18 - Physiological Labour Flashcards
At what week gestation can expulsion of the fetus be classified as labour (parturition)?
Labour is the process where fetus, placenta and membranes are expelled through the birth canal. Takes around 18hrs
- 24 weeks, before this it is miscarriage
- Before the 37th week it is pre-term labour
What are the three stages of labour?
1st Stage: onset of regular contractions. creation of birth canal, release of structures normally retaining fetus, enlargement and realignment of cervix and vagina. Latent up to 4cm then active then transition at 8cm
2nd stage: Expulsion of the fetus at 10cm
3rd stage: expulsion of the placenta and changes to minimise blood loss from mother
How do we decide how we are going to manage labour?
- The lie, presentation and position of the fetus
- Baby should lie longitudinally in a cephalic presentation well flexed so diameter of about 9.5cm
What do we mean by the lie and presentation of the fetus?
- Lie: relationship of the long axis of the fetus with the long axis of the uterus. commonest lie is longitudinal
- Presentation: which part of fetus is adjacent to pelvic inlet. If baby longitudinal presenting part may be cephalic or podalic (breech)
What is the pelvic inlet bounded by and how big is this area normally?
- Posterior: sacral promontory
- Lateral: iliopectinal line
- Anterior: superior pubic rami (pub symphysis)
True diameter is around 11cm but softening of the pelvic ligaments may allow expansion to occur
What happens to the cervix during labour?
Cervical softening (ripening)
Reduction in collagen and increase in GAGs to decrease aggregation of collagen fibres so collagen bundles loosen
Influx in inflammatory cells and nitric oxide output, all dilating the cervix
Changes triggered by prostaglandins E2 and F2a
What happens to the uterine smooth muscle in labour?
- Myometrium gets thicker in pregnancy due to increased cell size and glycogen deposition
- Rise in calcium concentration from action potentials spreading across gap junctions from pacemakers in the smooth muscle so actin and myosin contract
- Braxton Hicks are when frequency of contractions decreases from every 30 minutes in early pregnancy but high amplitude
What causes onset of labour?
- Increased synthesis of prostaglandins: cause release of calcium from intracellular stores
- Increased sensitivity to oxytocin by fall in progesterone: lowers threshold for triggering action potentials
Both work to increase force and frequency of contractions
What is the Ferguson reflex?
- Sensory receptors in cervix and vagina feed back to hypothalamus promoting oxytocin release
- Positive feedback makes contractions more forceful and frequent
What is brachystasis?
- Uterine smooth muscle contracts but then does not relax fully allowing fundal region to shorten
- Pushes presenting part into birth canal until it engages
How can we induce labour and why may fetus go into distress during contractions?
- Give prostaglandins
- Reduction in placental flow during contraction so lowered fetal heart rate
What are the sequence of events in the second stage of labour and how long does it take?
- When cervical dilation is at 10cm, usually 1hr in multiparous and 2hrs in primigravida
1. Descended head flexes as it meets pelvic floor to reduce diameter of presentation
2. Internal rotation
3. Flexed head descends to vulva stretching the vagina and perineum
4. Head crowns and as it emerges it rotates back to original position and extends
5. Shoulders rotate followed by head and shoulders are delivered followed by rest of fetus
What happens in the third stage of labour?
- When fetus is removed there is a powerful uterine contraction which separates the placenta
- Placenta and membranes usually then delivered in 10 minutes
- Contaction of uterus along with oxytocin drug compresses blood vessels and stops bleeding
What are the first things that happen to the baby when it is born?
- Takes its first breath in response to temperature change and delivery trauma
- Fall in pulmonary vascular resistance and pulmonary arterial pressure, increasing left atrial pressure. Closes the shunts
What tool is used to assess the wellbeing of a neonate?
- APGAR score when initially born and then 5 minutes later
- Higher number on 1-10 the healthier the baby
What are the three P’s of labour?
- Power: uterine contractions which can be assessed by frequency, amplitude and duration
- Passage: birth canal
- Passenger: size and presentation of fetus
Failure of labour to progress can be due to any of these factors. Plot a partogram
What are some of the changes that occur 2-3 weeks before labour?
Lightening means the head is dropping and may get sharp shooting pains
What are the factors needed to mark the onset of labour contractions?
- At least 2 contractions every ten minutes
- Contractions so painful that you cannot speak
- Fundal dominance
What needs to happen to the cervix before it dilates?
- Effacement in primigravida
What is a show?
As cervix dilates the mucoid operculum may come away and be streaked with blood
What are forewaters?
- Waters in front of the babies head when it engages
- Next contraction after losing it can be painful
What are some physiological changes to the mother during labour?
What are some the signs of the second stage of labour?
What are some risk factors for post-partum haemorraghe and what do you need to do when the placenta has been delivered?
- Need to make sure all the placenta is delivered with all the membranes to reduce risk of haemorraghe
What is the definition of post partum haemorraghe and how does the body physiologically try to avoid this?
- Loss of more than 500ml of blood after delivery
- Blood is hypercoagulable, more plasma and uterine contractions
At what point in pregnancy would asymmetrical growth restriction occur?
Later stages
Why does smoking lead to a restriction in O2 delivery to the fetus?
- Carbon monoxide prevents oxygen disocciating from Hb to the fetus
- Vasoconstriction of the placental vessels
What maternal and fetal landmarks are used to asses fetal head position in the birth canal?
- Ischial spines
- Fontanelles of the baby
What is parturition and delivery?
Parturition: transition from a pregnancy to a non-pregnancy state
Delivery: method of expulsion of the fetus
