L7 Labour Flashcards

1
Q

Outline the stages of labour:

A
  • Latent phase: precursor to labour with irregular contractions, effacement and dilation of cervix
  • First stage: regular contractions, effacement and dilation of cervical os to full dilation (classed as labour from 4cm)
  • Second stage: Full dilation of cervix (10cm) to birth
  • Third stage: Birth to expulsion of placenta and membranes
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2
Q

When does labour occur?

A
  • Generally between 37 and 42 weeks
  • Mean 39.6 weeks in humans
  • Only 3- 5% of women actually deliver on their EDD (280 days from first day of last period)
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3
Q

What two major physiological changes are important for expelling the fetus?

A
  • Softening/ripening of cervix -> goes from providing a structural function to acting as a birth canal
  • Change to myometrial tone: Allows coordinated contractions to increase uterine pressure (assisted later on by contractions of striated muscles in abdominal wall)
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4
Q

Describe the ideal positioning of baby and placenta before delivery:

A
  • Baby should drop in final stage of gestation, so head is lined up with cervix at bottom of uterus
  • Back of baby should be facing out instead of being back-to-back with mum
  • Labour is longer and more difficult if back-to-back
  • Normal placenta: facing upper surface of endometrium (placenta praevia: facing cervix)
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5
Q

What is known about the cause of labour?

A
  • Not fully known
  • Hypothesised that the ripe placenta releases a labour-inducing substance that triggers the process (1931)
  • The placenta has a lifespan and becomes increasingly infracted by 43 weeks (gritty, calcified, impaired function)
  • Still a lot of complexity and lack of clarity
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6
Q

Why is it important to maintain myometrial quiescence during pregnancy?

A
  • During normal cycling, myometrium frequently contracts; important to avoid this to prevent spontaneous abortion
  • Therefore, the capacity for contractility is dampened by circulating hormones throughout gestation
  • Simultaneously, significant proliferation and hypertrophy of smooth muscle is taking place in anticipation for delivery
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7
Q

What hormones are involved in maintaining myometrial quiescence during pregnancy and what are their mechanisms in this process?

A
  • hCG (inhibits MGJ formation -> slower signal transduction)
  • Progesterone (inhibits circulating oestrogen which would stimulate MGJ formation)
  • Corticotrophin-releasing hormone
  • Relaxin (acts on all smooth muscle, particularly important for softening of pelvic joints ready to stretch during delivery)
  • Oxytocin (stimulates synthesis of relaxatory PGs until hCG levels drop)
  • Melatonin (supresses formation of myometrial oxytocin receptors)
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8
Q

What 3 types of stimuli support initiation of labour?

A
  • Mechanical
  • Hormonal
  • Inflammatory
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9
Q

Outline the key signals for activation of the myometrium (mechanical factors for labour induction):

A
  • Uterine stretch -> activation of myometrium
  • Substantial increase in oxytocin receptors in myometrium and endometrium -> increased MGJs -> fast signal transduction, greater contractility
  • Endocrine pathway (HPA axis) -> increased cortisol
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10
Q

Outline the composition and function of the myometrium (how is it different to normal smooth muscle?):

A
  • Bundles of smooth muscle cells (myocytes) embedded a connective tissue matrix (collagen) which coordinate transmission of the forces generated by myocyte contraction
  • Myocyte activity is carefully coordinated by gap junctions (metabolic and electrophysiological communication) -> functional syncytium
  • Unlike in other smooth muscle, in myocytes, the actin filaments interact with teh entire length of the myosin filaments -> greater shortening at each contraction -> facilitates cervical effacement and dilation as well as involution of uterus after delivery
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11
Q

What muscle layers surround the uterus?

A
  • Inner layer: latitudinal (sphincter action downwards)
  • Middle layer: spirals
  • Outer layer: longitudinal (squeezes down)
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12
Q

Myometrial gap junctions: basic structure and function, stimulation and inhibition

A
  • Symmetrical portions of plasma membrane from adjacent cells
  • Intracellular channels facilitating rapid propagation of APs
  • Formation is stimulated by oestrogen, certain PGs and melatonin
  • Formation is inhibited by progesterone, hCG and relaxin
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13
Q

What is oxytocin and where is it stored?

A
  • Hormone and neurotransmitter
  • Produced by hypothalamus, passes down neuronal axons and is stored in posterior pituitary until required
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14
Q

How does increasing oestrogen concentration in late pregnancy influence the myometrium?

A
  • Promotes formation of oxytocin receptors
  • Promotes formation of MGJs
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15
Q

Outline the Ferguson reflex:

A
  • Distension of cervix and vagina produce neuronal stimuli (engagement of baby’s head, enhanced by movement during labour)
  • Results in short pulses of oxytocin release from P pituitary
  • Creates fundal dominance (progressive conductance of electrical activity from fundus to cervix)
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16
Q

What hormones does the placenta secrete to induce labour?

A
  • Key: Oestrogen
  • Relaxin (important for uterine quiescence but mechanism in labour induction unclear)
  • CRH levels increase towards labour, potentiating effects of PGs and oxytocin on uterine contractility -> increased PG production by decidua and membrane (also stimulates fetal adrenal gland)
  • Activin A, follistatin
17
Q

How may the fetus be involved in communicating with the placenta and uterus to initiate labour?

A
  • Sheep models: adrenocotricotrophic hormone and resulting cortisol production involved in labour onset
  • Upon maturation of fetal HPA axis, adrenal gland becomes more sensitive to ACTH, stimulating cortisol production -> initiates uterine changes and promotes lung maturation of the fetus
  • Fetal adrenal gland simultaneously stimulated by rising CRH from placenta towards end of pregnancy -> fetal adrenal gland releases DHEAS (oestrogen precursor) -> raising oestrogen
18
Q

How does oxytocin enact its neurotransmission?

A
  • Floods MGJ
  • Activates receptor-operated calcium channels
  • Ca2+ release into myometrial cells -> contraction of myocyte
19
Q

What are prostaglandins?

A
  • Inflammatory markers and chemical mediators
  • Synthesised on site -> local effect
  • Strong myometrial stimulants, enhancing neurotransmission of oxytocin
  • Also important in cervical ripening
20
Q

When are PGs released?

A
  • Production of PgE2 and PgF2 in the placenta and decidua is stimulated by oestrogen dominance in late pregnancy via cytokines -> also triggers release
  • Synthesis also increases during labour
  • Act to induce labour by increasing fetal cortisol, increasing oestrogen etc
21
Q

Give 3 key inflammatory mediators in labour onset with their main function:

A
  • Cytokines: Stimulate synthesis of PGs
  • Interleukins: Increase collagenic activity of cervix
  • Nitric oxide: stimulate release of PgE2 from fetal membranes
22
Q

What inflammatory changes take place in cervix before labour?

A
  • Hypertrophic and inflammatory reaction occurs at term
  • Neutrophil polymorphonuclear leucocytosis (interleukin mediated)
  • Sometimes called cervical ripening
  • Associated with reduced collagen concentration, increased water content
  • Key mediators: cytokines, interleukins, nitric oxide -> loss of collagen, release of PGs
23
Q

Hallmarks of labour:

A
  • Involuntary process
  • Intermittant and regular contractions -> fetal axis pressure
  • Typically painful
  • Fundal dominance
24
Q

Average duration of stages of labour in first-time mother (primigravidae):

A
  • 1: 12 -14 hrs
  • 2: 1 - 2 hrs
  • 3: 20-30 mins or 5-15 mins with active engagement
  • 1st and 2nd stage around halved in multigravidae but 3rd stage the same
25
Q

What is cervical effacement?

A
  • Cervix shortens
  • Usually comes before dilation in primips but can be simultaneous in multips
  • Effacement may begin 2-3 weeks before end of pregnancy
  • Occurs as a result of contraction and retraction of uterine muscle
26
Q

What are the 3 main processes in the second stage of labour?

A
  • Contractions (uterine activity)
  • Secondary powers (pelvis)
  • Feta axis pressure
27
Q

How does fetal axis pressure influence maternal anatomy during delivery?

A
  • Fetus displaces soft tissues of pelvis as it descends -> bladder pushed up into abdominal cavity (in front) and rectum flattened into sacral nerve (at back), the levator ani muscles of pelvic floor are thinned and displaced (sideways)
  • Perineal body stretched and thinned
  • Pressure from contractions exert pressure down fetal spine to head -> aids descent
28
Q

What are some presumptive signs of second stage of labour?

A
  • Expulsive uterine contractions
  • Rupture of forewaters (however, baby can still be born ‘en caul’
  • Dilation and gaping of anus
  • Anal cleft line (mechanism unknown)