8. Parturition Flashcards
what are the 2 aims of contractions during labour
- cervix dilation
2. push foetus through birth canal
describe in general terms the 3 stages of labour
Stage 1 (activation):
- interval between onset of labour and full dilation of cervix (10cm)
- involves creation of birth canal and descent of foetal head into it
Stage 2 (stimulation):
- time between full dilation of cervix and delivery
- involves changes in uterine contractions to expulsive, descent of foetus through birth canal and delivery
Stage 3 (involution):
- from completed birth of baby to complete expulsion of placenta and membranes
- involves expulsion of placenta and sustained contraction of uterus (for haemostasis and reducing size)
which 2 processes must occur for creation of birth canal
- increased myometrial excitability and uterine contractions
- cervical ripening
which hormone stimulates uterine contractions - where is it released from
OXYTOCIN - pulsatile release from posterior pituitary gland (controlled by hypothalamus)
how are the actions of oxytocin inhibited until term
- progesterone
- relaxin
- low no. of oxytocin Rs in myometrium
describe 4 changes that must occur to increase myometrial excitability and initiate contractions
- small progesterone decrease towards end of term
- increased oestrogen:
- increase SM gap junctions (communication)
- increase PG synthesis
- increase myometrium oxytocin Rs - mechanical stretching of uterine SM as gestation advances - releases PGs
- increased prostaglandins:
- increase SM gap junctions (communication)
- increase calcium influx across membrane and Ca release from intracellular stores
- increase myometrial sensitivity to oxtocin
what is the Fergusson reflex
i. PG and oxytocin stimulate uterine contractions…
ii. contractions stimulate PP release of oxytocin and placental release of PGs
= positive feedback loop
what is cervical ripening
During late pregnancy (wks 36-40), cervix gradually softens: thins, shortens and draws up (= effacement) under influence of oestrogen, prostaglandins and relaxin.
Changes involve decreased collagen/ground substance ratio:
i. increase hyaluronic acid content… increase water
ii. enzymatic collagen degradation
iii. decreased bridging between collagen fibres
what causes ripened cervix to dilate
uterine contractions
what is the birth canal - which structures does it include
Potential space through which foetus is delivered. Includes:
- pelvis - determines max size (inlet typically 11 cm)
- soft tissues - cervix, vagina and perineum
why do uterine contractions allow pressure inside the uterus to increase
Myometrial fibres contract but only partially relax… permanent partial shortening of muscle fibres allow contraction and retraction… uterine capacity progressively reduced… increased pressure
`which 3 features describe foetal presentation
- Lie: longitudinal or transverse
- Attitude: flexion or extension
- Presentation:
- frank breach (butt 1st feet by head)
- full breach (butt 1st knees crossed)
- footling breach (1 foot 1st)
what are the active and passive features of stage 2
Passive = descent and rotation of head
Active = maternal effort to expel foetus and achieve birth
which features stimulate placental separation
- foetal delivery causes marked reduction in uterus size due to ongoing powerful contraction and retraction… decreased size of placental site… inelastic placenta sqeezed by contraction… blood tracks between placenta and decidus, completing separation
name 3 factors which minimise post-partum blood loss
- powerful contraction/retraction of uterus… constrict blood vessels running through myometrium
- pressure exerted on placental site by walls of contracted uterus - apposition - once placenta and membranes delivered
- blood clotting mechanism
