Labour
- Interplay between;
- The ‘powers’ of the uterus – contractions
- The ‘passages’ of the birth canal
- The ‘passenger’ – the fetus
Anatomical points
- Pelvic inlet in wider in the transverse than AP diameter
- Pelvic outlet is wider in the AP than in the transverse diameter
- Pelvic measurements may widen during labour due to pelvic ligament laxity
- Moulding may reduce the absolute measurements of the fetal skull during labour
- The degree of flexion of the fetal head (the attitude) determines the diameter of the fetal skull presenting to the pelvis
The onset of labour
- Onset can be defined as regular contractions bringing about progressive cervical dilation
- ‘show’ or SROM does not indicate onset
Stages of Labour
- First stage – time from diagnosis of labour to full cervical dilation (10cm). This can be divided into two phases, the latent phase which is the time between onset of labour to dilation of 3-4cm during which time the cervix becomes fully effaced. The second phase of the first stage is the active phase between the end of the latent phase and full dilatation. Should dilate at 1cm per hour on average.
- Second stage – the time from full dilation to delivery of the fetus. This may be subdivided into two phases, the passive phase when there is no urge to push and the active second stage causing a reflex urge to ‘bear down’
- Third stage – the time from delivery of the fetus to delivery of the placenta. Normally takes 5-10 mins. Management involves controlled cord traction. Syntometrine (oxytocin and ergotamine) increase uterine contraction and prevent bleeding.
Mechanisms of Labour
- Refers to the series of changes in the position and attitude that the fetus undergoes during its passage through the birth canal. In terms of the vertex presentation and the gynaecoid pelvis this can be described as;
- Engagement - head normally enters the pelvis in the transverse position. Engagement is said to occur when the widest part of the presenting part has successfully passed through the inlet. In nulliparous women engagement generally occurs prior to labour, for multiparous women this is not the case. Engagement is described in terms of the number of fifths of fetal head palpable in the abdomen. If less than 2/5 are palpable then the head is said to be engaged.
- Descent - descent of the fetal head is required before flexion, internal rotation and extension can occur
- Flexion – occurs passively and is due to pressure from surrounding structures. It is important because it minimises the presenting diameter of the fetal head
- Internal Rotation – on reaching the levator ani muscles the presenting part will be encouraged to rotate anteriorly so that the saggital suture now lies in the AP diameter of the pelvic outlet
- Extension – this results in the occiput escaping from underneath the symphysis pubis and distension of the vulva. This is known as ‘crowning’
- Restitution – once the head is delivered it aligns itself with the shoulders which have entered the pelvis in the oblique position. Restitution is the rotation of the occiput through 1/8th of a circle
- External Rotation – the shoulders rotate into the AP plane and the occiput rotates trhough a further 1/8th of a circle to be in the transverse plain
- Delivery of the shoulders and fetal body – anterior shoulder is delivered first followed by the posterior shoulder. Lateral traction can be applied pulling the fetal head downwards to help release the anterior shoulder from the pubic symphysis.
Fetal Assessment in Labour
- Observation of the colour of the liquor – fresh meconium staining (green) and heavy bleeding are signs of fetal compromise
- Intermittent auscultation of the fetal heart using a Pinard stethoscope or a hand held Doppler ultrasound.
Normal should be a rate of 110-160, baseline variablility of 10-25bpm, 2 accelerations in 20 mins and no decelerations - Continuous external fetal monitoring using cardiotocography (CTG)
- Fetal scalp blood sampling
- With each contraction placental blood flow and oxygen transfer are interrupted
Normal labour is taken as being
- Spontaneous onset
- Single cephalic presentation
- 27-42 weeks gestation
- no artificial interventions
- unassisted spontaneous vaginal delivery
- duration of <12hr in nulliparous women and <8hr in multiparous women
Abnormal Labour
- Labour considered abnormal is it is showing poor progress, there is fetal compromise, there is malrepresentation, a uterine scar or labour has been induced.
Poor progress in labour
Due to,
- Inefficient uterine action – characterised by weak and infrequent contractions. 4-5 contractions every 10 mins is considered ideal. Treatment is via maternal rehydration, ARM and intravenous oxytocin.
- Cephalo-pelvic disproportion (CPD)Â - due to a large head, small pelvis or both
- Malpresentation – breech and face presentations may fail to apply the cervix. Risk of uterine rupture
- Abnormalites of the passages – bony pelvis, uterus (fibroids in the lower section) and cervix (cervical dystocia)
Patterns of Abnormal progress in labour
- Prolonged latent phase
- Primary dysfunctional labour – poor progress in active phase of labour most commonly due to inefficient uterine contractions but can result from CPD and malposition
- Secondary Arrest – occurs when initial progress in active phase is good but slows after about 7cm dilation. Most commonly due to fetal malpositions, malrepresentations and CPD
The use of oxytocin
- Relatively safe in nulliparous women. Less safe in multiparous women due to risk of unterine hyperstimulation, fetal compromise and uterine rupture in the face of obstruction
- Careful consideration is required in breech presentation or previous C-section
Risk factors for fetal compromise in labour
- Placental insufficiency – IUGR or pre-eclampsia
- Prematurity
- Postmaturity
- Multiple pregnancy
- Prolonged labour
- Uterine hyperstimulation
- Precipitate labour
- Intrapartum abruption
- Cord prolapse
- Uterine rupture
- Maternal diabetes
- Cholestasis of pregnancy
- Maternal pyrexia
- Chorioamnionitis
- Oligohydramnios
Uterine Scarring
- Mostly following a previous C-section – incision is in the lower segment
- Increased risk of uterine rupture or partial rupture. Risk is higher if it is an upper uterine scar
- Rupture is particularly likely to occur
- Late in the first stage of labour
- With induced or accelerated labour
- In association with a large baby
- Signs of uterine rupture are
- Severe lower abdominal pain
- Vaginal bleeding
- Haematuria
- Cessation of contractions
- Maternal tachycardia
- Fetal compromise
Malpresentations
- Breech presentation – increased risk of cord compression and prolapse. Also mechanical difficulties in the delivery of the head/shoulders
- Face presentation – due to complete extension of the fetal head. Results in late engagement and slow progression
- Shoulder presentation – risk of cord prolapse or uterine rupture. Results for fetus being in transverse or oblique lie. Cause by placenta praevia, pelvic tumour or uterine anomaly
Induction of Labour
Common indictions
- Post dates – 12 days or more from EDD
- Fetal growth restriction
- Other evidence of placental insufficiency e.g. oligohydramnios
- Pre-eclampsia
- Other maternal hypertensive problems
- Deteriorating maternal illness
- Prolonged prelabour rupture of membranes (increased risk of ascending infection – chorioamnionitis)
- Unexplained antepartum haemorrhage
- Diabetes
- Twin pregnancy continuing after 38 weeks
- Rhesus isoimmunisation
- Intrauterine death
- Evidence that pregnancies extending beyond 42 weeks are at higher risk if still births, fetal compromise, meconium aspiration and mechanical problems at labour
Methods
- Between 1 to 3 doses of vaginal prostaglandin
- ARM
- syntocinon