the neck is through one-eighth of the circle in occipito anterior position, quarter of the circle in occipito transverse and three-eighth of a circle in complete internal rotation of occipito posterior position. In internal rotation, when the head rotates through one-eighth of a circle, there is no movement of shoulders. The neck can sustain only one-eighth of a circle twist. When the twist in the neck is quarter of a circle the shoulders rotate through one-eighth of a circle and if the twist is through three-eighth of a circle the shoulder rotates through quarter of a circle.
Extension
In the second stage of labour （when the cervix is fully dilated）, two forces act on the head. Uterine contractions and abdominal muscles contractions exerting downwards force and the pelvic floor muscles exerting upward and forward resistance. As a result of these counter forces, the forward force acts to deliver the head by extension. The chin slides over the edge of the perineum and becomes separated from the chest wall, i.e. the head becomes extended. The vaginal outlet is stretched and crowning occurs. With progressive distension of the perineum the occiput gradually appears and the head is delivered by further extension as the occiput, bregma, forehead, nose, mouth and finally the chin pass successively over the perineum.
Restitution
The movement of detwisting of the neck, to release the torsion it attained during internal rotation is visible externally as a movement of the head, in a direction opposite to that of the internal rotation （45°）—for example in right occipitoanterior the shoulders （after restitution）are engaged in right oblique diameter （opposite to vertex which engages in left oblique diameter）while the shoulders are in the antero-posterior diameter in the occipito transverse position. The occiput points to the maternal thigh of the corresponding side to which it originally lies.
External Rotation
In the internal rotation of the shoulder the movement of the shoulder is by 1/8th of a circle. The engaging bisacromial diameter thus comes into relation with the antero-posterior diameter of the pelvic outlet. This is visible externally by the movement of the head by 1/8th of a circle. This is external ro tation of the head. It occurs in the same direction as restitution. Now the shoulders are in A-P axis. The anterior shoulder escapes under the pubic arch, while the posterior shoulder sweeps over the perineum.
4. Premonitory Signs of Labour
Premonitory signs refer to symptoms experienced before the onset of true labour. These are:
·“Lightening” or descent of the foetal head into the pelvis. This occurs about 10 to 14 days before birth especially in primigravidas. In multigravida it is more likely to occur after labour begins
·Braxton Hicks contractions
·Cervical softening, effacement and occasional dilatation of the cervix to 1 to 2 cm
·Increased vaginal discharge
·Sciatic nerve pressure
·Greater frequency of urination
·Occasional rupture of the membrane.
Show
The mucus plug in the cervical canal during pregnancy contains accumulated cervical secretions, and may be expelled when the cervix softens in the last days of pregnancy. Pressure of the descending presenting part of the foetus causes minute capillaries in the cervix to rupture and this blood mixes with the mucus creating a pink tinge. It should be differentiated from a substantial discharge of blood which may indicate an obstetric complication.
Rupture of Membranes
Occasionally rupture of membranes is the first indication of an approaching labour. Pregnant women should report to the concerned health care personnel when the membranes rupture.
5. Onset of Labour
·Painful uterine contractions （regular, rhythmic uterine contractions lasting for about 45 seconds. With a frequency of at least 2 per 15 minutes）
·Slight uterine haemorrhage—show Commencing dilatation of the internal os
·Effacement of the cervix
·Formation of bag of waters
·Spontaneous rupture of membranes.
6. Stages of Labour
Labour is the process of delivery of the foetus from the uterus through the vaginal route. Labour has been divided in four stages:
First Stage
The first stage starts from the onset of regular uterine contractions accompanied by the start of effacement and dilatation of the cervix to a full dilatation of the cervix （10 cm）. Average duration in nulliparas is 8 -12 hours and in multiparas is 3 -8 hours.
Second Stage
Second stage starts from full dilatation of the cervix and ends with expulsion of the foetus from the birth canal. Average duration in primiparas is 1-2 hours and in multiparas 0.5-1 hour.
Third Stage
Third stage starts after the delivery of the baby to the delivery of placenta and membranes. The duration is 20-30 min or 5-15 min if actively managed in multis/primis. This stage of labour also includes the control of haemorrhage.
Fourth Stage
This stage begins after the delivery of the placenta and lasts up to two hours after delivery. This is a crucial period when the women may die without proper observation of postpartum haemorrhage （from a relaxed uterus or trauma）or a haematoma which is increasing in size （episiotomy or tear）. The American Academy of Paediatrics and the American College of Obstetricians and Gynaecologists recommend taking of maternal blood pressure and pulse immediately after delivery and every 15 minutes in the first hour after birth besides observing the size of uterus and examining the perineum for any bleeding or swelling.
7. Conduct of Normal Labour
7.1 First Stage of Labour
The first stage starts from the onset of labour to full dilatation of the cervix （diameter 10 cm）. On an average in a primigravida patient, it lasts for around 8-12 hours while in a multigravida, for 3-8 hours. It is subdivided into:
（1）Latent Phase of the First Stage
The latent phase commences with maternal perception of regular contractions and slow cervical dilatation and ends at a cervical dilatation of 2-3 cm. Here the progress of labour is assessed with the Bishop score. The cervix should change at a minimum of 1 Bishop score point an hour if the labour is to end within a reasonable time. A score of 11 indicates the onset of active labour.
（2）Active Phase of the First Stage
It starts from a cervical diameter of 2-3 cm to a full cervical dilatation （10 cm）. For a normal progress of labour, the rate of cervical dilatation should be ＞ 1 to 1.2 cm/hour in nulliparous and ＞1.5 cm/hour in multiparous women. The active phase has an initial acceleration phase, then a phase of maximum slope and finally a deceleration phase. If the rate of progress is less than that of the normal, one should rule out:
·Hypotonic uterine contractions,
·Cephalopelvic disproportion （CPD）,
·Excessive sedation
·Foetal malpositions.
In cases with hypotonic uterine contractions an augmentation with oxytocin should be tried. If the patient is still making no progress for 2-4 hours or even earlier than that if the maternal or foetal well being appears to be getting compromised despite having achieved an adequate contraction pattern on a maximum oxytocin dose （appropriately used）, CPD or malpositions should be ruled out and operative delivery should be considered.
Principles of Management
·Noninterference
·Monitor maternal and foetal wellbeing
·Monitor progress of labour
·Detect any abnormality at the earliest.
Non interference means as far as possible we let the delivery occur by itself without actively interfering to deliver a part or whole of the foetus.
Maternal Wellbeing
This is ensured by observing the vital parameters such as pulse, BP, temperature, respiratory rate, etc. every 4 hourly. Sips of fluids like tea, fruit juice, soup and plain biscuits are allowed in low risk-pregnancies; IV fluids are administered in cases where the labour is prolonged. Solid foods are withheld in active labour because:
·Emptying of the stomach is delayed
·Vomiting/aspiration may occur
If the need for an operative delivery arises, then the risk of aspiration and other post-operative complications （Mendelson's syndrome）has to be borne in mind.
Oral ranitidine 150 mg should be given 6 hourly to a woman in labour （if there is a chance that operative delivery is needed later on）. During the early stages of labour, the mother is allowed to stay out of bed （move about）if membranes are intact, but once the membranes are ruptured the mother's movement should be restricted. Per abdominal examination is done to monitor the descent of the head, foetal heart sounds and any distension of the urinary bladder. Monitoring of the uterine contractions is carried out by the palm of the hand lightly placed on the abdomen. Time of onset, duration and intensity of each uterine contraction is noted. During a uterine contraction, the thumb cannot indent the uterus. Bed rest in a lateral position is preferred as it lifts the uterus away from the great vessels, thus preventing a compromise in the blood supply to the foetus.
Per-vaginal examination is performed every 4 hours to check the progress of labour needless tosay, before a PV examination the patient should be explained about the procedure. During first stage, FHR is monitored every 30 mins. The suprapubic region should be checked in every abdominal examination for bladder fullness. Bladder distension should be avoided. The patient should be encouraged to pass urine at least every 2 hours. Adequate pain relief in the form of parenteral analgesics or epidural analgesia can be offered. However one must bear in mind that reassurance is still one of the best analgesics. Relaxation techniques taught during the antenatal period are of a great help. Amniotomy or artificial rupture of membranes （ARM）is performed early in labour once the head is engaged, the cervix is well-applied and dilatation is ＞ 3 cm unless:
i. There is spontaneous rupture of membranes
ii. The presentation is unknown, floating or unstable
iii. The cervix is ＜ 3 cm dilated.
7.2 Second stage of Labour
The second stage starts from a full dilatation of the cervix to the delivery of the baby. It lasts on an average for 1 to 2 hours in primigravidas and in 0.5-1 hours in multigravidas. In the absence of foetal compromise, maternal distress, rupture of membranes or other indications for termination there is no ur gency in delivering the patient.
Principles of Management
·Assist in maternal expulsion of the foetus
·Prevent perineal injuries.
When the patient has reached stage II: Labour-assessment at least every 30 minutes is done to assess descent and rotation of the presenting part. If the patient is making appropriate progress one can anticipate vaginal delivery. Foetal descent should be ＞ 1 cm per hour. If the progress of labour is not adequate, evaluate:
·Uterine contractions.
·Foetal position if occipito transverse or occipito posterior.
·Rule out cephalopelvic disproportion.
·Evaluation of fluid balance is done, correction of dehydration if any should be done.
·Unless contraindicated, oxytocin augmentation for failed stage II is advocated.
When the above measures fail, operative vaginal delivery including vacuum extraction or forceps delivery should be considered unless contraindicated. Caesarean section should be considered in cases of failed progress and non-descent of head. （Here foetal poles may be palpable on P/A assessment.）
·The patient should be lying down with legs half flexed at the time of bearing down and a dorsal lithotomy position should be maintained （with or without stirrups）at the time of delivery. Shoulders should be raised.
·An episiotomy should be given at the crowning of head, if the doctor conducting delivery thinks that the perineum is likely to tear, especially in cases of nulliparous women and in-strumental deliveries. Local anaesthesia using 1 per cent lignocaine should be given.
·The Partograph: A graphic monitoring of the progress of labour. This is an essential tool for decision-making during labour.
Not to be advocated:
·Fundal pressure ·Ironing or stretching of perineum.
7.3 Third Stage of Labour
The third stage starts after the delivery of the baby and lasts up to the delivery of the placenta and the membranes. The duration in usually of 5-15 mins. If actively managed but it can be as long as 15-20 mins, without intervention. Management can be expectant （traditional）or active.
Watchful Expectancy.
A hand is placed over the fundus and signs of placental separation are looked for which are as follows:
（1）The shape of uterus becomes globular and it be-comes firmer. This is the earliest sign of placental separation.
（2）There is a sudden gush of blood.
（3）The uterus rises in the abdomen slightly more than its previous position because the placenta separates and fills the lower uterine segment and a part of the vagina and pushes the uterus up.
（4）There is true lengthening of the umbilical cord which indicates that the placenta is descending. Once separation is confirmed the patient is asked to bear down while gentle traction is applied on umbilical cord. If the membranes threaten to tear, they are held with sponge holding forceps and gently pulled out.
Active Management
Active management helps to achieve early haemostasis. The disadvantage is a slightly higher incidence of retained placenta （1-2%）. Inj. methyl ergometrine 2 mg I/V or 125 mg of PGF2α is administered at the delivery of the anterior shoulder unless contraindicated as in cases of multiple pregnancy, Rh-ve mothers, PIH, heart disease, bronchial asthma, etc. In such cases IM oxytocin 10 unit can be given. Following delivery of the baby, the placenta is expected to separate and deliver spontaneously. If not, it is delivered by controlled cord traction. Needless to say, after delivery of the neonate one should exclude multiple pregnancy before using these pharma-cological measures.
7.4 Fourth Stage of Labour
Post-delivery the patient's vital signs should be monitored every 15 min. for the first hour and then every hour in the labour room itself till she is stable. Breast feeding is advocated within half hour after delivery. The uterine fundus should be palpated to see if uterus is well-contracted or relaxed. The time of first void （of urine）should be noted. The episiotomy should be inspected for any haematoma. Excessive bleeding is to be looked for besides monitoring pulse, respiration and blood pressure. If the condition of the patient remains stable, she could be transferred to the ward two hours after her delivery.
（李彩虹）