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Flashcards in Repro 10 Deck (61):

When is labour viable?

After 24 weeks. Before this, it is spontaneous abortion. Before 37 weeks, birth is pre-term or premature.


Where does the uterus extend to by 36 weeks?



How wide is the girth of the umbilicus of the mother at 24 weeks?

60cm. After this, the girth increases by approximately 2.5cm per week until term at which point it is approximately 100cm.


How is the period of the first stage of labour defined?

Interval between the onset of labour and full cervical dilation


What are the phases of the first stage of labour?

Latent: Slow cervical changes to approx 4cm dilation. Varying duration
Active: Faster rate of cervical change - 1-1.2cm/hr with regular uterine contractions


What is the lie of the foetus?

The relationship of the foetus long axis to the long axis of the maternal uterus. Should be parallel - longitudinal.


What is the presentation of the foetus?

Describes which part is adjacent to the pelvic inlet. Usually head (cephalic) or buttocks (podalic) aka breech. Presenting part may be in a variety of positions.


What does the clinical management of labour depend on?

Lie and presentation of the foetus


What is the typical position of the presenting part of the foetus?

Flexed - vertex presents to the pelvic inlet. In this case, diameter of presentation is typically 9.5cm


What is the required diameter of the birth canal?



What limits the diameter of the birth canal?

Pelvic inlet:
Bound posteriorly by the sacral promontory, laterally by the ilio-pectinal line and anteriorly by the superior pubic rami and the upper margin of the pubic symphysis.


What usually the true diameter of the pelvic inlet?



How might the diameter of the pelvic inlet be increased?

Softening of the pelvic ligaments may allow some expansion


How is the foetus normally retained in the uterus?

The cervix is closed and relative quiescence of the myometrium


How is the birth canal created?

Cervix dilates and is retracted anteriorly


What process is seen exteriorly as the "waters breaking"?

Foetal membrane rupture releasing amniotic fluid. This occurs at some point during dilation and retraction of the cervix


How does cervical dilation occur?

Cervical ripening (structural changes) facilitate it and forceful contractions of the uterine smooth muscle cause effacement and then dilation


Why is cervical ripening required?

Would tear without these structural changes.


When does cervical ripening occur?

From 36 weeks


What changes occur in the cervix during ripening?

Marked reduction in collagen and marked increase in glycosaminoglycans (GAGs) which decrease the aggregation of collagen fibres. Keratin sulphate increases at the expense of derma ton sulphate. Consequently, collagen bundles loosen.
Influx of inflammatory cells and increase in nitric oxide output.


What triggers cervical ripening?

Prostaglandins, namely E2 and F2x


How does the myometrium size change during pregnancy?

Becomes much thicker, primarily due to increased cell size (10 fold) and glycogen deposition


What causes the co-ordinated contractions of the myometrium?

Action potentials spread from cell to cell via specialised gap junctions. Some sm cells are capable of spontaneous depolarisation and action potential so can act as pacemakers.


Where are the pacemakers in the myometrium?

In the top of the uterus at the poles. Spreads to fundus and then down. More powerful contraction at the top than the bottom.


What effect does oestrogen have on the smooth muscle cells of the myometrium?

Increases gap junction communication between smooth muscle cells, increasing contractility.


When do contractions of the myometrium begin?

Always spontaneously active. In early pregnancy, may occur every 30 mins but at low amplitude so not felt (maybe flutter). As pregnancy continues, the frequency falls but with some increase in amplitude, potentially producing noticeable contractions - Braxton-Hicks' contractions. None of these contractions are normally forceful enough to have any effect on the foetus but can induce pain and be mistaken for labour.


What causes the onset of labour?

Prostaglandins and oxytocin.


What are prostaglandins?

Lipid paracrine hormones


What is the function of prostaglandins in labour?

Enhance the release of calcium from intracellular stores


Where are prostaglandins produced?

Mostly endometrium
Also myometrium, decidua and placenta
Amnion increases prostaglandin release in 3rd trimester


How is prostaglandin synthesis controlled?

Changing the release of phosphlipase from liposomes. This is mostly causes by change in the oestrogen/progesterone ratio (fall in progesterone relative to oestrogen).
Prostaglandin release may also be stimulated by the action of oxytocin


What inhibits oxytocin?

Relaxin and progesterone
Few oxytocin receptors


What is the function of oxytocin?

Lowers the threshold for triggering action potentials, increasing smooth muscle excitability.


What is oxytocin?

Peptide hormone


Where is oxytocin produced?

Posterior pituitary. Release controlled by hypothalamus


What is the effect of reducing progesterone levels/increasing oestrogen levels on oxytocin?

Increases the receptor population making cells more sensitive to low circulating levels of the hormone.


What is the Ferguson reflex?

Sensory receptors in the cervix and vagina are stimulated by contractions
Excitation passes via afferent nerves to the hypothalamus, promoting massive oxytocin release
This positive feedback makes contractions more forceful and frequent


What is brachystasis?

At each contraction of the uterine smooth muscle, fibres shorten and do not relax fully. The uterus, particularly the fundus region therefore shortens progressively. This pushes the presenting part into the birth canal and stretches the cervix over it


How can labour be induced?



What does evidence from animals suggest about the initiation of labour?

Increase in prostaglandin production and oxytocin sensitivity triggered by a fall in progesterone levels relative to oestrogen, triggered by rising production of cortisol by the foetus as it matures.


What evidence suggests a rise in cortisol is not the trigger to initiation of labour in humans?

Foetal cortisol does rise prior to birth but foetuses that do not produce cortisol are born


Why does the foetal heart rate need to be monitored during labour?

Increasingly forceful uterine contractions may temporarily reduce placental blood flow and so reduce oxygen supply to the foetus. This may lead to brief reductions in foetal heart rate that need to be monitored. If the reductions in flow are greater than usual, larger "dips" occur, as the foetus becomes distressed.


How does the foetus orientation change during birth?

The descended head flexes as it meets the pelvic floor, reducing the diameter of presentation. There is then internal rotation . The sharply lexed head descends to the vulva so stretching the vagina and perineum. The head is then delivered. (crowning) and as it emerges it rotates back to its original position and extends. The shoulders then rotate followed by the head and the shoulders deliver followed rapidly by the rest of the foetus


How is the period of the second stage of labour defined?

From 10cm dilation of the cervix to complete delivery of the foetus.


Define the period of the third stage of labour.

From complete delivery of the baby to complete expulsion of the placenta


How long does the third stage of labour take?

Usually 5-15 mins but up to one hour is considered normal


How is the placenta removed from the uterus?

Powerful uterine contractions occur after the birth of the baby that squeeze the inelastic placenta. It sheers off and is positioned into the upper part of the vagina or lower uterine segment.


Why is the third stage of labour dangerous?

Massive potential for haemorrhage due to the large blodd supply to the placenta - usually 500-800 ml/min.


How is bleeding controlled in the third stage of labour?

1. Powerful contraction/retraction of the uterus especially of interlacing muscle fibres (living ligaments) which constrict blood vessels running through the myometrium
2. Pressure exerted on placental site of walls by contracted uterus (apposition after placenta and membranes delivered)
3. Blood clotting mechanism


What are the 3 p's of labour?



What are the powers involved in labour?

Myometrial contraction
Contraction and retraction of the myometrial muscle fibres causes progressive shortening particularly in the fundus


What aspects of uterine contractions are assessed during labour?



What forms the passage in labour?

Bony pelvis and soft tissues - cervix, vagina and perineum


What plane of the pelvic inlet is shortest?

Anterior-posterior plane


What plane of the pelvic outlet is narrowest?



What implications does the width of the pelvic inlet and outlet have on the delivery of the baby?

Head must flex, extend and rotate as it descends through the pelvis


What aspects of the passenger need to be considered during labour?

Lie: transverse/longitudinal
Attitude: Flexed/extended
Presentation: Frank breech/full breech/single footing breech
Size and number


How does the foetal skull different from adults that helps with labour?

Cranial sutures not yet fused therefore moulding of the foetal cranium may occur during labour


Why might labour fail to progress?

Inadequate power
Inadequate pasage (abnormal pelvis, rigid perineum)
Abnormalities of the passenger (too big, presentation)


How is labour plotted?

Graphically on a partogram


Describe the potential attitudes of the foetus.

Vertex: suboccipitobregmatic (9.5cm)
Sinciput: suboccipitofrontal (10cm)
Brow: mentovertical (13.8cm)
Face: submentobregmatic (9.5cm)