L7 Labour Flashcards
Outline the stages of labour:
- 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
When does labour occur?
- 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)
What two major physiological changes are important for expelling the fetus?
- 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)
Describe the ideal positioning of baby and placenta before delivery:
- 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)
What is known about the cause of labour?
- 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
Why is it important to maintain myometrial quiescence during pregnancy?
- 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
What hormones are involved in maintaining myometrial quiescence during pregnancy and what are their mechanisms in this process?
- 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)
What 3 types of stimuli support initiation of labour?
- Mechanical
- Hormonal
- Inflammatory
Outline the key signals for activation of the myometrium (mechanical factors for labour induction):
- 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
Outline the composition and function of the myometrium (how is it different to normal smooth muscle?):
- 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
What muscle layers surround the uterus?
- Inner layer: latitudinal (sphincter action downwards)
- Middle layer: spirals
- Outer layer: longitudinal (squeezes down)
Myometrial gap junctions: basic structure and function, stimulation and inhibition
- 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
What is oxytocin and where is it stored?
- Hormone and neurotransmitter
- Produced by hypothalamus, passes down neuronal axons and is stored in posterior pituitary until required
How does increasing oestrogen concentration in late pregnancy influence the myometrium?
- Promotes formation of oxytocin receptors
- Promotes formation of MGJs
Outline the Ferguson reflex:
- 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)
What hormones does the placenta secrete to induce labour?
- 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
How may the fetus be involved in communicating with the placenta and uterus to initiate labour?
- 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
How does oxytocin enact its neurotransmission?
- Floods MGJ
- Activates receptor-operated calcium channels
- Ca2+ release into myometrial cells -> contraction of myocyte
What are prostaglandins?
- Inflammatory markers and chemical mediators
- Synthesised on site -> local effect
- Strong myometrial stimulants, enhancing neurotransmission of oxytocin
- Also important in cervical ripening
When are PGs released?
- 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
Give 3 key inflammatory mediators in labour onset with their main function:
- Cytokines: Stimulate synthesis of PGs
- Interleukins: Increase collagenic activity of cervix
- Nitric oxide: stimulate release of PgE2 from fetal membranes
What inflammatory changes take place in cervix before labour?
- 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
Hallmarks of labour:
- Involuntary process
- Intermittant and regular contractions -> fetal axis pressure
- Typically painful
- Fundal dominance
Average duration of stages of labour in first-time mother (primigravidae):
- 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
What is cervical effacement?
- 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
What are the 3 main processes in the second stage of labour?
- Contractions (uterine activity)
- Secondary powers (pelvis)
- Feta axis pressure
How does fetal axis pressure influence maternal anatomy during delivery?
- 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
What are some presumptive signs of second stage of labour?
- Expulsive uterine contractions
- Rupture of forewaters (however, baby can still be born ‘en caul’
- Dilation and gaping of anus
- Anal cleft line (mechanism unknown)