Labour And Delivery Flashcards
What is parturition?
Transition from pregnant to non-pregnant state (birth)
What is labour?
Physiological process by which a foetus is expelled from the uterus to the outside world
What is delivery?
The method of expulsion of the foetus, transforming foetus to neonate
What changes (vaguely) are involved in labour?
Involves sequential integrated changes in uterine decidua (internal lining) and myometrium
Changes in the cervix tend to precede uterine contractions.
What two things do the contractions during labour need to achieve?
Dilation of the cervix
Pushing the foetus through the birth canal.
What happens in the first stage of labour?
Creation of the birth canal and descent of the foetal head into it.
It is the time period between the onset of labour and full dilation of the cervix.
There are two phases:
- Latent: onset of labour with slow cervical dilation but softening. This lasts a variable amount of time.
- Active:Faster rate of change and regular contractions.
What happens in the second stage of labour?
Changes in uterine contractions to expulsive, descent of foetus through the birth canal and delivery.
Clinically this is the time between full (10cm) dilation of the cervix and delivery.
There are both passive (descent and rotation of the head) and active (maternal effort to expel the foetus and achieve birth) parts to this.
What happens in the third stage of labour?
expulsion of the placenta and contraction of the uterus.
The third stage starts with the completed birth of the baby and ends with the complete expulsion of placenta and membranes.
Usually lasted between 5-15 mins, Up to 30-60 mins may be normal depending on circumstances.
How does labour start?
Uterine musculature becomes progressively more excitable
Why does labour start?
Prostaglandins promote labour but we don’t know if it is associated with a rise in cortisol like labour is in animals.
What are prostaglandins?
Biologically active lipids
Local hormones
Produced mainly in myometrium and decidua
Production controlled by oestrogen:progesterone ration
What role do prostaglandins have in labour?
Powerful contractor of smooth muscle and are also involved in cervical softening.
Increase in oestrogen:progesteone ratio and mechanical damage stimulates prostaglandin synthesis.
Placenta, decidua, myometrium and membranes cal all synthesis prostaglandins.
Increased synthesis of prostaglandins by amnion in third trimester.
Levels of prostaglandins in amniotic fund rise very early in labour.
Cervical ripening is due to oestrogen, relaxin and prostaglandins breaking down the connective tissue.
What role does progesterone have in labour?
Inhibits contractions - so a fall in progesterone levels facilitates myometrial excitability.
What effect does oestrogen have in labour?
Oestrogen increases gap junctional communication between smooth muscle cells - increases contractility.
What effect does oxytocin have in labour?
Initiates uterine contractions.
Action inhibited in pregnancy by progesterone, relaxin and low number of oxytocin receptors.
Pregnancy = increased number of gap junctions to aid communication between muscle cells and coordinate uterine activity.
At 36 weeks there is an increased number of oxytocin receptors in the myometrium - therefore the uterus can respond to pulsatile release of oxytocin from the posterior pituitary gland.
How oxytocin controlled?
It is secreted by the posterior pituitary and controlled but the hypothalamus.
What other factors can help to trigger labour?
Cervical stretching releases prostaglandins.
Foetal effects?
Infection, bleeding - feed into the central mechanisms triggering contractions.
How does the cervix change during labour (broadly)?
Cervix retains foetus for most of pregnancy - tough, thick ad made of collagen.
Needs to soften - ‘cervical ripening’ - This will make it soft and open
How and why does the cervix rip?
The cervix is made from collagen in a proteoglycan matrix.
Ripening is triggered by prostaglandins (PGE2 and PGF2a) and involves: -Reduction in collagen -Increase in GAGs Increase in hyaluronic acid Reduced aggregation of collagen fibres
What cervical changes occur during labour (more detailed)?
The cervix undergoes effacement (thinning) and dilation
Relaxin increases which causes changes in the cervix - collagen ground substance ratio, enzymes degrade collagen.
This occurs over a period of weeks - it is evident from week 36
It means that the labour service offers less resistance to the presenting part.
How does the size of the birth canal change?
In a normal presentation the head is the biggest part and the diameter of presentation is 9.5cm.
Maximum size of birth canal is determined by the pelvis - pelvic inlet is typically 11cm but, softening of the ligaments may increase it.
How does the pelvic floor, vagina and peineum change?
The stretching of the fibres of the lavator ani and the thinning of the central portion of the perineum transforms to almost transparent membranous structure.
How does myometrium help labour?
Contraction and retraction
Myometrial fibres contract but only partially relax. This means that the myometrial muscle does not return to its original size and causes permanent partial shortening of the muscle fibres.
This means that the uterine capacity is progressively reduced so the pressure inside the uterus become stronger and stronger.
How is the force generated to push baby out?
The myometrium is much thickened in pregnancy.
Force is generated as the intracellular [Ca2+] rises due to action potentials. It is triggered spontaneously ‘pacemakers’.
