Labour: mechanism Flashcards
What is the diagnosis of labour and what are the different stages?
Diagnosis: painful contractions lead to cervical dilatation.
Stage 1: initiation to full cervical dilatation
Stage 2: Full cervical dilatation to delivery of foetus.
Stage 3: delivery of foetus to delivery of placenta.
What 3 mechanical factors determine the process during labour?
- The degree of force expelling the foetus (the powers)
- The dimensions of the pelvis and resistance of soft tissues (the passage)
- The diameters of the foetal head.
Once labour is established, how often are contractions and how long are they?
Contractions are 45-60 seconds every 2-4 minutes.
How do contractions of the uterus cause cervical dilatation?
The contractions pull the cervix up (effacement) and causes dilatation, aided by the pressure of the head as the uterus pushes the head down into the pelvis.
Who is poor uterine activity more common in?
Nulliparous women and induced labours.
What are the transverse and anterio-posterior planes like in each of the 3 principle planes?
Inlet: transverse diameter (13cm) is wider than the AP diameter (11cm).
Mid-cavity: almost as round as the transverse and AP is similar (11cm).
Outlet: Transverse diameter (11cm) is smaller than the AP (12.5cm).
Which structure is the decent of the head vaginally referred to?
The ischial spines.
What is the level of descent of the head referred to?
Station. It is measured in relation to the ischial spines in cm.
What does cervical dilatation depend on?
Contractions of the uterus, the pressure of the head on the cervix and the ability of the cervix to soften and allow distention.
What shape is the head of the foetus and how can you tell what angle it is being expelled at?
It is oblong. You can tell the angle based on which fontanelles and sutures you can feel
What are the different fontanelles called and where are they?
Anterior fontanelle (bregma) - lies above the forehead. Posterior fontanelle (occiput) - lie on the back of the top of the head.
What is the attitude?
The degree of extension/flexion of the head on the neck in the foetus.
What is the ideal attitude? What is it called? What is the presenting diameter?
Maximal flexion, keeping the head bowed.
It is called vertex presentation.
Presenting diameter is 9.5cm.
What are the examples of extension presenting attitude?
Much larger.
Brow presentation = 13cm. Called brow because the brow suture line is presenting on delivery.
Face presentation = more and cannot be delivered vaginally.
What position is the head of the foetus in as it descends down the pelvis?
Inlet: the sagittal suture is transverse so the head fits in.
Outlet: the sagittal suture must be vertical for the head to fit.
Therefore, the head must rotate 90* during labour. This is usually with the occiput anterior. In 5% of deliveries, the occiput is posterior and more difficulty is encountered.
What happens if the head does not rotate through the pelvis?
Delivery is not possible without assistance.
How does the head fit through the pelvis other than by rotation? What is this called?
The sutures allow the head to be compressed as the bones can come together and even overlap slightly.
This is called moulding.
What are the contractions throughout the third trimester called?
Braxton Hicks
What is prostaglandin production important for in starting labour?
- Reducing cervical resistance
2. Increasing release of hormone oxytocin from the posterior pituitary gland.
What is effacement?
When the normally tubular cervix is drawn up into the lower segment until it is flat. This is commonly accompanied by a ‘show’ or mucus plug from the cervix and/or rupture of the membranes.
At what length is the cervix fully dilated? Which stage of labour does this signal the end of?
10cm.
End of first stage of labour.
What are the 2 phases of the first stage of labour?
Latent phase - cervix dilates slowly for the first 4cm and may take several hours.
Active phase - average cervical dilatation is 1cm.hour in nulliparous women and 2cm/hour in multiparous women.
What are the 2 stages of the second stage of labour? Who long should each last?
Passive stage - lasts from full dilatation until the head reaches the pelvic floor and the women has the desire to push. This may last a few minutes. The rotation and flexion are commonly completed.
Active stage - when the mother is pushing with contractions - there is an irresistible urge to bear down, although epidural may prevent this. This stage usually takes 40 minutes for a nulliparous woman and 20 minutes for a multiparous woman.
If the active stage is taking over an hour, spontaneous delivery is unlikely.
After the head is delivered, what happens?
The baby turns 90* back again so it is in the position it entered the pelvis in - this allows the shoulders to be delivered. The rest of the body then follows.
How long is the third stage of labour? What happens?
From the delivery of the foetus to the delivery of the placenta. Usually 15 minutes. There is usually 500mL of blood loss. Uterine muscles contract to compress the blood vessels formerly supplying the placenta.
What happens to the perineum during labour?
It often tears - 2/3 of nulliparous women and 1/2 multiparous women.
