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
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)
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)
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)
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
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
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)
8
Q
What 3 types of stimuli support initiation of labour?
A
- Mechanical
- Hormonal
- Inflammatory
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
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
11
Q
What muscle layers surround the uterus?
A
- Inner layer: latitudinal (sphincter action downwards)
- Middle layer: spirals
- Outer layer: longitudinal (squeezes down)
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
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
14
Q
How does increasing oestrogen concentration in late pregnancy influence the myometrium?
A
- Promotes formation of oxytocin receptors
- Promotes formation of MGJs
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)
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
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
26
What are the 3 main processes in the second stage of labour?
* Contractions (uterine activity)
* Secondary powers (pelvis)
* Feta axis pressure
27
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
28
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)
