Definition and Stages of Labour

Definition: Regular uterine contractions along with effacement and dilatation of the cervix.

 

These processes can occur simultaneously in a parous woman, but will only occur in sequence (effacement followed by dilatation) in primigravids. The entire process (from 4cm dilatation and in the presence of regular painful contractions) takes 8 to 18 hours for primigravids and around 5.5 (up to 12) hours for parous women.

 

Labour can be divided into 3 stages

  1. Onset of labour until cervix is fully dilated (vaginal examination to be performed every 4 hours; cervix dilates at a rate of 1cm/hour, but 0.5cm/hour is also considered normal)
  2. Cervical dilatation until the head has been fully delivered
  3. Delivery of baby until expulsion of the placenta and membranes

Mechanism of Normal Labour

Effacement and Dilatation of cervix

The cervix ripens towards end of pregnancy and during labour it becomes effaced ('spread' over the head of the fetus, losing its cylindrical shape to become ‘flat’ as it becomes incorporated into the lower uterine segment) and dilated. Full dilatation is around 10cm. The position of the fetus' head as it passes through the birth canal is described in terms of the occiput to mother’s pelvis.

Spontaneous Vaginal Delivery

 

Delivery of the head: The two widest points of the fetus need to pass through the birth canal: the head (along the antero-posterior plane) and the shoulders (transverse plane). Assuming a longitudinal lie and cephalic presentation, the head enters the pelvis in the right or left occipito-transverse position (ROT/LOT). Uterine contractions cause the head of the fetus to flex (chin towards chest), allowing the minimum head diameter to be presented during delivery. The V-shaped pelvic floor at the level of the ischial spines caused the head to rotate 90° to the occipitoanterior/posterior position (the former being more common), allowing the head to pass through this level and be delivered.

 

Delivery of the shoulders and rest of the fetus: At this point the shoulders enter the pelvis in the transverse position, and rotate into the antero-posterior plane as they pass through the V-shaped pelvic floor. Thus once the head has been delivered, the anterior shoulder is delivered using downward traction of the head and lateral traction of the fetal trunk aids passage of the anterior shoulder from the pubic arch. If a woman has consented to an IM oxytocic to reduce the likelihood of post-partum haemorrhage, it is given once the anterior shoulder has been delivered. It cannot be given before in case of shoulder dystocia. The posterior shoulder is delivered by upward lateral traction of the trunk, followed by the rest of the fetus.

 

Delivery of placenta and membranes: Uterine contractions separate the placenta from the uterus (indicated by passage of small volume of dark blood and ‘lengthening’ of the cord) after which the placenta can be delivered by gentle traction of the cord + upwards pressure on the body uterine fundus (to reduce possibility of uterine inversion). Ensure that the entire placenta has been passed and the vagina and perineum are inspected for tears. 

Movement during spontaneous vaginal delivery

Analgesia

  • Some patients obtain relief from TENS (transcutaneous electrical nerve stimulation)
  • Entonox  (50% Nitrous Oxide and 50% Oxygen; an inhaled analgesic) is commonly used, but does not provide complete relief; side effects include nausea, vomiting, drowsiness
  • Opioid analgesia (IM Diamorphine) is effective but can potentially lead to nausea and vomiting so should be prescribed with an anti-emetic. Opioids can cause transient respiratory depression in the fetus.
  • Regional anaesthesia:

1. Epidural – a bolus of local anaesthetic (usually bupivacaine) introduced into the epidural space using an epidural catheter and can be ‘topped up’ if necessary; does not prolong 1st stage of labour and is not associated with increased rate of Caesarian sections, but does increase length of 2nd stage of labour. Fetal monitorning (CTG) is needed for 30 minutes after initial dose and each bolus. Side effects include hypotension caused by a sympathetic blockade - managed by placing patient in left lateral position, providing Oxygen and administration of IV fluids.

2. Spinal – drugs introduced into the subarachnoid space (intrathecal); only a single dose is given leading to a dense blockade lasting 2-4 hours; Side effects similar to those of an epidural and are managed similarly. To be used for operative procedures, prolonged labour or management of labour-related complications.

 

Instrumental/Operative Delivery

Commonest indications are failure to progress and fetal distress; choice of method depends on cervical dilatation, stage of labour and specific criteria. For any instrumental/operative delivery, ensure maternal consent has been obtained and appropriate analgesia provided. Instrumental vaginal delivery can be performed using Forceps or Ventouse. Operative delivery refers to a Caesarian section.

Types of forceps:

  1. Low-cavity outlet forceps (Wrigley’s) – used when head is on perineum
  2. Mid-cavity forceps (Haig Ferguson, Neville-Barnes, Simpson’s) – used when fetus’ sagittal suture is on the AP plane
  3. Kielland’s forceps – rotational delivery to occipitoanterior or occipitoposterior position

Ventouse involves the use of silastic cup or a metal vacuum extractor cup, placed on the midline or slightly anterior to the posterior fontanelle of the baby’s head (to encourage flexion) and is pulled in time with mother’s contractive effort.  This method is associated with increased chances of failure compared to forceps, but less maternal perineal or vaginal trauma. This technique however can lead to cephalohaematoma or retinal haemorrhage in the baby, and can cause a ‘chignon’ (a transient, raised area on the baby’s scalp due to the vacuum exerted by the device).

Caesarian section can be performed electively before onset of labour (indications: placental abruption/praevia, severe pre-eclampsia, transverse lie or breech presentation) or emergency during labour (performed if cervix is not fully dilated but there is fetal distress/failure to progress). Lower uterine caesarian section (LUCS) commonly performed, as opposed to the traditional vertical incision because of better healing, fewer post-operative complications and rates of uterine rupture subsequently.

Caesarian sections may cause significant morbidity from:

  • Thromboembolic disease
  • Haemorrhage
  • Infection
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