Lecture 15: Normal Birth Flashcards
What are the two components that define labour:
Uterine activity and cervical change which leads to the expulsion of the fetus and placenta
Contraction (uterine) + Shortening + Dilation (Cervical)
What are the key players of parturition?
Uterus
Cervix
Hormones
Why is the uterus amazing?
- Relaxed & quiescent - but massive growth
- Decides when to start labour
- Highly coordinated, forceful activity (but with relaxation to allow sustained blood flow)
- Remains contracted (allows sufficient tension on cervix)
What are the layers of myometrium?
Outer layer (long. fibres) Middle layer (mesh-like fibres) Inner layer (circular fibres)
Describe the structure of myometrium: what enhances or decreases contractile force:
Interaction between actin and myosin filaments cause contraction (Ca dependent)
Oxytocin and prostaglandins promote Ca uptake into cell / freeing from intracellular stores
Progesterone, cAMP, b adernergic agents inhibitory
What triggers myometrial activity:
Myometrium is spontaneously active smooth muscle, produce regular contractions without hormonal or nervous input
Myocyte contraction are phasic in nature, describe what influences this:
- Resting tone with cycles of discrete, intermittent contractions
- Varying frequency, amplitude and duration
- Contractions are modulated by
- > Cell surface receptors
Coordination of contractions facilitated by GAP junctions
Describe the effect of progesterone, estrogen, oxytonin, prostaglandins on the uterus:
Progesterone, prostaglandins (PgI2) is a relaxant.
Estrogen, oxytonin, prostaglandins (PgE2, F2) are all uterotonic.
What are the physiological processes of labour:
Quiescence -> Activation -> Stimulation -> Involution
What are the anatomical changes associated with physiological processes of labour:
Quiescence: Myocyte hyperplasia/trophy. Ligametns laxity, cervix softens
Activation: Lower segments form
Stimulation: Cervix shortens and effaces, membranes rupture, coordinated uterine activity.
Involution: Retraction, remodelling
Describe the changes of the uterus in pregnancy:
- Myometrium is tranquil
- Increase in capacity (volume)
- First weeks of pregnancy: Myometrial hyperplasia - then, from stretch-induced myometrial activity
Describe the physiological changes seen in quiescence:
- Uterine contractions poorly synchronised.
- Low amplitude
- Low Hz, ‘painless’ Braxton Hicks
- Cervix softens but remains firm and closed
What factors contribute to quiescence:
Progesterone is the major contributor
What is the goal of activation in labour?
Priming for labour
- Myocytes becomes more responsive
- Occurs without servical change
- Last 6-8 weeks of pregnancy
Myocytes become more responsive in the physiological process known as activation in labour, describe the upregulation of myometrium-1 and how this contributes:
Increase in contractile associated proteins (CAPS)
i.e oxytocin and Pg receptors
CAPS control contractility of myocytes
- Enhanced interactions between actin and myosin
- Increase excitability of ind. cells
- Promote crosstalk between muscle cells
Myocytes become more responsive in the physiological process known as activation in labour, describe the upregulation of myometrium-2 and how this contributes:
- Gap junctions allow AP to propogate through myometrium
- Many types by connexin 43 most important
Whats the role of progesterone in labour?
Maternal/fetal/amniotic conc. elevated through birth
- > Removing CL and thus Prog actually induces preg. loss
- > Prog. treatment from early (but not late) pregnancy can reduces rates of PTB in well selected women
- > Giving progesterone antagonist causes cervical effacement and labour…
Progesterone promotes myometrial relaxatation and quiesence. Therefore its withdrawal is essential for all functions. But must be timed.
What is the role of oxytocin?
- Not essential for induction of labour
- Its conc. doesnt actually change, but there is a 200 fold increase in receptor concentration. It promotes contraction of myometrium.
What is the role of estrogen in labour?
- uterotonic but does not directly cause contractions.
- Upregulates gap junctions and uterotonic receptors. (inc. oxytocin)
- Estrogen produced in placenta, but placenta lacks precursors (fetal adrenals provide abundant DHEAS)
= Fetus has role in myometrial priming and sitmulation
What is the role of prostaglandins and labour?
PG E2/F2 uterotonic / cervical ripening.
Its likely to contorl the final pathway of onset of labour.
- > Increase in uterine synthesis at onset
- > Administration results in labour
- > Inhibition of PG synthesis suppresses myometrium
Summarise what the stimulation physiological stage of labour is:
A release from inhibitory effect of pregnancy on myometrium “the breaks are off”
Describe the physiological stimulation phase of labour
- Mature fetus, timing right.
- Pelvic ligaments softened
- Uterus excitable and contractions coordinated.
- Cervix soft and easily dialted
- Membranes may rupture (more PGs and possible mechanical benefits)
PROSTAGLANDINS FACILITATE THIS
What must happen for activation to become stimulation?
A positive feedback cascade i.e prostaglandins
What facilitates the stimulation stage of labour?
Prostaglandins
Describe the ripe cervix:
- Collagen degrades
- Increased spacing between fibres
- Wavy fibres, allow immune cell infiltration
- Glands proliferate
Prostaglandins are important in labour, what is the implication of this from a generic viewpoint? What bigger picture are they a part of?
Inflammation is important in labour
What role do the membranes play in labour?
- Retain strength i.e collagen
- Site of PG production
- MMPs degrade collagen and weaken - PGs, Inflammation, stretch, shear force, increase production
I.e rupturing membranes increase PG production
What triggers the positive feedback cascade that leads to the stimulation phase?
Unsure, but in essence procontraction things outweight anti-contraction signals.
How might time of day influence labour?
Melatonin (spike in night pregnancies)
How can fetal stress influence labour?
Fetal stress -> ACTH -> cortisol -> Dec. placental;
(progesterone) Increases placental (estrogen, prostaglandins) -> this leads to contraction and retractions of myocytes -> stretch on cervix / lower uterus
+ive feedback to release oxytoinin = Inc contractions and retractions of myocytes
insert station dilation slide
39
What is involution?
- Placental separation
- Cleave through the decidua basalis
- Contractions to prevent postpartum haemorrhage
- Increased uterine sensitivity to oxytonin
Later ‘involution’ returns woman to non-pregnant state
How can one help induce labour?
- Progesterone antagonists
- Prostaglandins
- > might be caused by artificial ruptured membranes, stretch and sweep or giving PGE2
How can a stalled labour be helped?
Nipple stimulation
Support person
Amniotomy
Syntocinon
How can post partum haemorrhage be prevented?
- Oxytonin
- Prostaglandin
- Carboprost
How can pre-term labour be prevented?
Vaginal progesterone
Cervical cerclage (stitches)
Treating infection
How can contractions be slowed?
Block Ca channels
Beta agonists i.e salbutimol
Block oxytonin receptors
Block PG synthesis i.e indomethacin
What is the key hormone putting breaks on myometrium?
Progesterone
How is activation acheived?
Activation/priming involves upregulation of gap junctions and CAPS (oestrogen/oxytonin receptors)
How does stimulation occur?
With removal of factors that promote quiescence
- Progesterone withdrawal, inflammation, fetal signals, stretch
What is the stimulation cascade characterised by?
- Postive feedback loop, resulting in sychronous contractions
- Changes of the composition of the cervix mean that it dilates as the fetal head descends
Insert diagram of shunting
Slide 48
