Flashcards in Labour Deck (37):
What is spontaneous abortion?
Labour before 24 weeks
What is pre-term labour classed as?
Before 37 weeks
What is term labour classed as?
What happens in general in each of the stages of labour?
First: creation of the birth canal
Second: expulsion of the fetus
Third: expulsion of the placenta, contraction of the uterus
After how many weeks is the uterus palpable?
When does it reach the umbilicus?
When does it reach the xiphisternum?
What is the lie of the fetus usually?
Fetus is normally flexed and longitudinal in relation to the long axis of the uterus
What is the presentation of the fetus?
The head part is usually adjacent to the pelvic inlet (cephalic presentation)
Can sometimes be the buttocks (podalic presentation)
What is the diameter of the head at presentation?
What determines the maximum size of the birth canal?
What is the diameter of the pelvic inlet?
How is the birth canal created?
Expansion of soft tissues including the cervix, vagina and perineum to about 10cm
What is the structure of the tough and thick cervix?
Collagen fibres embedded in a proteoglycan matrix
Describe the process of cervical ripening (the structural changes)
Reduction in collagen production
Increase in glycosaminoglycans (GAGs) - disrupts the matrix
Reduces aggregation of collagen fibres so that they uncoil
What stimulates cervical ripening?
Prostaglandins E2 and F2x which are locally diffused from the uterus
How does the myometrium get bigger?
Smooth muscle cells increase in size 10 fold
Get glycogen deposition
What generates force in the myometrium?
An action potential produced by the cells spontaneously depolarising causes an increase in intracellular calcium concentration
How does the myometrium have a uterine contraction?
Action potentials can spread from cell to cell via gap junctions
How do contractions of the myometrium change?
Early pregnancy - contractions of low amplitude every 30 minutes
As pregnancy continues, frequency falls and amplitude increases
Can cause Braxton-Hicks contractions
What makes the contractions more forceful and frequent for labour and how?
Prostaglandins - more calcium per action potential
Oxytocin - more action potentials by lowering the threshold
What are prostaglandins?
Biologically active lipids which act as local hormones
What controls the production of prostaglandins?
Controlled by the progesterone:oestrogen ratio
-prog>oestrogen, prostaglandins low
Also stimulated by oxytocin
What initiates labour?
Progesterone:oestrogen ratio decreases
-relative fall in progesterone
Get an increase in prostaglandins which ripen the cervix and promote uterine contractions
How is response to oxytocin increased?
Fall in progesterone relative to oestrogen increases the oxytocin receptor population making cells more sensitive to low levels of oxytocin
(Oxytocin acts by binding to receptors on SMCs)
What is the Ferguson reflex?
As contractions increase, sensory receptors in the cervix and vagina are stimulated by contractions and so get excitation via afferents nerves to the hypothalamus to increase secretion of oxytocin
Why is the Ferguson reflex good?
Increases frequency and force of contractions
What is brachystasis?
On each contraction, muscle fibres shorten but do not relax fully
Therefore the uterus, particularly the fundus, shortens progressively
Pushes the presenting part into the birth canal and stretches the cervix over it - descent of presenting part
What is thought to cause the fall in progesterone and increase in oestrogen to initiate labour?
Rising production of cortisol by the fetus as it matures
However some fetuses are born who don't produce cortisol
Why does fetal heart rate need to be monitored during labour?
Uterine contractions can temporarily reduce placental blood flow, decreasing oxygen to the fetus
Causes brief reductions in fetal heart rate
If reductions in flow are greater than usual, get larger dips and the fetus becomes distressed
When does the second stage of labour begin?
When cervical dilation reaches 10cm
How long does the second stage of labour last?
1 hour in multiparous women
2 hours in primigravida
What is labour?
The expulsion of products of conception after 24 weeks
Describe the positions of the fetus during the second stage of labour
Head flexes as it meets pelvic floor
Sharply flexed head descends into vulva
Head is delivered (crowning)
As it emerges it rotates back to original position and extends
Shoulders rotate, followed by head
Shoulders delivered, followed by the rest
What happens in the third stage of labour?
Powerful contraction of uterus
Separates the placenta and positions in into the upper part of the Bahia and lower uterine segment
Placenta and membrane then expelled in 10 mins
Now is maternal blood loss reduced?
The massive uterine contraction also compresses the blood vessels and reduces bleeding
Enhanced by gibing the oxytocic drug
What triggers the fetus to take the first breath?
Drop in temperature
What does the first breath cause?
Reduction in pulmonary vascular resistance
Reduces pulmonary arterial pressure and increases left atrial pressure relative to right
Closes the foramen ovale
Rising pO2 causes ductus arteriosus to constrict, establishing the adult circulation
Sphincter in ductus venosus constricts - blood to liver through hepatic sinusoids