Parturition

Question 1

What is labour?

Answer 1

Expulsion of products of conception after 24 weeks:

• Spontaneous abortion - before 24 weeks
• Pre-term - before 37 weeks
• Term - 37-42 weeks

Parturition

Question 2

What are the stages of labour?

Answer 2

First stage
- Creation of birth canal
Second stage
- Expulsion of fetus
Third stage
- Expulsion of placenta
- Contraction of uterus

Question 3

Describe the change in size of things over pregnancy.

Answer 3

Fetus, placenta, uterus increase dramatically in size

• Uterus palpable by 12 weeks
• Reaches umbilicus c20 weeks
• Xiphisternum by 36 weeks

Question 4

What needs to be assessed towards the end of pregnancy?

Answer 4

'Lie'
- Relationship to long axis of uterus
- Normally longitudinal
- Fetus normally flexed
'Presentation'
- Which part is adjacent to pelvic inlet
- Normally head (cephalic)
- Sometimes buttocks (podalic) (also known as a breach)
'Position'
- Orientation of presenting
- Most commonly;
   - Longitudinal lie
   - Cephalic presentation
   - Vertex to pelvic inlet at minimum diameter

Question 5

Tell me about the size of the birth canal.

Answer 5

In normal presentation:
- Head biggest part
- Diameter of presentation 9.5cm
Maximum size of birth canal determined by pelvis
- Pelvic inlet typically 11cm
- Softening of ligaments may increase it

Question 6

Tell me about the creation of the birth canal.

Answer 6

Expansion of soft tissues:
- Cervix
- Vagina
- Perineum
- To about 10cm
Needs
- Structural changes
- A lot of force

Question 7

What are the structural changes associated with the creation of the birth canal?

Answer 7

Cervix retains fetus for most of pregnancy
- Tough, thick
- Collagen
Needs to soften
- 'Cervical ripening'

Question 8

Describe cervical ripening.

Answer 8

Cervix collagen in proteoglycan matrix
Ripening involves:
- Reduction in collagen
- Increase in glycosaminoglycans 
- Reduces aggregation of collagen fibres
Triggered by prostaglandins
- PG E2 and F2x

Question 9

How is the force generated to create the birth canal, and to give birth?

Answer 9

Myometrium

• Smooth muscle
• Much thickened in pregnancy
• Force when intracellular [Ca2+] rises
• Due to action potentials (burst of action potentials)
• Triggered spontaneously
  
  • ‘Pacemakers’

Question 10

Tell me about uterine contractions.

Answer 10

Occur throughout pregnancy

• Early
  
  • Low amplitude, every 30 min
• Late
  
  • Less frequent
  • Higher amplitude
  • ‘Braxton-Hicks’ contractions

Question 11

How is contractibility controlled in pregnancy?

Answer 11

Contractions are made more forceful and frequent by:

• Prostaglandins
  
  • More Ca2+ per action potential
• Oxytocin
  
  • More action potentials
    
    • Lower threshold

Question 12

What are prostaglandins? (in relation to pregnancy)

Answer 12

Biologically active lipids
Local hormones
Produced mainly in endometrium
Production controlled by oestrogen progesterone ratio:
- Low (progesterone>oestrogen)
   - Low prostaglandins
- High (oestrogen >progesterone)
   - Increased prostaglandins

Question 13

What happens when there is a relative fall in progesterone?

Answer 13

Increases prostaglandins

• Ripen cervix
• Promote uterine contractions

Question 14

Tell me about oxytocin.

Answer 14

Secreted by posterior pituitary
Controlled by hypothalamus
Increased by afferent impulses from cervix and vagina
- ‘Ferguson reflex’
Acts on smooth muscle receptors
- More receptors if oestrogen/progesterone ratio high

Question 15

Tell me about the onset of labour.

Answer 15

More prostaglandins

• Cervix ripens
• Uterine contractions more forceful
• Brachystasis
  
  • Uterus relaxes less than contracts
  • Fibres shorten in body of uterus
  • Drives presenting part to cervix

Question 16

Tell me about the changes in the cervix towards onset of labour.

Answer 16

Cervix thins and flattens
- ‘Effacement’
Ferguson reflex stimulates Oxytocin release
- Contractions more forceful and more frequent
Cervix begins to dilate
Rupture of the amnion
- ‘breaking of the waters’

Question 17

How wide is the cervix at the end of the first stage?

Answer 17

Cervix dilated to 10cm

May take many hours

Question 18

Tell me about the initiation of labour.

Answer 18

Prostaglandins promote labour
BUT
- Fetuses with no adrenals get born
- No consistent evidence of progesterone oestrogen 
  changes
So situation is unclear

Question 19

Tell me about the second stage of labour.

Answer 19

Relatively rapid
- Up to 1hr but can be very fast
Urge to ‘bear down’ - to use abdominal muscles
Presenting part appears in birth canal

Head rotates internally
Head stretches vagina and perineum
- Risk of tearing
   - Episiotomy
Head delivered
Heads rotates and extends
Shoulders rotate
Shoulders deliver
Followed rapidly by rest

Question 20

Tell me about the third stage of labour.

Answer 20

Effect of uterine contractions dramatically increased by expulsion of fetus
Uterus contracts down hard
Shears of placenta
And expel it
Normally within 10 minutes

Question 21

What is the importance of uterine contractions?

Answer 21

When the placenta is sheered off a lot of blood vessels are sheered open as well
Compresses blood vessels
Reduce haemorrhage
Enhanced by giving oxytocic drug

Question 22

Tell me about establishing independent life after birth.

Answer 22

Neonate takes first breath
- Multiple stimuli
   - Trauma
   - Cold
Reduces pulmonary vascular resistance
Increases arterial pO2

Left atrial pressure exceeds right
- Foramen ovale closes
Ductus arteriosus contracts in response to raised pO2
Fetal converts to adult circulation