Labour: mechanism

Question 1

What is the diagnosis of labour and what are the different stages?

Answer 1

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.

Question 2

What 3 mechanical factors determine the process during labour?

Answer 2

1. The degree of force expelling the foetus (the powers)
2. The dimensions of the pelvis and resistance of soft tissues (the passage)
3. The diameters of the foetal head.

Question 3

Once labour is established, how often are contractions and how long are they?

Answer 3

Contractions are 45-60 seconds every 2-4 minutes.

Question 4

How do contractions of the uterus cause cervical dilatation?

Answer 4

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.

Question 5

Who is poor uterine activity more common in?

Answer 5

Nulliparous women and induced labours.

Question 6

What are the transverse and anterio-posterior planes like in each of the 3 principle planes?

Answer 6

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).

Question 7

Which structure is the decent of the head vaginally referred to?

Answer 7

The ischial spines.

Question 8

What is the level of descent of the head referred to?

Answer 8

Station. It is measured in relation to the ischial spines in cm.

Question 9

What does cervical dilatation depend on?

Answer 9

Contractions of the uterus, the pressure of the head on the cervix and the ability of the cervix to soften and allow distention.

Question 10

What shape is the head of the foetus and how can you tell what angle it is being expelled at?

Answer 10

It is oblong. You can tell the angle based on which fontanelles and sutures you can feel

Question 11

What are the different fontanelles called and where are they?

Answer 11

Anterior fontanelle (bregma) - lies above the forehead.
Posterior fontanelle (occiput) - lie on the back of the top of the head.

Question 12

What is the attitude?

Answer 12

The degree of extension/flexion of the head on the neck in the foetus.

Question 13

What is the ideal attitude? What is it called? What is the presenting diameter?

Answer 13

Maximal flexion, keeping the head bowed.
It is called vertex presentation.
Presenting diameter is 9.5cm.

Question 14

What are the examples of extension presenting attitude?

Answer 14

Much larger.
Brow presentation = 13cm. Called brow because the brow suture line is presenting on delivery.
Face presentation = more and cannot be delivered vaginally.

Question 15

What position is the head of the foetus in as it descends down the pelvis?

Answer 15

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.

Question 16

What happens if the head does not rotate through the pelvis?

Answer 16

Delivery is not possible without assistance.

Question 17

How does the head fit through the pelvis other than by rotation? What is this called?

Answer 17

The sutures allow the head to be compressed as the bones can come together and even overlap slightly.

This is called moulding.

Question 18

What are the contractions throughout the third trimester called?

Answer 18

Braxton Hicks

Question 19

What is prostaglandin production important for in starting labour?

Answer 19

1. Reducing cervical resistance

2. Increasing release of hormone oxytocin from the posterior pituitary gland.

Question 20

What is effacement?

Answer 20

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.

Question 21

At what length is the cervix fully dilated? Which stage of labour does this signal the end of?

Answer 21

10cm.

End of first stage of labour.

Question 22

What are the 2 phases of the first stage of labour?

Answer 22

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.

Question 23

What are the 2 stages of the second stage of labour? Who long should each last?

Answer 23

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.

Question 24

After the head is delivered, what happens?

Answer 24

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.

Question 25

How long is the third stage of labour? What happens?

Answer 25

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.

Question 26

What happens to the perineum during labour?

Answer 26

It often tears - 2/3 of nulliparous women and 1/2 multiparous women.