[34] Definitions, Mechanisms, and Diagnosis of Labour Flashcards
What is labour also known as?
Parturition
What is the definition of labour?
The process whereby the products of conception are expelled from the uterine cavity after the 24th week gestation
What is ‘term’ labour?
When labour occurs between 37-42 weeks
What percentage of mothers deliver at ‘term’?
93-94%
What is pre-term labour?
When labour occurs from 24-36+6 weeks
What percentage of mothers deliver pre-term?
7-8%
What is it called when the contents of the uterus are delivered before 24 weeks?
Miscarriage
Why is miscarriage different to labour?
The fetus is not viable (although som pre-term births may not survive either)
What is prolonged labour in primigravida?
> 24 hours
What is prolonged labour in multigravida?
> 16 hours
What is prolonged labour associated with?
Increased fetal and maternal morbidity and mortality
What happens before labour?
Pre-labour phase
How long can the pre-labour phase last?
Days - weeks
How is labour divided in clinical management?
Into 3 stages
What are the three stages of labour called?
- First stage
- Second stage
- Third stage
Imaginative isn’t it
What marks the start of the first stage of labour?
Onset of regular painful contractions and cervical changes
What marks the end of the first stage of labour?
When the cervix reaches full dilation and is no longer palpable
What can the first stage of labour be further divided into?
- Early latent phase
- Active phase
What happens in the early latent phase of the first stage of labour?
Cervix becomes effaced and shorter from 3cm in length and dilates up to 3 cm
What happens in the active phase of the first stage of labour?
Cervix dilates from 3cm to full dilation (or 10cm)
What marks the start of the second stage of labour?
Full cervical dilation
What marks the end of the second stage of labour?
Delivery of the fetus
What can the second stage of labour be divided into?
- Pelvic or passive phase
- Active phase
What happens in the pelvic/passive phase of labour?
The head descends from the pelvis
What happens in the active phase of labour?
The mother gets a stronger urge to push and the foetus is delivered by the force of the uterine contractions and maternal bearing down
What happens in the third stage of labour?
Delivery of the placenta and membranes
What hormonal changes take place to initiate the onset of labour?
- Progesterone withdrawal
- Increase in oestrogen
- Increase in prostaglandin action
What is likely to be involved in regulating the hormonal changes that naturally induce labour?
Corticotrophin-releasing hormone (CRH)
Where is CRH produced?
Placenta
Where in the placenta is CRH produced?
Syncitiotrophoblasts
What happens to the number of syncitiotrophoblast nuclei throughout gestation?
Increase
What is the effect of the number of syncitiotrophoblast nuclei increasing during gestation?
There is an exponential increase in the levels of maternal and fetal plasma CRH
What are the effects of CRH on the placenta?
- Increases production of oestrogen
- Decreases production of progesterone
What is the effect of CRH on the membranes?
Increased synthesis of prostaglandins
What molecule is increased by the changes in prostaglandin, progesterone and oestrogen levels throughout pregnancy?
Connexin 43
What does connexin 43 promote?
- Connectivity of the uterine myocytes
- Increases myocyte excitability
What is the result of connexin 43’s effects on the uterine myocytes?
Generalised uterine contractions
How do contractions of uterine myocytes differ to striated muscle contraction?
They shorten and remain so (striated muscle returns to normal length upon relaxation)
What other placental hormones can affect myometrial contraction?
- Relaxin
- hCG
- Activating A
Why is the integrity of the cervix essential throughout gestation?
To retain the products of conception
What cells are found in the structure of the cervix?
Myocytes and fibroblasts
How does the integrity of the cervix change towards term?
It becomes soft and stretchable
What causes the cervix to become soft and stretchable towards term?
Increase in leukocyte infiltration and a decrease in the amount of collagen due to increased proteolytic enzyme activity
What changes to the connective tissue of the cervix cause softening and stretching?
Increased hyaluronic acid to reduce affinity of fibronectin for collagen
What is the softening and stretching of the cervix called?
Ripening of the cervix
What changes to cervical and uterine physiology are required in labour?
- Reduced cervical resistance
- Increased frequency, duration and strength of uterine contractions
Are contractions present in pregnancy before labour?
Yes
Describe the pre-labour contractions experienced throughout pregnancy?
Painless and irregular
How do pre-labour contractions progress throughout pregnancy?
They are minimal at the start and become more frequent as pregnancy progress towards labour
What is uterine contractile activity regulated by?
The fetoplacental unit
What causes the increased contractile activity at the end of gestation?
Down-regulation of factors that keep the uterus and cervix inactive and upregulation of procontractile influences
What is the result of the changes in uterine contractile activity at the end of gestation?
Increased frequency, duration and strength of contractions
What is the effect of progressive uterine contractions on the cervix in labour?
Causes effacement and dilatation
What happens to the upper uterine segment in labour?
There is shortening of the myometrial fibres
What happens to the lower uterine segment in labour?
Stretching and thinning
What happens to the junction between the upper and lower uterine segments in labour?
It rises within the abdomen
What may happen to the upper and lower uterine junction if labour becomes obstructed?
It may become visible at the level of the umbilicus
What is it called when the uterine junction is visible in obstructed labour?
A retraction ring
Where do contraction impulses start and move in the uterus?
In the fundal region and spread downwards through the myometrium
Where are uterine contractions stronger and longer?
At the fundus
Why is fundal dominance in uterine contractions important for labour?
It allows for progressive effacement and dilatation of the cervix
What happens as the uterus and round ligament contract in labour?
The axis of the uterus straightens and pulls the longitudinal axis of the foetus towards the anterior abdominal wall in line with the inlet of the true pelvis
Why is it important that the axis of the uterus and foetus align with the inlet of the true pelvis?
To promote descent of the presenting part as the foetus is pushed into the pelvic cavity
What does the bony pelvis consist of?
- Ilium
- Ischium
- Pubis
- Sacrum
- Coccyx
What are the ilium, ischium and pubis known as?
The paired innominate bones
What joins the innominate bones anteriorly?
Symphysis pubis
What do the innominate bones articulate with posteriorly?
Sacrum
What is the joint between the sacrum and the innominate bones known as?
The sacroiliac joint
What does the sacrum articulate with superiorly and inferiorly?
- 5th Lumbar vertebrae
- Coccyx
What is the bony pelvis divided into?
- False pelvis
- True pelvis
What separates the false and true pelvis?
The pelvic brim
What is the true pelvis divided into?
- Pelvic inlet
- Mid-pelvis
- Pelvic outlet
What is the anterior border of the pelvic inlet?
Superior surface of the pubic bones
What is the posterior border of the pelvic inlet?
Promontory and alae of sacrum
Where is the mid-pelvis?
At the level of the ischial spines
What is the anterior border of the pelvic outlet?
Lower border of the symphysis
What forms the lateral border of the pelvic outlet?
Ischial tuberosities
What forms the posterior border of the pelvic outlet?
Tip of the sacrum
How do the pelvic joints adapt during labour?
There is softening of the sacroiliac ligaments and pubic symphysis
What does softening of the pelvic joints in labour allow?
Expansion of the pelvic cavity
How does the fetus adapt to allow the progress of labour?
There is changes of the head diameter through flexion, rotation and moulding
How do the soft tissues of the pelvis adapt to labour?
They are more distensible to help reduce the risk of tearing of the perineum and vaginal walls during birth of the head
What pelvic soft tissues become more distensible in labour?
- Pelvic floor
- Vaginal orifice
- Perineum
In which diameter is the pelvic inlet larger?
Lateral
What does the larger lateral pelvic inlet diameter promote?
The head to engage in the pelvis in the transverse position
Why does passage of the head and trunk in delivery follow a well-defined pattern?
- The upper pelvis strait is transverse
- The middle pelvic strait is circular
- The outer pelvic strait is anteroposterior
In what percentage of cases does the fetal head present by the vertex?
95%
What are the processes involved in the fetal head adapting to the various segments of the pelvis?
- Descent
- Flexion
- Internal rotation
- Extension
- Restitution
- External rotation
- Delivery of the shoulders
Why is descent an important process of delivery?
It is a prerequisite for birth of the baby
When does engagement (full descent) of the head occur in most primigravid women?
Before the onset of labour
When may engagement of the head not occur before labour?
In multifarious women
When does flexion of the head occur in labour?
As it descends and meets the medially and forward sloping pelvic floor
What is flexion of the fetal head in labour?
Where the chin is brought into contact with the fetal thorax
How does flexion affect the diameter of presentation?
It provides the smallest diameter by changing from occipito-frontal diameter to suboccipitobregmatic diameter
What is the average occipito-frontal diameter?
11.5cm
What is the average suboccipitobregmatic diameter?
9.5cm
When does internal rotation of the head occur in labour?
When the head reaches the pelvic floor
What happens in internal rotation of the head in labour?
The occiput rotates anteriorly from the lateral position towards the pubic symphysis
What causes internal rotation of the head in labour?
Force of contractions being transmitted via the fetal spine to the head at the point where the spine meets the skull
When does the head extend in labour?
As the base of the occiput comes into contact with the inferior rami of the pubis
When does maximal distension of the periueum and introitus occur?
During final expulsion of the head
What is ‘crowning’ in labour?
When the head is seen at the introitus and does not recede between contractions
What is restitution of the head in labour?
Rotation back into line with the normal relationship to the fetal shoulders after delivery of the head
When does external rotation of the fetal head occur in labour?
As the shoulders reach the pelvic floor and they rotate into the anteroposterior diameter of the pelvis
What is external rotation of the head in labour?
When the head accompanies the shoulders to take the anteroposterior diameter of the pelvis to look laterally at the maternal thigh
When does final expulsion of the fetal trunk occur in labour?
Upon delivery of the shoulders
Which shoulder is delivered first in labour?
Anterior shoulder
How is the anterior shoulder delivered?
Traction posteriorly on the fetal head so it emerges under the pubic arch
How is the posterior shoulder delivered?
Lifting the head anteriorly over the perineum
What occurs after the delivery of the shoulders?
Rapid delivery of the remainder of the trunk and lower limbs
What happens in the third stage of labour?
The uterine muscle contracts, shears off the placenta and pushes it into the lower segment and vault of the vagina
Why can determining the onset of labour be difficult?
Contractions may be irregular and may start and stop with no cervical change
What is contractions without cervical change called?
‘False labour’
How is the duration of labour determined?
- Progress of contractions
- Cervical changes
- Descent of the head
What are the clinical signs of the onset of labour?
- Regular, painful contractions
- Passage of blood-stained mucus from the cervix
- Rupture of fetal membranes
How do the contractions of labour change as it progresses?
Become more frequent and longer, producing progressive cervical dilation
What is the passage of blood stained mucus from the cervix in labour called?
The ‘show’
Is the ‘show’ on its own an indicator of labour?
No
When does rupture of fetal membranes occur?
Can be at onset but may vary
What is PROM?
Pre-labour rupture of membranes?
What is the definition of PROM?
Rupture of membranes followed by a latent period of > 4 hours before onset of painful contractions
What is it called if PROM happens in the pre-term period?
Pre-term pre-labour rupture of membranes (PPROM)
How can the request and duration of contractions be determined?
By palpation or external tocography
What is needed to assess the strength of contractions in labour?
Intrauterine pressure catheters
What frequency of contractions suggests labour has started?
Two contractions of >20 seconds in 10 minutes
What is the normal resting tonus in labour?
~10-20 mmHg
What happens to the normal resting tonus as labour progresses?
It increases slightly
What happens to the intensity of contractions throughout labour?
They increase
Where should contractions be recorded?
On the partograph
How are contractions recorded in the partograph?
Based on frequency and duration
What are the classical signs of placental separation seen in the third stage of labour?
- Trickling of bright blood
- Lengthening of the umbilical cord
- Elevation of the uterine fundus with the abdominal cavity
How long does the third stage of labour usually take?
5-10 minutes
When is a diagnosis of a retained placenta made?
If not expelled within 30 minutes
Is a retained placenta a sign of normal third stage labour?
No, it’s abnormal. This seems a bit obvious tbh
