Physiology of 1st Stage Flashcards
Describe the uterus during pregnancy
- Uterus begins to grow by hyperplasia and then after 4 months hypertrophy
- Myometrium consists of bundles of myometrial cells separated by connective tissue
What is hyperplasia?
Increased amount of organ tissue due to cell proliferation
Describe the uterus at term
- Muscle fibre density highest in fundus, reducing until cervix where there is more connective tissue than muscle
- Uterine muscle consists of longitudinal, circular and spiral muscle fibres (used as ligatures)
What does the contractile strength of the uterus relate to?
- The proportion of muscle
- Upper segment contracts stronger to push baby down
Describe the 2 segments of the uterus
- Towards the end of pregnancy
- Upper segment = formed from fundus body
- Lower segment = formed from isthmus and cervix
What is the isthmus?
Narrow passage/ organ that connects 2 larger parts
What is quiescence?
Uterine muscle has spontaneous contractibility and is never completely quiet so low intensity contractions always occur
What happens approximately 6 weeks prior to labour?
Intensity of quiescence increases; these are called Braxton-Hicks and are not associated with cervical effacement or dilation
What is cervical effacement?
Shortening, softening and thinning of the cervix
What are the 2 main functions of the uterus?
- To grow but remain quiescent (inactive)
- To commence powerful contractions at the right time
What might women notice at the end of pregnancy?
- Mood swings/ surges of energy
- Walking may become more difficult
- Relief of pressure at fundus
- Lightening
- Increased pressure in pelvis
- Nesting
What is lightening?
SFH starts to reduce as baby starts to descend towards the birth canal
Why does dilation occur?
As a result of uterine action and the counter-pressure applied by the intact bag of membranes or presenting part (or both)
How does the cervix change prior to labour?
- Cervix is rigid in pregnancy
- Connective tissue will soften
- Partial dilation of external os evident from 24 weeks but individuals vary = cervical assessment unreliable indicator of labour
- At term, 90% of cervix is water
What are the 2 elements of cervical softening?
- Increased vascularity and water content
- Structural changes in connective tissue
Describe effacement
- If softening has taken place, contractions pull on cervix, stretching it
- Effacement takes place before regular contractions
- Shortens and thins cervix so both os disappear
- Leads to inclusion of cervical canal
- Operculum (mucous plug) becomes dislodged
- Longitudinal fibres allow cervical dilation without presenting part pressure
What is a ‘show’?
Blood-stained mucoid discharge (operculum) seen in early labour and small loss of red blood during transitional stage
What happens if an unripe cervix attempts to dilate?
Can cause damage to collagen fibres which can lead to miscarriage
What hormones are involved in the initiation of labour?
- Cortisol
- Progesterone
- Oestrogen
- Prostaglandins
- CRH
- Oxytocin
- Relaxin
What is CRH?
Corticotrophin Releasing Hormone
What effect does cortisol have?
- Produced by anterior pituitary of foetus
- Production increases towards term
- Affects and reduces maternal progesterone production
What effect does progesterone have?
- Inhibits contractions in pregnancy
- Local changes in concentration not reflected in maternal blood; foetal membranes increase cortisol levels to reduce progesterone
What effect does oestrogen have?
- Slight rise in levels makes uterus more sensitive to oxytocin at term (receptors become unblocked and more sensitive)
- Stimulates oxytocin receptors in myometrium and gap junctions to form
- Encourages placenta to release prostaglandins to soften and efface cervix
Describe the oestrogen:progesterone ratio
- Changing ratio of oestrogen and progesterone is important for effective contractions in labour
- Increasing oestrogen/ decreasing progesterone leads to release of phospholipase A2 which releases arachidonic acid which stimulates prostaglandin synthesis
What effect do prostaglandins have?
- Occur in placenta, foetal membranes, decidua, myometrium and cervix
- Important in labour progress
- In late pregnancy, prostaglandin synthesis stimulated by coitus, VE, membrane sweep, amniotomy and labour
- Exogenous prostaglandins will ripen the cervix and induce labour
What is an amniotomy?
Artifical rupture of membranes
What effect does oxytocin have?
- Used to induce labour
- Endogenous production by nipple stimulation can initiate labour
- Receptors in myometrium increase in pregnancy
- Rapid production in labour
- Also synthesised by decidua
What is the Ferguson reflex?
Pressure in the vagina/ cervix increases oxytocin production and causes the uterus to contract/ retract
What effect does relaxin have?
- Inhibits myometrial contractility and softens joints
- Early fall in levels (high in 1st trimester)
- Associated with preterm labour
- Promotes cervical ripening towards end of pregnancy
What factors may initiate parturition?
- Response to environment
- Familial (genetic)
- Time of lunar month/ ovarian cycle
- Circadian rhythm (sleeping pattern)
- Mammals labour best at time of day they are usually resting (at night)
What is parturition?
Childbirth
When should the midwife be contacted?
When regular, rhythmic uterine contractions are experienced that are uncomfortable and/or painful
Describe myometrial contractility
- Myometrial cells packed with long bundles of actin and myosin; slide past each other to enable muscle shortening for contractions
- Non-pregnant uterus contracts in response to adrenaline/ noreadrenaline
- Noreadrenaline causes contractions, adrenaline inhibits contractions
- Paraplegies go into labour so contractions are under hormonal control not nervous
What are gap junctions?
- Gaps that pass messages between myometrial fibres
- Form from bundles of protein called connexions
- Allow rapid transmission due to reduced electrical resistance = fibres contract together
Describe gap junctions
- Number of gap junctions increases throughout pregnancy to 1000/cell
- Greater density of gap junctions if labour occurs spontaneously at/ before term
- More gap junctions = larger area of muscle contracting = greater intrauterine pressure changes
What pressure changes occur during contractions?
- Increase in pressure of 20mmHg can be felt manually
- Pressures can rise as high as 75mmHg in 2nd stage
Describe how contractions occur
- Involuntary
- Amount of elastin rises throughout pregnancy; this enables contraction and retraction during labour and after birth
- Upper segment becomes gradually shorter and thicker
- Contractions strongest at top of fundus
- Must be rest between contractions to allow myometrium and foetus to be reoxygenated
What is retraction?
Unique property of uterine muscle, to remain slightly shortened following a contraction
Describe the coordination of contractions
- Uterus exhibits pacemaker-like activity
- Contractions begin from each side of fundus near the cornuae
- Contractile waves strongest and last longer at fundus where there is highest density of muscle fibres
- Peak is reached simultaneously all over uterus and then fades together - known as fundal dominance
Describe the polarity of the uterus
- 2 segments of uterus work in neuromuscular harmony during contractions
- Upper pole (segment) contracts strongly and retracts to expel foetus
- Lower pole (segment) contracts slightly and dilates to allow expulsion
What is the retraction ring?
- Ridge formed between upper and lower segments
- Normal for all labour, but if exaggerated ridge is noted abdominally above symphysis, can be sign of mechanically obstructed labour; known as Bandl’s ring
How are the forewaters and hindwaters formed?
- Chorion detaches from uterine wall due to stretching of lower segment and effacement
- Membranes extruded through cervical opening
- Head of foetus acts as ‘ball valve’
- Presenting part separates fore and hind waters
What is foetal axis pressure?
- Forewaters spread pressure of contractions over cervix’s aiding dilation
- Hind waters cushion baby from contractions
- Force of contractions transmitted through foetal body to cervix is called foetal axis pressure
Describe membrane rupture
- Frequently occurs when cervix between 8-10cm as bag of fore waters descends
- Also associated with collagen degradation of membranes
- Increases prostaglandin release
- Associated with onset of stronger, coordinated contractions
What must be the situation for labour to progress well?
- Soft cervix
- Adequate oxytocin receptors
- Adequate gap junctions
- Adequate prostaglandin production
- Mother not stressed
What would aid the progress of labour?
- Well-fitting presenting part on cervical os
- Foetus in optimal position
- No obstruction to foetal descent
Describe effective contractions
- Regular with relaxation inbetween
- Gradually increasing strength and frequency
- Frequent but <4 / 10 mins
- Long-lasting but not over 1 min in 1st stage
What is an LSCS?
Lower Segment Caesarean Section
What is augmentation?
Labour starts naturally but help is needed to make labour progress and have stronger contractions
What is os short for?
Orifice
