8. Parturition

Question 1

what are the 2 aims of contractions during labour

Answer 1

1. cervix dilation

2. push foetus through birth canal

Question 2

describe in general terms the 3 stages of labour

Answer 2

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)

Question 3

which 2 processes must occur for creation of birth canal

Answer 3

1. increased myometrial excitability and uterine contractions
2. cervical ripening

Question 4

which hormone stimulates uterine contractions - where is it released from

Answer 4

OXYTOCIN - pulsatile release from posterior pituitary gland (controlled by hypothalamus)

Question 5

how are the actions of oxytocin inhibited until term

Answer 5

• progesterone
• relaxin
• low no. of oxytocin Rs in myometrium

Question 6

describe 4 changes that must occur to increase myometrial excitability and initiate contractions

Answer 6

1. small progesterone decrease towards end of term
2. increased oestrogen:
   - increase SM gap junctions (communication)
   - increase PG synthesis
   - increase myometrium oxytocin Rs
3. mechanical stretching of uterine SM as gestation advances - releases PGs
4. increased prostaglandins:
   - increase SM gap junctions (communication)
   - increase calcium influx across membrane and Ca release from intracellular stores
   - increase myometrial sensitivity to oxtocin

Question 7

what is the Fergusson reflex

Answer 7

i. PG and oxytocin stimulate uterine contractions…
ii. contractions stimulate PP release of oxytocin and placental release of PGs

= positive feedback loop

Question 8

what is cervical ripening

Answer 8

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

Question 9

what causes ripened cervix to dilate

Answer 9

uterine contractions

Question 10

what is the birth canal - which structures does it include

Answer 10

Potential space through which foetus is delivered. Includes:

1. pelvis - determines max size (inlet typically 11 cm)
2. soft tissues - cervix, vagina and perineum

Question 11

why do uterine contractions allow pressure inside the uterus to increase

Answer 11

Myometrial fibres contract but only partially relax… permanent partial shortening of muscle fibres allow contraction and retraction… uterine capacity progressively reduced… increased pressure

Question 12

`which 3 features describe foetal presentation

Answer 12

1. Lie: longitudinal or transverse
2. Attitude: flexion or extension
3. Presentation:
   - frank breach (butt 1st feet by head)
   - full breach (butt 1st knees crossed)
   - footling breach (1 foot 1st)

Question 13

what are the active and passive features of stage 2

Answer 13

Passive = descent and rotation of head

Active = maternal effort to expel foetus and achieve birth

Question 14

which features stimulate placental separation

Answer 14

• 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

Question 15

name 3 factors which minimise post-partum blood loss

Answer 15

1. powerful contraction/retraction of uterus… constrict blood vessels running through myometrium
2. pressure exerted on placental site by walls of contracted uterus - apposition - once placenta and membranes delivered
3. blood clotting mechanism

Question 16

name the 4 main causes of post-partum haemorrhage

Answer 16

4 Ts:

1. lack of uterine Tone (most common) - myometrial SM doesn’t contract sufficiently to clamp shut placental arteries… slow and steady blood loss
2. Trauma - damage to genital structures, e.g. due to medical instruments
3. Tissue - placental fragments retained in uterine cavity, preventing contraction and causing uterine atony
4. Thrombin-related - blood-clotting condition, e.g. genetic (von Willebrand disease) or obstetric (eclampsia, placental abruption) - can lead to DIC

Question 17

what is Sheehan’s syndrome

Answer 17

very rare pituitary failure as result of AP necrosis from lack of blood supply