Parturition Flashcards
What is meant by parturition?
It is the action of giving birth
What type of placentation is present in humans?
What does this mean?
Where is the placenta found?
Haemochorial placentation
Maternal blood is in direct contact with chorionic villi
The placenta is within the endometrium
What happens to the placenta as pregnancy progresses?
It is effective from the end of the first trimester
It senesces as pregnancy progresses
What are the 3 key roles of the placenta?
- hormone production
- acquisition of nutrients and removal of toxins
- gas exchange
What are the blood vessels of the placenta?
The umbilical vein and umbilical arteries are within the umbilical cord
The maternal venules and arterioles feed into maternal blood pools
What are the 2 parts of the placenta?
Foetal portion (chorion)
Maternal portion
What is the structure of the placenta like?
It consists of a few layers of cells that separate maternal from fetal blood
Folding villi give it a large surface area
What is the relation of the maternal blood pools to the foetal capillaries?
The foetal capillaries project from the umbilical artery (and vein) and are surrounded by maternal blood pools
What is the structure of a terminal villus on the foetal side of placental circulation?
Why is it like this?
The terminal vessels form a convoluted knot supplied by straight capillaries
The terminal dilatations mean blood flow is slower at the end of the capillaries, giving more time for exchange of metabolites between foetal and maternal blood
How does the oxygen saturation curve compare for foetal Hb?
The oxygen saturation curve is to the left
Why is foetal oxygen content higher than maternal oxygen content?
- foetus has higher Hb concentrations
2. HbF has a higher affinity for oxygen
How does foetal Hb concentration compare to maternal Hb concentration?
Foetal [Hb] is 17 g/dl
Maternal [Hb] is 12 g/dl
Why does HbF have a higher affinity for oxygen than HbA?
HbF contains 2 alpha and 2 gamma chains
2,3 DPG binds preferentially to beta chains in HbA - causing it to release more oxygen
What causes the foetal Hb curve to shift even further to the left?
- reduced binding of 2,3-DPG to gamma chains
- increased pCO2 and relative acidosis on maternal side
“double-Bohr” effect
When does foetal lung surfactant begin to be produced?
Where is it produced from?
Type II pneumocytes
They begin to produce surfactant from 24 weeks of pregnancy
What is the composition of foetal lung surfactant?
- phospholipids - PC and PG
2. apoproteins - SP-A, B, C, D
What is the function of foetal surfactant?
It decreases the surface tension at the air-liquid interface in the alveoli
This enables them to remain open at end-expiration
What stimulates production of foetal lung surfactant?
Fetal glucocorticoids and thyroid hormones (less so)
What does lack of foetal lung surfactant lead to?
Why is there a lack of surfactant?
Neonatal respiratory distress
Lack of surfactant may be due to prematurity or infection
What are the 2 therapies in place is there is a lack of foetal lung surfactant?
- promote production antenatally by administering maternal glucocorticoids
- replace in neonatal period through artificial surfactant
e. g. Curosurf
What is the myometrium and what is its main function?
The middle layer of the uterine wall that consists mainly of smooth muscle cells
Its main function is to induce uterine contractions
What are the main structures in the myometrium?
- fasciculi
- smooth muscle bundles
- communicating bridges
How is an electrical potential generated in the myometrium?
Any cell can act as a pacemaker and generate an action potential
How is depolarisation mediated in the myometrium?
It is mediated by Ca2+ ions
Intercellularly through gap junctions
Intracellularly via Ca2+ channels and intracellular stores
How are action potentials propagated in the myometrium?
There can be a rapid action potential wave (global)
Then a slower intercellular calcium wave (local)
Why is it important that the myometrium has an electrical potential?
It enables coordinated and sustained contractions
Pregnancy requires a contraction that lasts 45-60 seconds
How does myometrial activity change during pregnancy and why?
There is a gradual preparation for labour
- rise in resting membrane potential
- increased intercellular coupling through gap junctions
- connexin expression
What is the “diagnosis” of labour?
There are regular painful contractions
AND
Progressive effacement (opening/shortening) and dilatation of the cervix
AND
descent of the presenting part (head of the baby)
What are the endocrine influences on parturition?
- release of cytokines
- cytokines lead to release of prostaglandins
- prostaglandins lead to release of oxytocin
- labour
How is the foetus able to take oxygen from the mother’s blood?
Foetal haemoglobin has a much higher affinity for oxygen than maternal Hb
How does the frequency of contractions change as pregnancy progresses?
At 28 weeks, contractions interval is 30 minutes
The contraction interval becomes progressively smaller as pregnancy progresses
How many days, roughly, does pregnancy last?
280 days and slightly onwards
There is a large range of durations of pregnancy
What are the endocrine influences that lead to cytokine production in labour?
- progesterone
- oestrogens
- DHEAS
- CRH (corticotrophin releasing hormone)
These are part of the stress response
What are the stages in labour?
Firstly there are several stages of labour
This is followed by the cardinal movements of labour
What is the diameter of the cervix during the first stage of labour?
How long does it last?
From 4 - 10 cm
It lasts for 12 - 24 hours
What happens during the first stage of labour?
- contractions of uterine muscles force baby towards cervix
- cervical opening thins and widens as contractions of lengthwise uterine muscles pull it open
How long does the second stage of labour last for?
When does it start?
It lasts from 1 - 4 hours
It starts when the cervix is dilated to 10 cm
How are uterine contractions aided during the second stage of labour?
Uterine contractions are aided by mother’s involuntary pushes with abdominal wall muscles
What else happens during the second stage of labour?
- head moves through dilated cervix and birth canal
2. shoulders and rest of body move through cervix and birth canal
How long does the third stage of labour last for?
Up to 1 hour
What happens during the third stage of labour?
The placenta separates from the uterine wall and is delivered through the vagina
What is meant by the cardinal movements of labour?
The way in which the pelvis is shaped means that the baby has to make certain movements in order to be able to get out through the pelvis
What are the cardinal movements of labour?
what the baby needs to do
- descent
- engagement (fixed into the pelvis so it doesn’t come back up)
- flexion
- internal rotation (so the smallest diameter of its head passes through the pelvis)
- extension (to get round the corner during birth)
- external rotation
- expulsion
In what ways does the baby have to adapt to life outside the uterus?
- foetal circulation is different to that of the adult
- the placenta (not the lung) is the site of gas exchange
- foetal adaptations must be reversed at birth
Why is oxygenated blood entering into the right ventricle of a foetus?
Oxygenated blood is coming from the umbilical veins
What 2 structures are present in foetal circulation that allow blood to bypass the lungs?
- ductus arteriosus
2. foramen ovale
How is neonatal wellbeing assessed?
APGAR score
Appearance
Pulse
Grimace
Activity
Respiration
How does APGAR scoring work?
Each category is measured at 1. 5 and 10 minutes
Two points are awarded for each category
What % of deliveries are classed as preterm?
What determines whether a baby is preterm?
10% of pregnancies are preterm
Preterm pregnancies mean birth occurs before 37 weeks
What are the negative consequences of preterm births?
Higher risk of mortality and morbidity
How many babies are born preterm each year?
60,000 babies per year
What are the common morbidities affecting preterm births?
- respiratory distress syndrome - O2 dependence
- intraventricular haemorrhage - cerebral palsy
- necrotising enterocolitis - malabsorption
- retinopathy - blindness
How can the risk of preterm birth morbidities be reduced?
- corticosteroids
- ventilation
- artificial surfactant
- magnesium sulphate (to prevent cerebral palsy)
What are the underlying reasons for why preterm births occur?
- uterine capacity
- cervical weakness
- placental abruption (comes away from uterine wall too early)
- local or systemic infections
How can bacteria travel to affect the foetus?
it will travel up the vagina and through the cervix
It affects the chorion and amniotic fluid leading to chorioamnionitis
What is a septate uterus?
How does it affect pregnancy?
There is a septum in the centre of the uterus
This leads to a reduced capacity of the uterus that cannot withstand a full-term pregnancy
How can the shortened cervix be identified on ultrasound?
it has an hourglass shape
This is due to membranes bulging from the cervix into the vagina
What is meant by birth asphyxia?
The baby is not getting enough oxygen during birth
With each contraction of labour, what happens to blood flow?
- compression of myometrial arteries
2. this leads to cessation of flow to the placental bed
What is the result of blood flow cessation to the placental bed during birth?
- lack of gas exchange
- relative foetal hypoxia during contraction
- anaerobic metabolism in baby
- gradual lactic acidosis over time
What are the consequences of birth asphyxia?
- lactic acidemia
- tissue acidosis
- hypoxic-ischaemic encephalopathy
- cerebral palsy
When is fetal distress, leading to birth asphyxia, more likely?
- less reserve (low birth weight)
- long labour
- impaired placental function
What is a placental abruption and what can it lead to?
Where the placenta comes away from the uterine wall earlier than it should
It can lead to preterm birth
What happens to the mother and the baby during placental abruption?
The baby experiences distress due to lack of oxygen
The mother will bleed severely
What are the management methods for birth asphyxia?
- foetal heart rate monitoring
- measurement of foetal scalp pH
- monitor ST segment changes in fECG
- expedite delivery by cesarean section
Under what conditions does birth asphyxia have a good prognosis?
it umbilical cord pH > 7.00
AND
BE is better than -12 mmol/l
