Repro Session 10 Flashcards
What is parturition?
Scientific term used to describe than transition from the pregnant state to the non pregnant state at the end of gestation
What is labour as a non-scientific term?
Describes part of parturition when both the cervix and uterus have been remodelled
What term might lay people use to mean the same as parturition despite it having a different scientific meaning?
Labour
What is the term used if parturition occurs at >24 weeks gestation?
Spontaneous abortion
What is the term used if parturition occurs at 24-36 weeks of gestation?
Pre-term
What is the term used if parturition occurs at 37-42 weeks of gestation?
Term
What is the term used if parturition occurs at >42 weeks of gestation?
Post-term
Why are pregnancies >42 weeks gestation induced?
To avoid foetal and maternal mortality
What are the 3 stages of labour?
Creation of the birth canal; expulsion of the foetus; expulsion of the placenta and contraction of the uterus
At what stage during gestation is the uterus palpable?
12th week
Where can the fundus be palpated at 20 weeks gestation?
Umbilicus
Where will the fundus extend to at 38 weeks?
Xiphisternum
Why does the fundal height reduce at term?
Foetus adopts position for birth
What is foetal lie?
Relationship of foetal long axis to the long axis of the uterus
What is foetal lie usually?
Longitudinal (foetal and maternal vertebral columns //)
Is the foetus normally flexed or extended?
Flexed
What is foetal presentation?
Which part of the foetal head is adjacent to the pelvic inlet
What term is used to describe a presentation when the head is next to the pelvic inlet?
Cephalic
What does podalic presentation mean?
Foetal buttock is adjacent to pelvic inlet
What does breech presentation describe?
Presentation where anything except the foetal crown is adjacent to the pelvic inlet
What can be said about foetal lie if neither the head nor buttocks is palpable at the pelvic inlet?
Must be either in transverse or oblique lie
Where is the foetal head in oblique lie?
Iliac fossa
Where is the foetal head in transverse lie?
Flank
What is the foetal vertex?
Relationship of foetus along its axis determining the orientation of the presenting part
What is the most common foetal positioning for birth?
Longitudinal lie, cephalic presentation and vertex to pelvic inlet at minimum diameter
What is the largest diameter in normal presentation of the foetus?
AP diameter of head at 9.5 cm
What is the typical diameter of the pelvic inlet?
11 cm
What determines the birth canal diameter?
Pelvic inlet
What does cervical dilatation rely on?
Contractions, pressure of foetal head on cervix, ability of cervix to soften and allow distension
What tissues other than the cervix must change to allow for the second stage of labour?
Soft tissues of vagina and perineum
What causes cervical ripening?
Prostaglandins and hormonal influences
What does the oestrogen:progesterone ratio need to be to allow for the prostaglandins necessary for cervical ripening?
High
What are the effects of the relative drop in progesterone seen in labour?
Cervical ripening and promotion of contractions
What are the results of prostaglandin and hormonal action in cervical ripening?
Decreased collagen, increased glycosaminoglycans, increased hyaluronic acid, decreased aggregation of collagen fibres
What causes the decrease in cervical collagen during cervical ripening?
Expression of collagenases in the cervix
What causes an increase in hyaluronic acid during labour?
Rubbing of foetal head on membranes creating blisters
What is the action of decorin in cervical ripening?
Unwinds collagen strands allowing for further enzymatic degradation
Why do uterine contractions occur throughout reproductive life?
Action of pacemaker cells at the top of the fundus
Can uterine contractions during the normal menstrual cycle be detected by the woman?
No, amplitude is too small
What change in uterine smooth muscles causes progression of uterine contractions?
Thickening
Describe uterine contractions in early pregnancy.
Low amplitude every 30 mins as in the normal menstrual cycle
Describe the uterine contractions seen in mid-pregnancy.
Slightly higher amplitude than normal, progressing to Braxton-Hicks contractions
What are Braxton-Hicks contractions?
‘Practise’ labour contractions that pull together muscle fibres to form a syncytium so that when labour occurs it responds as a whole to one stimulus
What is the difference between Braxton-Hicks contractions and those seen in labour?
In labour there is contraction-retraction where the muscle fibres shorten with each contraction but this does not happen in Braxton-Hicks
When might Braxton-Hicks contractions be mistaken by the mother for labour?
In a first pregnancy
Describe the uterine contractions seen in early labour.
Variable with raised amplitude and reduction in fibre length after each contraction
Describe the uterine contractions seen in late labour.
More frequent and high amplitude
What is the clinical definition of labour?
More than 3 contractions each lasting a minute within the space of 10 minutes
What causes an increase in amplitude of uterine contraction during labour?
Prostaglandin synthesis by the myometrium and decidua of the uterine isthmus increasing calcium movement with each action potential
What increases the frequency of uterine contractions during labour?
Why toxin release from the uterine fundus lowering action potential threshold
What happens in the Ferguson reflex?
Afferent impulses from the cervix and vagina cause positive feedback on the hypothalamus and posterior pituitary resulting in oxytocin release
What is brachystasis?
Where uterine myometrium relaxes less than it contracts causing muscle fibre shortening
What is the result of brachystasis in labour?
Drives presenting part of foetus to the cervix
What causes induction of oxytocin receptors on the uterus?
Oestrogen
What is the result of increased oxytocin receptor number?
Stimulation of uterus to contract and placenta to produce prostaglandins
What happens to the cervix during effacement?
Thins and flattens
What is the latent phase of cervical dilatation?
Slow dilation of cervix up to 3 cm taking several hours
What is the active phase of cervical dilatation?
Much faster rate of cervical dilation from 3 cm onwards with onset of pain and amniotic membrane rupture (if it hasn’t already)
What diameter is achieved by the end of the active phase of cervical dilatation?
10 cm
How long do the latent and active phases of cervical dilatation usually take altogether?
24 hours
How is labour initiated in sheep?
Rise in foetal cortisol –> decreased placental progesterone –> increased prostaglandins and myometrial sensitivity to oxytocin –> high oestrogen:progesterone ratio
Why can the mechanism of labour initiation identified in sheep not be applied to humans?
Human foetuses with congenital adrenal hypoplasia are born and there is no consistent evidence of an increase in the oestrogen:progesterone ratio in humans
How is surfactant linked with the initiation of labour in humans?
Thought that surfactant protein A activates macrophages in the endometrium causing prostaglandin synthesis in the myometrium
How long does the second stage of labour usually take?
Up to an hour
What does the duration of the second stage of labour depend on?
Parity
What does the second stage of labour describe?
Period of full dilatation of the cervix to delivery
What happens to the likelihood of spontaneous delivery of the second stage of labour takes more than an hour?
Decreases
What is the passive stage of labour?
From full dilatation to the desire to push
What happens in the passive stage of labour?
Internal rotation of foetal head and flexion of foetal neck so that presenting part appears in the birth canal
How long does the passive stage of labour last?
Usually a few minutes but can be much longer
What is the active stage of labour?
Mother pushing
What causes an irresistible desire to bear down during the active stage of labour?
Pressure of foetal head on pelvic floor
Why is episiotomy not recommended?
Healing will not be along line of weakness thus increasing the risk of poor healing +/- fistula formation
Describe the passage of the foetus during delivery.
Head delivers in the transverse plane it entered the pelvis, then rotates and extends, shoulders follow and rotate to be delivered with the aid of lateral body flexion anteriorly and posteriorly for everything else to follow
Compare the time taken for delivery of the foetus in nulliparous vs multiparous mothers.
40 vs 20 mins
What is the third stage of labour?
Time from delivery of foetus to placental delivery
How are uterine contractions involved in the third stage of labour?
Their effect is rapidly increased by the expulsion of the foetus so that it contracts hard down to almost its pre-pregnancy size causing shearing of the placenta
How can shearing of the placenta be encouraged?
Gentle tug on the umbilical cord
What do uterine contractions following the expulsion of the placenta reduce the risk of?
Haemorrhage
How can haemorrhage risk be reduced clinically?
Oxytocin drug +/- manual fundal massage
Why is the first cry used clinically to determine establishment of independent life of the foetus?
Shows that the first breath in must’ve been taken in order to cry
What stimuli can cause the first breath to be taken?
Trauma, cold, light, noise
What is the effect on arterial pO2 of taking the first breath?
Increases from 4 kPa to 13 kPa
What does clamping of the umbilical cord lead to in order to establish independent life?
Ductus venosus closure
What does taking the first breath lead to in order to establish independent life?
Reduced tissue resistance in the lungs
What is the implications for blood flow to the lungs by decreased vascular resistance in establishing independent life?
Increases
What causes foramen ovale closure to establish independent life?
p(R heart) decreases and p(LA) increases
What causes closure of the ductus arteriosus?
Pressure imbalance causing a reversed flow that exposes its smooth muscle cells to higher pO2 –> contraction
Why is failure of ductus arteriosus closure non-pathological in the adult?
Pressure differences keep it occluded
What does labour mean when used scientifically?
Physiologic process by which a foetus is expelled from the uterus to the outside world involving sequential integrated changes in the uterine decidua and myometrium
What tends to precede uterine contractions in labour?
Changes to uterine cervix
What are the 3 Ps of labour?
Powers, passenger and passage
What is contraction and retraction?
Contraction of myometrial fibres followed by partial relaxation causing permanent shortening of the fibres
Describe the movement of contractions across the uterus.
Originate from 2 poles moving to the fundus and upper uterus to progress down to the lower segment
Where are forces of myometrial contraction more powerful?
Upper segment of uterus
What is the result of retraction in myometrial contraction?
Decreases uterine capacity resulting in increased intrauterine pressure
What are important passenger considerations in labour?
Size, number and position
What is transverse foetal lie?
Where the long axis of the foetus is perpendicular to that of the mother
What is foetal attitude?
Whether the foetal head is flexed or extended
What is the ideal attitude of a foetus in labour?
Maximal flexion
Describe the foetal position in frank breech presentation.
Buttocks presenting with extended knees
Describe the foetal position in full breech.
Buttocks presenting with 2 flexed knees
What is footling presentation?
Where the foot is adjacent to the pelvic inlet
What causes alteration of the collagen:ground substance ratio in the cervix during labour?
Increased levels of relaxin
From what time is enzymatic degradation of collagen in the cervix evident?
36 weeks
How does enzymatic degradation of collagen in the cervix result in cervical effacement and dilatation?
Reduction of resistance to presenting part
What causes effacement of the cervix in mid-late pregnancy?
Oestrogens, prostaglandins and relaxin
What causes effacement and dilatation of the cervix in late pregnancy to term?
Retraction of myometrium
Describe the gynecoid pelvis shape.
Wide pelvic inlet and forepelvis, straight side walls, wide pelvic outlet (subpubic arch)
Describe the anthropoid pelvic shape.
Narrow transverse inlet, wide AP inlet, divergent forepelvis, narrow side walls
Describe the android pelvic shape.
Narrow forepelvis, convergent side walls, narrow pelvic outlet
What population is the platypelloid pelvic shape commonly seen in?
African females
What is the impact on engagement of the foetus in mothers with a platypelloid pelvis?
Late
Describe the platypelloid pelvic shape.
Narrow AP diameter, straight forepelvis, wide side walls and pelvic outlet
What forms an almost transparent membranous structure seen in the maternal pelvic floor during childbirth?
Stretching of levator ani fibres and thinning of the central perineum
What can be carried out if the process of labour gets stuck?
Caesarean section, forceps or vacuum extraction
When does placental separation occur?
3rd stage of labour
How long does placental separation usually take?
5-15 mins but anything up to an hour is considered normal
What causes a huge decreases in uterine size following delivery of the foetus?
Ongoing contraction and retraction
What is the effect of a reduction in the placental site size due to contraction and retraction of the uterus?
Inelastic placenta is squeezed
What happens to the blood in intervillous spaces during placental separation?
Forced back into veins of decidua basalis
What inhibits drainage of blood from the intervillous space into the maternal circulation?
Retracted uterus
Describe the veins in the decidua basalis during placental separation.
Tense and congested under pressure by underlying muscle
What completed placental separation as contraction and retraction continue?
Blood tracking between placenta and decidua
Why must bleeding during placental separation be controlled?
Normal blood flow is 10-15% of maternal cardiac output
What forms ‘living ligatures’ in placental separation?
Contraction and retraction causing interlocking of muscle fibres
What applies pressure on the placental site once placenta and membranes have been delivered?
Apposition of uterus
What occurs in the sinuses and torn vessels of placental separation that limits bleeding?
Blood clotting
What is the most common cause of maternal mortality?
Pre-eclampsia/eclampsia
Give some causes of maternal mortality other than pre-eclampsia/eclampsia.
PPH, obstructed labour, unsafe abortion, pulmonary TB and HIV, ruptured uterus
What does rupture of foetal membranes result in that works with phospholipase to increase prostaglandin levels?
Disruption of decidual and membrane lysosomes
What increases excitability of uterine musculature during labour?
Changes to oestrogen:progesterone ratio, oxytocin from maternal posterior pituitary and foetus
What increases uterine muscle contractility in labour?
Mechanical stretching of smooth muscle, oestrogen increasing gap-junctional communication between cells
What can inhibit progesterone during labour?
Foetal glucocorticoids and placenta
What stimulates prostaglandin synthesis?
Increased oestrogen:progesterone ratio and mechanical damage
What structures can synthesis prostaglandins?
Placenta, decidua and myometrium
Why are prostaglandin levels raised in the amniotic fluid early in labour?
Amnion increases synthesis during T3
What inhibits oxytocin action during pregnancy?
Progesterone, relaxin, low receptor number
What happens at 36 weeks that allows the uterus to respond to pulsatile release of oxytocin from the maternal posterior pituitary gland?
Increased number of receptors in myometrium
What does completion of cervical ripening stimulate in the hypothalamus?
Stimulation of posterior pituitary to release oxytocin
