Parturition is transition from the pregnant to the non-pregnant state (birth)
Labour is the physiologic process by which a fetus is expelled from the uterus to the outside world
Delivery is the method of expulsion of the fetus, transforming the foetus to neonate
Identify the 3 stages of labour
- First stage: creation of the birth canal
- Second stage: descent of foetus and delivery
- Third stage: delivery of placenta
Describe the first stage of labour physiologically and clinically
- Physiologically: multiple changes resulting in creation of the birth canal and descent of the fetal head
- Clinically: interval between onset of labour and full dilatation of the cervix
Identify and describe the two phases in the first stage of labour
- Latent: onset of labour with slow cervical dilatation but softening (variable duration)
- Active: faster rate of change & regular contractions
Describe the second stage of labour physiologically and clinically
- Physiologically: changes in uterine contractions to expulsive, descent of the foetus through the birth canal and delivery
- Clinically: the time between full (10cm) dilatation of the cervix and delivery
Identify the passive and active efforts involved in the second stage of labour
- Passive – descent and rotation of the head
- Active – maternal effort to expel the foetus and achieve birth
Describe the third stage of labour physiologically and clinically
- Physiologically: expulsion of the placenta and contraction of the uterus
- Clinically: starts with completed birth of the baby and ends with complete expulsion of placenta and membranes
How long does the third stage of labour take?
Usually lasts between 5 and 15 minutes (may be 30-60 min in certain circumstances)
In 5 steps, describe the mechanism of labour
⇒ Head flexion and internal rotation
⇒ Crowning – head stretches through perineal muscle and skin
⇒ Head extension and external rotation
⇒ Shoulders rotate and deliver
⇒ Body rapidly delivers afterwards
How does labour start?
Labour is initiated by cervical “ripening” wherein the uterine musculature becomes progressively more excitable
What causes cervical ripening?
Cervical ripening is due to oestrogen, relaxin and prostaglandins (PGE2, PGF2α) breaking down the connective tissue
In 4 steps, describe the physiological process of cervical ripening
⇒ Reduction in collagen
⇒ Increase in glycosaminoglycans
⇒ Increases in hyaluronic acid
⇒ Reduced aggregation of collagen fibres
What promotes labour in humans?
Prostaglandins promote labour
What are prostaglandins and what do they do?
Prostaglandins are powerful contractors of smooth muscle and are also involved in cervical softening
Describe the structure, production and control of prostaglandins
- Structure: biologically active lipids (local hormones)
- Production: placenta, myometrium and decidua
- Control: production controlled by oestrogen:progesterone ratio
What stimulates prostaglandin synthesis?
- Increase in oestrogen: progesterone ratio and mechanical damage stimulates prostaglandin synthesis
- Increased synthesis of prostaglandins by amnion in third trimester and prostaglandin levels rise very early in labour
Explain how a rise in the oestrogen:progesterone (relative decrease in progesterone) stimulates myometrial contractility
- Progesterone inhibits contractions
- Oestrogen increases contractility by increasing gap junctional communication between smooth muscle cells
Describe the function and inhibition of oxytocin
- Oxytocin initiates uterine contraction
- Inhibited in pregnancy by progesterone, relaxin and a low number of oxytocin receptors
Describe the secretion and control of oxytocin
- Secreted by posterior pituitary
- Controlled by hypothalamus
Describe the processes facilitating the release and action of oxytocin
- Increased by afferent impulses from cervix and vagina (Ferguson reflex)
- Acts on smooth muscle receptors in myometrium
- More receptors if oestrogen:progesterone high
Cervical effacement and dilation are required to create the birth canal.
What does this involve?
- Dilation is the opening of the cervix (measured in centimeters)
- Effacement is the thinning of the cervix (measured in percentage)
What is the birth canal?
The birth canal is the passage through which the foetus passes during birth, formed by the cervix, vagina, and vulva
How do we assess the birth canal?
- Look at size of birth canal
- Normal presentation diameter = 9.5 cm
Changes to which structures are necessary to form the birth canal?
- Pelvic floor
Describe the changes which occur to form the birth canal
The stretching of the fibres of the levator ani and the thinning of the central portion of the perineum transforms it to an almost transparent membranous structure
Describe the special properties of the uterine smooth muscle which facilitate labour
- Myometrial fibres contract but only partially relax
- Myometrial muscle does not return to its original size after contraction (retraction)
What is the effect of the contraction and retraction of the myometrium during labour?
Permanent partial shortening of the muscle fibres leads to a progressive reduction of uterine capacity and a build up of pressure
Foetal attitude refers to the relation of the foetal head to the spine.
Identify different some types observed