Chapter 7 Normal labour and delivery

David T Y. Liu, Pamela M. Thwaites

CHAPTER CONTENTS

Myometrial activity: pregnancy 45

Myometrial activity: labour 46

Uterine work 46

Intrauterine pressure 46

General comments 47

Effect of uterine activity on the cervix 47

Before labour 47

In labour 48

Effect of uterine activity on the fetus 48

Management of the delivery 48

Position 48

Aseptic conditions 48

Medical attendant 48

Delivery of the head 49

Delivery of the shoulders 49

Umbilical cord 49

Management of the third stage 51

Delivery of the placenta 51

Effect of uterine activity on the mother 52

Management guidelines 52

MYOMETRIAL ACTIVITY: PREGNANCY

During pregnancy the uterus is usually in a quiescent state. In the third trimester women experience low amplitude, poorly synchronised Braxton Hicks or practice contractions. Substances which contribute to the inhibition of myometrial activity include progesterone from placental syncytiotrophoblast and chorion; myometrial parathyroid hormone-related peptide; and nitric oxide and relaxin from the myometrium, decidua, chorion and amnion in addition to prostacyclins. The process of myometrial contraction requires activation of calmodulin, a calcium-binding protein with calcium ions which then in turn activates the enzyme myosin light chain kinase to produce adenosine triphosphate to power actin and myosin filaments to slide over each other to produce shortening. Inhibitors of myometrial activity act by increasing intracellular levels of cyclic nucleotides to prevent release of calcium ions from intracellular stores or by reducing myosin light chain kinase activity.

Towards term a number of processes occur which predispose to activation or preparation of the myometrium for onset of labour. Formation of gap junctions by increases in contraction-associated proteins such as connexin-43 enhance cell to cell coupling. Receptors for oxytocin and stimulatory prostaglandins are also increased. These changes are associated with myometrial stretch when the uterus enlarges and with the higher levels of oestrogens derived partly from placental conversion of fetal dehydroepiandrosterone (DHEAS) where they also exert a local oestrogen effect. In addition to increased DHEAS production, activation of the fetal hypothalamic–pituitary–adrenal axis in late pregnancy results in more fetal cortisol biosynthesis. Fetal cortisol competes to reduce the local progesterone effect and stimulate synthesis of corticotrophin-releasing hormone from placenta and fetal membranes for production of prostaglandins by the latter structures.

Page 46

MYOMETRIAL ACTIVITY: LABOUR

Initiation of labour remains unclear, but prostaglandins have been implicated in myometrial contraction. Isoforms of phospholipase A₂ or C, activated by varying requirements for calcium ions, liberate arachidonic acids from membrane phospholipids. Prostaglandin synthase in amnion and chorion convert arachidonic acids to primary prostaglandins. For prostaglandin E₂ (PGE₂) there are four main receptor subtypes labelled from EP-1 to EP-4. These receptors are distributed in the myometrium in varying concentrations. Stimulation of EP-1 and EP-3 results in contractions whereas stimulation of the other two receptors leads to relaxation. These together with corticotrophin-releasing hormone-related cyclic AMP in the lower uterine segment contribute to fundal dominance.

Onset of labour is associated with a substantial increased myometrial sensitivity to oxytocin stimulation and a three- to five-fold increase in oxytocin production by chorio-decidual tissue. Oxytocin raises concentrations of free calcium in the myocytes to promote myometrial contraction.

Fetal membranes covering the internal cervical os exhibit a decreased production of the 15-hydroxyprostaglandin dehydrogenase enzyme which metabolises prostaglandins. More local prostaglandin production is available for cervical effacement and dilatation. This effect is supplemented by release of collagenase through increased cytokine activity.

Onset of labour is most likely between midnight and 0500 hours when maternal secretion of oxytocin peaks and the myometrium is most sensitive to oxytocin and prostaglandin. These changes in steroid and protein concentrations return to non-pregnant levels 48–72 hours post partum.

Uterine work

Myometrial contraction exerts a pull in circular and longitudinal directions (Figure 7.1)

Figure 7.1 Myometrial contraction exerts pull in circular and longitudinal directions.

The term fundal dominance is used to describe travel of myometrial contractions from the fundus of the uterus towards the cervix. The starting point or pacemaker for myometrial activity is situated near the point of insertion of the fallopian tubes into the uterus. The spread of myoelectrical activity through the uterus requires 1 minute. Approximately another minute is needed for adequate relaxation. Blood vessels must traverse the myometrium to reach the placenta. Contractions occurring more than once every 2 minutes contribute to poor myometrial relaxation and reduce blood and hence oxygen supply to the fetus. When two contractions are noted every 10 minutes for an hour consider possible onset of labour. In established labour the rate of contractions ranges between 3 and 5 per 10 minutes.

In early labour the uterus is not working at maximum capacity. This may be because the number of myometrial cells contracting is limited or contraction is poorly synchronised or of short duration. As labour progresses contractions becomes more efficient and uterine work increases. Capacity for work cannot, however, increase indefinitely. A stable phase for ability to work results once maximal work output is achieved for the individual mother. When the stable phase is achieved (Figure 7.2a) additional stimulation by oxytocics is of little value and can be harmful. Uterine work has been expressed as:

• Classically in Montevideo units which are intensity (mmHg) times number of contractions per 10 minutes. The intensity is taken as the height reached by the recorded contraction from the resting tone (Figure 7.2b).

• Kilopascals per 15 minutes. Included as a display in contemporary models of fetal monitors when intrauterine transducers are used. The range for normal labour is 700–1500 kPa per 15 minutes.

Figure 7.2 (a) Graph of uterine activity with stable phase (plateau) for individual woman. (b) Montevideo unit = intensity (amplitude of recorded contraction) × contractions per 10 minutes. The Alexandra unit, a refinement, takes into consideration duration of the contraction.

Intrauterine pressure

The viscoelastic myometrium always exerts pressure on the amniotic fluid. The resting pressure or tone is usually 6–12 mmHg. Amniotic fluid is not compressible.

Measurement is achieved by:

Page 47

• Insertion of a fluid-filled polythene tube into the uterine cavity. This tube is connected to a transducer capable of measuring changes in hydrostatic pressure.

• Insertion of a catheter-tipped pressure transducer into the uterine cavity (Figure 7.3). This is a more costly but simpler system.

• Palpation and external (tocometers) assessment do not indicate intrauterine pressure.

Figure 7.3 Intrauterine pressure measured by a catheter-tipped pressure transducer placed above presenting part is less affected by movement.

Intensity of uterine contractions increases throughout pregnancy. Towards term pressures up to 30 mmHg may be recorded. During labour intrauterine pressure increases to levels of 60–80 mmHg. Contractions of this intensity are still detectable for 48 hours post partum but the frequency diminishes after 12 hours.

General comments

• Normal efficient uterine activity is associated with fundal dominance and regular synchronised contractions of good (more than 40 mmHg) intensity.

• Contractions should last between 40 and 60 seconds with an adequate interval in between when intrauterine pressure can return to resting tone.

• Uterine contractions are more efficient when women lie on their side.

• Primiparous labour is generally associated with contractions of greater intensity than those in multiparae.

• Early amniotomy shortens labour with no detrimental effect on fetal outcome nor an increase in assisted delivery. In uncomplicated labours, the woman’s preference must be considered. The increased uterine activity of labour exerts additional pressure on the amniotic fluid. Measurement of this intrauterine hydrostatic pressure (IUP) indicates the strength of the contraction.

• Epidural lengthens both first and second stage of labour, increased incidence of fetal malposition and vaginal instrumental delivery (incidence reduced by routine oxytocin).

Effect of uterine activity on the cervix

Before labour

In the weeks before labour that portion of the uterus between the internal os and the reflection of the uterovesical fold of the peritoneum is attenuated by stretch and Braxton Hicks contractions to form the lower segment. This allows the presenting part of the fetus, particularly in primiparous women, to settle or engage into the pelvis.

Page 48

In labour

Labour is traditionally divided into two stages:

• First stage – onset of labour to full cervical dilatation. This includes the latent and active phase of labour.

• Second stage – full dilatation to delivery of the baby. The duration approximates 60 minutes in primiparas and 30 minutes in multiparas. Management/intervention should not be based solely on these suggested times. A little more time can be allocated to avoid unwelcome interference if both the fetus and the mother are well. On the other hand with complications or fetal distress, elective assisted delivery or a shortened second stage is advised.

Effacement precedes cervical dilatation. Initially uterine activity is expanded to achieve effacement. This period, the latent phase of labour, takes on average 9 ± 6 hours in the primiparas and 5 ± 4 hours in multiparas. The cervix is usually up to 2 cm dilated when labour begins. At the end of the latent phase cervical dilatation is usually between 3 cm and 4 cm. Once effaced further uterine activity produces rapid cervical dilatation of at least 1 cm per hour in both parous and nulliparous women. This, the active phase of labour takes on average 6 hours to reach full cervical dilatation (Figure 7.4). Multiparous women approach labour with more cervical effacement hence a shorter latent phase and thus a shorter labour.

Figure 7.4 Partogram. Primipara and multipara showing latent phase and steep active phase to full cervical dilatation.

It is common practice to plot the rate of cervical dilatation to indicate the progress of labour. This is usually charted on a single sheet of paper designed for recording labour events over a 24-hour period. Provision is also made for registering fetal heart rate (every 30 minutes), uterine contractions (every 30 minutes), rate of head descent and medication. Cervical dilatation detected by vaginal examination is recorded every 2–4 hours. This graphic representation of parturition (partogram) summarises and depicts, visually, the events in labour (Figure 7.5). Cervical dilatation lagging 2–3 hours behind that expected from the normal graph (Figure 7.6), should suggest to medical attendants the need for reassessment.

Figure 7.5 Example of a partogram in use showing a concise summary of events in labour.

Figure 7.6 Partogram showing deviation (−) from the expected course.

Effect of uterine activity on the fetus

• Blood vessels traversing the myometrium to supply the placenta and fetus are compressed during uterine contractions. Delivery of nutrients and, particularly, oxygen is impeded or curtailed once the intrauterine pressure exceeds 40 mmHg. Increased myometrial tension or tone and rapid recurring contractions further reduce the capacity to supply the fetus and hence threaten hypoxic insult.

• Compaction or curling up of the fetus occurs with each contraction. Pressure is exerted on the presenting parts, such as the fetal head. During passage through the birth canal, the whole fetus is compressed and this can evoke vagal stimuli.

• The substantial increase in fetal catecholamine production helps switch off fetal lung liquid production.

• The fetus is gradually expelled into the vagina in the process of birth.

• The mechanism of labour is illustrated in Box 7.1.

Box 7.1 Mechanism of labour

Flexion and entry

Uterine contractions cause further flexion and entry of the fetal head into the pelvis, usually in the occipito-transverse (Figure 7.7).

Descent and internal rotation

Descent occurs to the level of the ischial spines when levator ani muscles assist internal rotation to align the sagittal sutures for delivery through the widest anteroposterior diameter of the outlet.

Extension and delivery of the head

Distension of the lower part of the vagina evokes reflexes which stimulate the urge to push. Pushing is achieved by the Valsalva response and contraction of diaphragmatic and abdominal muscles. The head stretches the vagina and vulva as it delivers. Extension occurs once the head passes beneath the symphysis pubis. Fetal membranes usually rupture before this stage (shown in sequence in Figure 7.8).

Restitution, external rotation and delivery of the shoulders

The head rotates to occipitolateral or restitutes to align naturally perpendicular to the shoulders. The shoulders, having entered the pelvis in the oblique diameter, rotate so the bisacromial diameter of the shoulder delivers in the anteroposterior diameter of the pelvic outlet. Further rotation of the head laterally accompanies rotation of the shoulders (Figure 7.9).

Delivery of the body

The shoulders deliver assisted by lateral flexion of the body. Once this is achieved the rest of the fetus delivers readily as the uterus contracts down (Figure 7.10).

Figure 7.7 The widest part of the inlet (a) (transverse diameter) and outlet (b) (anteroposterior diameter) is represented geometrically. The fetal head enters in the occipito-transverse then rotates to address anteroposterior diameter of outlet.

Figure 7.8 Delivery of the head by extension.

Figure 7.9 Restitution showing the relation of the sagittal axis of the fetal head to allow delivery of shoulders in the anteroposterior diameter of the pelvic outlet.

Figure 7.10 Delivery of the body by lateral flexion.

Management of the delivery

Position

The squatting posture is well suited to delivery. A woman adopting the lithotomy position propped up with pillows and her legs drawn back essentially achieves this posture but has the added advantage of allowing attendant access to assist delivery.

Aseptic conditions

Gloves and gown are mandatory for the attendant conducting the delivery. Women should be draped and the vulval area cleansed. Deliver onto sterile towels.

Medical attendant

The medical attendant is usually stationed on the woman’s right. Place the left hand on the fetal head as it ‘crowns’ (passage of the biparietal eminences through the vulva) to maintain flexion and prevent expulsive delivery.

The right hand is used to guard the perineum and cover the anus with a sanitary pad. If necessary assist extension of the fetal head by lifting the chin. Consider if an episiotomy is needed.

Page 49

Delivery of the head

Instruct the woman to pant once the head is crowned. This prevents forceful pushing, and repeated gentle increase in intra-abdominal pressure created by panting nudges the head out. Aspirate the mouth and then the nose of the baby at the first opportunity. Give 0.5–1 ml of Syntometrine (a combination of the synthetic oxytocic Syntocinon 5 units/ml and ergometrine maleate, 0.5 mg/ml) intramuscularly to the mother. The oxytocic effect is evident in 2.5 minutes (Syntocinon) to 7 minutes (ergometrine). It assists uterine contraction and placental separation and thereby controls blood loss. For mothers at risk of haemorrhage, give ergometrine intravenously (oxytocic effect in 1 minute).

Delivery of the shoulders

Once external rotation is completed, direct the head gently downwards to assist delivery of the shoulders. When the anterior shoulder appears beneath the symphysis insert a finger into the anterior axilla and lift the body upwards watching the perineum at the same time to avoid extension of the episiotomy or tearing of the perineum.

Umbilical cord

Cut the cord within 1 minute of birth. This interval allows additional transfusion of more than a third of the fetal blood volume. Do not wait for cessation of pulsation as the placenta may be separated and fetal exsanguination can result. Free any loose cord around the neck over the head or shoulders and proceed with the delivery. If the cord is tight around the neck or fetal resuscitation is anticipated clamp, cut the cord at once and deliver the baby.

Page 50

Page 51

Management of the third stage

The third stage is the internal between delivery of the fetus and delivery of the placenta. Placental separation is shown by:

• a lengthening of the cut cord

• a show of blood

• elevation of the fundus as the uterus contracts following separation of the placenta.

Delivery of the placenta

The placenta usually separates within 3 minutes and is delivered within 5 minutes after birth.

• Stand on the mother’s right side. The uterus should be contracted.

• The left hand is placed suprapubically. Straddle the uterus with the thumb on one side and the rest of the fingers on the other for better control (Figure 7.11). Press backwards towards the mother to align the uterus with the vaginal axis. Figure 7.12 shows straightening of the uterus.

• Grasp the cord with the right hand and apply gentle traction in line with the pelvic axis (approximately 45° to the horizontal). If the cord springs back once pressure is removed, placental separation is not complete. Wait. If the placenta is separated, controlled traction will deliver the placenta (Figure 7.13).

• Rotate the placenta to wind the membranes into a cord to assist their complete delivery.

• Inspect the placenta (Box 7.2), the membranes and the cord for abnormalities and completeness.

Figure 7.11 Placement of the hand for delivery of the placenta.

Figure 7.12 Alignment of uterus with vaginal axis assists the delivery of placenta.

Figure 7.13 Controlled cord traction facilitates delivery of the separated placenta.

Box 7.2 Placental inspection

• Inspect and document all macroscopic features. Request histological examination for obstetric complications, e.g. growth restriction, stillbirth to determine possible aetiology. In twin pregnancy, confirmation of chorionicity is important.

• Karyotyping and genetic examination assist with diagnosis of fetal abnormalities, unexpected outcome and growth below the third centile.

• Take swabs from the chorion when infection is suspected.

Page 52

Effect of uterine activity on the mother

Management guidelines

• Labour evokes feelings of anxiety and anticipation particularly in the primipara, where labour is lengthy and when complications are present. Emphasise teachings from antenatal classes and offer support.

• Pain threshold varies among individuals. Ensure comfort (Box 7.3) and avoid distress.

• Nurse the woman on her side or in a semi-reclining position (semi-Fowler) to assist labour and avoid compression of the inferior vena cava. Flexibility in attitude of the attendants is needed to accommodate individual preferences (check birth plans).

• Maintain energy requirements and electrolyte balance.

• Record the pulse, blood pressure and temperature at regular intervals.

• Special precautions are required if there is pre-existing maternal medical disease or complications (for example, supplementary hydrocortisone to cover stress of labour).

• Good communication and documentation is essential particularly when there is digression from standard practice.

Box 7.3 Administration of entonox

• Check that gas cylinders are not empty and that they have been stored at room temperature. The two gases may separate if stored at low temperatures (−7 °C).

• Ensure the mask fits over the nose and mouth with no air leaks from the slides of the mask.

• Analgesia begins in 20 seconds and is maximum at 45 seconds. The interval between onset of uterine contractions and sensation of pain is 20–30 seconds in the latent phase and 10–15 seconds in the active phase of labour. Use the mask at the beginning of contractions for the maximum analgesic effect at the height of the contraction.

• Instruct the mother to inhale deeply through the mouth and exhale rapidly to make the machine click. The gas mixture is only released from the demand valve in the machine when the mother inhales deeply.

• If the mother is particularly sensitive to nitrous oxide use the mask only when contractions become uncomfortable.

• For analgesia in the second stage, time contractions and breath 30 seconds before each contraction. Instruct women to take two to three quick deep breaths on the mask during a contraction prior to her pushing efforts.

• Women must be instructed to hold the mask themselves. Should they become anaesthetised the mask will fall away from the face and allow recovery.

Bibliography

Fox H. Pathology of the Placenta. London: WB Saunders, 1997.

Grammatopoulos DK, Hillhouse EW. Role of corticotrophin-releasing hormone in onset of labour. Lancet. 1999;354:1546-1549.

Howell CJ, Kidd C, Roberts W, et al. A randomised controlled trial of epidural compared with non-epidural analgesia in labour. British Journal of Obstetrics and Gynaecology. 2001;108:27-33.

Lye SJ, Ou CW, Teoh TG. The molecular basis of labour and tocolysis. Feto-Maternal Medical Review. 1998;10:121-136.

Patel FA, Clifton VL, Chawalisz K, et al. Steroidal regulation of prostaglandin dehydrogenase activity and expression in human term placenta and chorio decidua in relation to labour. Journal of Clinical Endocrinology and Metabolism. 1999;84:291-299.

Page 53

Petraglia F, Florio P, Nappi C, et al. Peptide signalling in human placenta and membranes: autocrine, paracrine, and endocrine mechanisms. Endocrine Reviews. 1996;17:156-186.

Sangha RK, Walton JC, Ensor CM, et al. Immunohistochemical localisation, mRNA abundance, and activity of 15-hydroxyprostaglandin dehydrogenase in placenta and fetal membranes during term and preterm labour. Journal of Clinical Endocrinology and Metabolism. 1994;78:982-989.

Sparey C, Robson SC, Bailey J, et al. The differential expression of myometrial connexin-43, cyclooxygenase-1 and -2 and Gsa proteins in the upper and lower segments of the human uterus during pregnancy and labour. Journal of Clinical Endocrinology and Metabolism. 1999;84:1705-1710.

Steer PJ, Carter MC, Gordon AJ, et al. The use of catheter-tip pressure transducers for the measurement of intrauterine pressure in labour. British Journal of Obstetrics and Gynaecology. 1978;85:561.

UK Amniotomy Group. A multi-centre randomised trial comparing routine versus delayed amniotomy in spontaneous first labour at term. British Journal of Obstetrics and Gynaecology. 1994;101:307-309.

Wu YW. Systematic review of chorio-amnionitis and cerebral palsy. Mental retardation and Developmental Disabilities Research Reviews. 2002;8:25-29.

Page 54