Mechanism of labour Flashcards
What are the stages of labour?
Diagnosis: painful contractions lead to dilatation of the cervix
First stage: initiation to full cervical dilatation
Second stage: full cervical dilatation to delivery of foetus
Third stage: delivery of foetus to delivery of placenta
What are the mechanical factors of labour?
o Power- the degree of force expelling the foetus
o Passage- the dimensions of the pelvis and the resistance of soft tissue
o Passenger- the diameters of the foetal head
What determines the power of labour?
• Once labour is established- the uterus contracts for 45-60secs every 2-4mins
this pulls the cervix up (effacement) and causes dilation
aided by the pressure of the head as the uterus pushes the head down into the pelvis
• Poor uterine activity is a common feature of nulliparous women and induced labour
rare in multiparous women
What affects the passage of the uterus?
Bony pelvis:
o Inlet- 13cm transverse diameter
o Mid-cavity
o Outlet- 12.5cm AP diameter
• Ischial spines are landmarks by which to assess the descent of the head of vaginal examination
Level of descent is the station, measured in cm in relation to the spine
station 0 means the head is at the level of the spines, while +2 is 2cm below and -2 is 2cm above
How does the passenger affect labour?
• The head is oblong in transverse section
• The anterior fontanelle (bregma) lies above the forehead
the posterior fontanelle (occiput) lies on the back of the top of the head, between these two is the vertex
• In front of the bregma is the brow
because the head is not round, several factors determine how easily it fits through the pelvic diameters
What is the attitude of the foetus?
the degree of flexion of the head on the neck- ideal is maximal flexion, keeping the head bowed
This is vertex presentation and the presenting diameter is 9.5cm running from the anterior fontanelle to below the occiput at the back of the head
• A small degree of extension results in a larger diameter- extension of 90o causes a brow presentation and a much larger diameter of 13cm, a further 30o extension is a face presentation
What is the position/rotation of heads during delivery?
• The position is the degree of rotation of the head on the neck
• If the sagittal suture is transverse, the oblong head will fit the pelvic inlet best
but at the outlet the sagittal suture must be vertical for the head to fit, the head must therefore rotate 90o during labour
• It is usually delivered with the occiput-anterior (OA) in 5% of deliveries it may be OP and more difficulty may be encountered, persistence of the OT position implies non-rotation and delivery without assistance is impossible
What are the terms used to describe the foetal head?
Presentation: part of the foetus that occupies the lower segment of the pelvis (cephalic/breech)
Presenting part: lowest part go the foetus palpable on vaginal examination- can be vertex, brow or the face- depending on attitude
Position: describes the head’s rotation- OT, OP, OA
Attitude: degree of flexion of head: vertex, brow or face
How is labour initiated?
• Involuntary contractions of uterine smooth muscle occur throughout the 3rd trimester, often felt as Braxon Hicks contractions
Prostaglandin production reduces cervical resistance and increasing oxytocin stimulates contractions
These arise in one of the pacemakers situated at each cornu of the uterus
effacement is when the normally tubular cervix is drawn up into the lower segment until it is flat, this is commonly accompanied by a ‘show’ or pink/white mucus plug from the cervix and/or rupture of the membranes causing release of liquor
What is the first stage of labour?
• This lasts for the diagnosis of labour until the cervix is dilated to 10cm (fully dilated)
• The descent, flexion and internal rotation described occur to varying degrees
If membranes haven’t ruptured they do now
• The latent phase, where the cervix usually dilates slowly for the first 4cm and may take several hours
• The active phase follows-average cervical dilation is at the rate of 1cm/h in nulliparous women and about 2cm/hr in multiparous women, the active first stage should not normally last longer than 16hrs
What is the second stage of delivery?
- This lasts from full dilation of the cervix to delivery- descent, flexion and rotation are completed and followed by extension as the head delivers
- The passive stage- lasts from full dilation until the head reaches the pelvic floor and women experience the desire to push, rotation and flexion are commonly completed, this stage may last a few minutes, but it can be much longer
- The active stage, when the mother is pushing, the presence of the head on the pelvic floor produces an irresistible desire to bear down, although spinal analgesia may prevent this
- The foetus is delivered, on average, after 40 minutes (nulliparous) or 20 minutes (multiparous)- this stage can be much quicker, but if it takes >1hr, spontaneous delivery becomes increasingly unlikely
What occurs during delivery?
As the head reach the perineum, it extends to come up out of the pelvis, the perineum begins to stretch and often tears, but can be cut if progression is slow or foetal distress is present
• The head then restitutes, rotating 90o to adopt the transverse position in which it entered the pelvis
• With the next contraction, the shoulder deliver- the anterior shoulder comes under the symphysis pubis first, usually aided by lateral body flexion in a posterior direction, the posterior shoulder is aided by lateral body flexion in an anterior direction, the rest of the body follows
What is the third stage of labour?
• This is the time from delivery of the foetus to delivery of the plaenta
it normally lasts about 15mins
normal blood loss is <500ml
• Uterine muscle fibres contract to compress the blood vessels formerly supplying the placenta, which shears away from the uterine wall
What are the degrees of perineal trauma?
• The perineum is intact in about ⅓ of nulliparous women and ½ of mutliparous women
o 1st degree- minor damage to the fourchette
o 2nd degree & episiotomies- perineal muscle
o 3rd degree- anal sphincter (1%)
o 4th degree- anal mucosa
How does the foetus move during delivery?
Engagement: head enters pelvis in OT position
Descent and flexion: head descends into the round mid-cavity and flexes as the cervix dilates. Station term used
Rotation: in the mid-cavity the head rotates 90o (internal) so the face enters the sacrum and the occiput is anterior OA. This allows it to pass through the pelvic outlet
Rotation completed further decent: the perineum distends
Extension and delivery
Restitution: head rotates 90o (external) to the same position in which it entered the inlet, facing either right or left
to enable delivery of the shoulders
