Normal Labour and Delivery

Introduction

Labour is the process by which regular, painful uterine contractions produce progressive effacement and dilatation of the cervix, leading to expulsion of the fetus, placenta and membranes. Normal labour is usually defined as spontaneous in onset, between 37+0 and 41+6 weeks, with a single fetus in a cephalic vertex presentation, progressing without intervention to a spontaneous vaginal birth.¹

Understanding what is normal is the prerequisite for recognising what is not. Almost every obstetric emergency is defined by reference to normal labour: delay in the first stage, delay in the second stage, malposition, malpresentation, fetal compromise and postpartum haemorrhage are all deviations from the sequence described here.

Physiology of the onset of labour

No single trigger initiates labour. The prevailing view is that the myometrium is actively held quiescent throughout pregnancy, principally by progesterone, and that labour begins when a series of changes overcomes that inhibition.

• Fetal signal - maturation of the fetal hypothalamic-pituitary-adrenal axis raises fetal cortisol, which drives placental corticotrophin-releasing hormone in a positive feedback loop
• Functional progesterone withdrawal - a shift in the ratio of oestrogen to progesterone activity, mediated at receptor level rather than by falling circulating progesterone in humans
• Oxytocin receptor upregulation - myometrial oxytocin receptor density rises dramatically near term, so the uterus becomes responsive to circulating oxytocin rather than oxytocin levels rising
• Prostaglandins - PGE2 and PGF2-alpha ripen the cervix by degrading collagen and increasing water content, and stimulate contractions
• Gap junctions - connexin-43 gap junctions form between myocytes, allowing the uterus to contract as a coordinated syncytium rather than as isolated cells
• The Ferguson reflex - stretching of the cervix and upper vagina triggers a neuroendocrine reflex release of oxytocin, providing positive feedback

Uterine contractions in labour are unusual among smooth muscle contractions in showing retraction: after each contraction the muscle fibres of the upper segment do not return to their original length. This progressively shortens the upper segment, pulls the lower segment and cervix upwards over the presenting part, and is the mechanism of effacement and dilatation.

The first stage

The first stage runs from the onset of labour to full cervical dilatation at 10 cm. It is divided into a latent and an established phase, and the distinction matters because women admitted in the latent phase have higher rates of intervention.

NICE defines delay in the established first stage as dilatation of less than 2 cm in 4 hours in a nulliparous woman, or less than 2 cm in 4 hours or a slowing of progress in a parous woman. Other features suggesting delay are descent and rotation of the head failing to progress, and a change in the strength, duration or frequency of contractions.¹

The mechanism of labour

The fetal head must negotiate a bony pelvis whose widest diameter changes orientation between the inlet and the outlet. At the pelvic inlet the transverse diameter is greater than the anteroposterior; at the outlet the reverse is true. The fetal head is itself oval, with a longer anteroposterior than transverse diameter, so it must rotate as it descends. This is the entire explanation for the cardinal movements.

The second stage

The second stage runs from full dilatation to the birth of the baby. It is divided into a passive phase, in which the cervix is fully dilated but there is no involuntary expulsive urge, and an active phase, in which the woman has an expulsive urge or is actively pushing.

Allowing an hour of passive second stage before active pushing in a nulliparous woman with an epidural permits the head to descend and rotate passively, and reduces the rate of instrumental delivery. Women should be encouraged to adopt whatever position they find comfortable and discouraged from lying supine, which reduces uteroplacental perfusion by aortocaval compression.

• Support the perineum and control the speed of the head at crowning to reduce trauma; warm compresses reduce third- and fourth-degree tears
• Episiotomy is not performed routinely; it is indicated for fetal compromise requiring expedited birth or for instrumental delivery, and should be mediolateral at 60 degrees from the midline
• Check for a nuchal cord after the head delivers, and either slip it over the head or, rarely, clamp and cut it
• Await restitution and external rotation, then apply gentle downward traction to deliver the anterior shoulder followed by upward traction for the posterior shoulder
• Note the time of birth and place the baby skin to skin with the mother

Monitoring in labour

Observations in a low-risk labour are deliberately minimal, with the intention of avoiding intervention that is not indicated.

Continuous cardiotocography is recommended where there are antenatal or intrapartum risk factors: suspected chorioamnionitis or sepsis, severe hypertension, oxytocin augmentation, significant meconium, fresh vaginal bleeding, or an abnormality detected on intermittent auscultation. Offering continuous monitoring to low-risk women increases the caesarean and instrumental delivery rate without reducing cerebral palsy.

Reading a cardiotocograph

The standard structured approach is summarised as DR C BRAVADO: Define Risk, Contractions, Baseline RAte, Variability, Accelerations, Decelerations, Overall impression. A normal baseline is 110-160 beats per minute with variability of 5-25 beats per minute. Early decelerations mirror contractions and reflect head compression. Variable decelerations vary in timing and shape and reflect cord compression. Late decelerations begin after the peak of the contraction and recover after it ends, and indicate uteroplacental insufficiency. Features are classified as reassuring, non-reassuring or abnormal, and the trace overall as normal, suspicious or pathological.²

Analgesia in labour

Women should be counselled accurately about epidurals. They provide much better pain relief than any alternative, and there is no evidence that they increase the caesarean section rate or cause long-term backache. They do prolong the second stage and increase the rate of instrumental vaginal birth, require intravenous access and more intensive monitoring, and cause hypotension, urinary retention and pruritus. Accidental dural puncture occurs in around 1 in 100 insertions and causes a postural headache in the majority of those affected.¹

The third stage

The third stage runs from birth of the baby to expulsion of the placenta and membranes. Signs of placental separation are a gush of blood, lengthening of the cord and the uterus rising and becoming globular. It is prolonged if not completed within 30 minutes with active management, or within 60 minutes with physiological management.

Active management is recommended for all women, but physiological management is a reasonable choice for a woman at low risk who requests it, with a clear plan to convert to active management if there is bleeding, if the placenta is not delivered within an hour, or if the woman wishes. Never apply controlled cord traction without a uterotonic and without a contracted uterus, because of the risk of uterine inversion.^1,3

Immediately after birth

• Skin-to-skin contact immediately and uninterrupted where possible; it improves thermoregulation, breastfeeding initiation and bonding
• Apgar score at 1 and 5 minutes, assessing appearance, pulse, grimace, activity and respiration
• Vitamin K offered to all babies, intramuscularly as a single dose or orally in a course, to prevent haemorrhagic disease of the newborn
• First feed within the first hour where possible, with support for breastfeeding
• Examine the placenta and membranes for completeness; retained fragments cause haemorrhage and infection
• Estimate blood loss by weighing swabs and measuring, rather than by visual estimation, which systematically underestimates
• Palpate the uterine fundus to confirm it is contracted, and inspect the perineum, vagina and cervix for trauma
• Newborn and infant physical examination within 72 hours, and the newborn blood spot screen at day 5

Perineal trauma

When labour is not normal

Failure to progress is traditionally analysed in terms of the three Ps. The powers may be inadequate, with hypotonic or poorly coordinated contractions, which is the commonest cause in a nulliparous woman and is treated with amniotomy and oxytocin augmentation. The passenger may be malpositioned, most often occipito-posterior, malpresenting, or simply large. The passages may be inadequate, either because of pelvic shape or, rarely, because of a soft tissue obstruction such as a cervical fibroid.

In a parous woman, poor progress should never be attributed to inadequate powers without careful thought, because a uterus that has already delivered a baby is usually capable of doing so again. Delay in a multipara raises the possibility of obstruction, malposition or malpresentation, and carries a real risk of uterine rupture if oxytocin is used uncritically.¹

The partogram is the tool that makes deviation visible. Plotting cervical dilatation against time, alongside descent, contractions, fetal heart rate, liquor and maternal observations, converts a series of individual assessments into a trend, and is what prompts the question of whether labour is progressing normally at the point at which something can still be done about it.