10) Birth Flashcards
Define the first stage of labour
Creation of birth canal Onset of labour > Full cervical dilation Latent phase - Onset > ~4cm dilation (slow) Active phase - Faster rate of cervical changes, 1-1.2cm/hour - Regular uterine contractions
Define the second stage of labour
Expulsion of foetus (up to 1hr, can be very fast) Urge to bear down Descended head flexes as it reaches pelvic floor - reduces presentation diameter Internal rotation Head stretches vagina & perineum - risk of tearing, episiotomy Head delivered, head rotates & extends Shoulders rotate & deliver Rest of baby rapidly follows
Define the third stage of labour
Expulsion of placenta
- sheared off by strong contractions of uterus
Contraction of uterus
- compresses blood vessels to reduce haemorrhage
Lasts 5-15 mins
What normal process is necessary to create a birth canal?
Expansion of soft tissues (up to 10cm)
Cervix - At some point of dilation, foetal membranes rupture, releasing amniotic fluid
Vagina
Perineum
Requires structural changes & a lot of force
What determines the size of the birth canal?
Normal presentation, head biggest part - diameter 9.5cm Max size determined by pelvis - pelvic inlet typically 11cm - softening of ligaments may increase this
What is the cervix made of?
Tough, thick collagen
Coiled to give greater structural strength
The cervix needs to soften for birth to occur. What is this called?
Cervical ripening
What is cervical ripening?
Cervix collagen in a proteoglycan matrix
Involves:
Reduction in collagen production (turnover altered)
Increase in glycosaminoglycans (disrupts the matrix)
Reduces aggregation of collagen fibres (uncoils)
What is cervical ripening triggered by?
Prostaglandins
PG E2 & F2x
Locally diffused from uterus
Once released, cervix is ready to be stretched
What happens to the myometrium during cervical ripening?
Smooth muscle, greatly increased in pregnancy
Force generated when intracellular [Ca2+] rises
Due to action potentials (bursts generate sustained contraction)
Triggered spontaneously (pacemakers)
Describe the uterine contractions throughout pregnancy
Progesterone suppresses myometrium’s contraction
Prevents labour occurring until proper time
Early - Low amplitude, every 30 mins
- generally speaking, mother not aware
Late - Higher amplitude, less frequent
- ‘Braxton-Hicks’ contractions
How are the contractions of the uterus made more forceful & frequent?
Prostaglandins (ripen cervix as well) - Increased intracellular [Ca2+] per action potential Oxytocin - More APs - Threshold lowered
What are prostaglandins?
Biologically active lipids, local hormones Produced mainly in endometrium Production controlled by Oestrogen:Progesterone ratio Progesterone>Oestrogen - Low prostaglandins - Throughout pregnancy Oestrogen>Progesterone - Increased prostaglandins - End of pregnancy
Where is oxytocin release from?
Secreted by posterior pituitary (controlled by hypothalamus)
Secretion increased by afferent impulses from cervix & vagina
- Ferguson reflex
- Mechanical stimulation of cervix due to prostaglandins increases oxytocin
Acts on smooth muscle receptors
- More receptors if Oestrogen>Progesterone
What is brachystasis?
Uterine smooth muscle relaxes less than it contracts
Unique feature of uterine smooth muscle
Fibres shorten in body of uterus
Drives presenting part of foetus to cervix
