Physiology Of The First Stage Of Labour

Question 1

What is the first stage of labour?

Answer 1

• it is from the onset of regular uterine contractions, accompanied by the effacement of the cervix and dilatation of the os to full dilatation of the os uteri

Question 2

Describe the different phases of the first stage of labour

Answer 2

• Latent Phase
  
  • -> prior to the active first stage
  • -> cervix dilates 0cm to 3/4cm
  • -> cervix shortens from 3cm to <0.5cm long
• Active Phase
  
  • -> undergoes more rapid dilatation
  • -> from 4cm dilated
  • -> rhythmic contractions
  • -> complete when cervix is fully dilated
• Transition
  
  • -> cervix around 8cm until expulsive contractions
  • -> brief lull in intensity of contractions

Question 3

What is spurious or false labour?

Answer 3

• women experience contractions prior to the onset of labour
• can be painful
• may be regular for a time
• no effacement or dilatation of the cervix
• no effect of contractions on cervix

Question 4

Describe the physiology of the first stage of labour

Answer 4

• labour is a physiological process rather than an event
  
  • -> pregnancy and labour are part of a continuum and cannot be considered as separate entities
• 2 major physiological changes
  
  • -> cervix undergoes structural change from a support to a birth canal
  • -> myometrial tone of uterus must change to allow coordinated contractions

Question 5

Describe the onset of spontaneous physiological labour

Answer 5

• determined by the complex interaction of maternal and fetal hormones
• process is not fully understood
• the onset is multifactorial (hormonal and mechanical)

Question 6

Describe the hormonal factors which influence the onset of labour

Answer 6

• increased DHAS (dehyroepiandosterone sulphate) production leads to synthesis of placental estradiol
• maternal oestrogen also rises sharply in last few weeks
• high levels of oestrogen cause uterine muscle fibres to display oxytocic receptors
• form gap junctions with each other
• increased ratio of oestrogen to progesterone leads to increased uterine contractility

Question 7

What are some of the factors that contribute to the onset of labour?

Answer 7

• placenta releases prostaglandin which produces enzymes (collagenase and elastin) which digest collagen in the cervix (helping it soften)
• evidence unclear that oestrogens and progesterones alter at the onset of labour
• balance of them does facilitate my myometrial activity

Question 8

What are the mechanical an neurohormonal factors that influence the onset of labour?

Answer 8

• uterine activity - may result in mechanical stimulation of the uterus and cervix
• overstretching of the uterus
• pressure from presenting part, well applied to cervix

Question 9

Describe the process of cervical effacement

Answer 9

• cervix is drawn up and gradually merges into the lower segment
• later in pregnancy or not until labour commences
• primigravida (complete effacement before dilatation)
• multigravida (effacement and dilatation may be simultaneous )
• mechanism
  
  • -> muscle fibres surrounding internal os drawn upwards by retracted uterine segment, cervix merges into lower uterine segment
  • -> or, external os taken up first, outward rolling movement, cervix thins from external os upwards, internal os affected last

Question 10

Other factors that can influence the onset of labour

Answer 10

• ‘show’
  - -> (operculum) bloodstained mucous, anything from days to a few hours before or after labour starts
  - -> blood from ruptured capillaries parietal decidua where chorion become detached
• membranes may rupture

Question 11

Describe the uterine action that occurs during the first stage of labour

Answer 11

• fundal dominance- each contraction starts in fundus, spreads across and downwards
• contraction lasts longest in the fundus, most intense, fades from all parts together
• permits the cervix to dilate and strongly contracting fundus to expel fetus

Question 12

Describe what contractions are like during the first stage of labour

Answer 12

• each labour is individual
• at onset ever 15-20 mins for 30 seconds
• fairy weak may not be felt by woman
• rhythmic regularity
• interval between lessens
• increased through latent phase into active phase
• at the end of the 1st stage occur every 2-3mins and are very powerful

Question 13

What is polarity?

Answer 13

• it is the neuromuscular harmony between 2 poles/segments of uterus throughout labour
• 2 poles act harmoniously
• upper -> contracts and retracts to expel fetus
• lower -> contracts slightly and dilates to allow expulsion

Question 14

What is retraction?

Answer 14

• contraction does not pass off completely
• muscle fibres retain some of the shortening instead of relaxing completely
• unique property of uterine muscle

Question 15

Describe the upper and lower segment of the uterus

Answer 15

• upper -> contraction and retraction, thick and muscular
• lower -> distension and dilatation
• no clear and strict division
• retracted longitudinal fibres in upper segment pulls on lower segment
• added by force of descending presenting part

Question 16

What is the retraction ring?

Answer 16

• it is the ridge between segments
• it rises as upper uterine segment contracts and retracts, lower segment thins out to accommodate descending fetus
• rises no further once cervix is fully dilated
• Bandl’s ring seen in obstructed labour, exaggerated, fetal compromise

Question 17

Describe cervical dilatation in the 1st stage of labour

Answer 17

• it is the process of enlargement of os uteri
• starts as a tightly closed arpeture
• needs to allow passage of the fetal head
• measured in cms
• occurs due to uterine action and pressure from the intact bag of membranes or presenting part
• needs pressure to be applied evenly
  
  • -> fundus to respond by contraction and retraction

Question 18

Describe the formation of forewaters

Answer 18

• lower segment forms and stretches
• chorion becomes detached, with increased intrauterine pressure loosed part of sac bulges into internal os
• depth 6-12mm
• well flexed head cuts off fluid in front of head
• pressure applied on hindwaters may help keep water intact - defence against infection

Question 19

Describe the rupture of membranes

Answer 19

• membranes will usually rupture around the end of 1st stage
• may rupture early is there is a malpostion e.g. OP
• if they do not rupture, appear at vulva as a bulging sac covering fetal dad (caul)

Question 20

What is fetal axis pressure?

Answer 20

• it is the force of fundal contraction transmitted to the upper pole of the fetus
• down long axis and applied by pp to cervix