Session 10: Birth

Question 1

How does pregnancy end? What kind of changes are invovled?

Answer 1

Pregnancies end with the expulsion of the products of conception. In humans, if this occurs after 24 weeks of gestation, the process is called labour.

[*] Before that time, it is commonly called spontaneous abortion.

[*] The biological term for this process is parturition.

[*] Labour that occurs before the 37th week of gestation is known as premature or pre-term labour.

[*] Term: 37-42 weeks

Labour involves the sequential integrated changes in the uterine decidua and myometrium. Changes in the uterine cervix tend to precede uterine contractions. The expulsion of the fetus requires a number of processes:

[*] The creation of a birth canal

• The release of the structures which normally retain the fetus in utero
• The enlargement and realignment of the cervix and vagina

[*] Expulsion of the fetus

[*] Expulsion of the placenta and changes to minimise blood loss from the mother

[*] These processes are known as the first, second and third stages of labour.

Question 2

Describe the First Stage of Labour

Answer 2

• Creation of birth canal
• Onset of labour => Full cervical dilation

Latent Phase

• Onset à ~4cm dilation (slow)
• Variable duration (can go on for days)

Active Phase

• Faster rate of cervical change, 1-1.2cm/hour
• Regular uterine contractions

Can take many hours – reached cervix is dilated to 10 cm.

Question 3

Describe the Second Stage of Labour

Answer 3

• Expulsion of fetus
• Relatively rapid
• Up to 1 hour but can be very fast
• Urge to bear down (maternal behaviour changes) – contraction of abdominal muscles
• The descended head flexes as it reaches the pelvic floor
• Reduces presentation diameter

Head rotates internally – to fit the dimensions of the birth canal
Head stretches vagina and perineum (very tough)

• Risk of tearing

Episiotomy to relieve pressure however this is not very common these days – tissues are allowed to tear naturally along natural lines => more likely to heal naturally)

• Head delivered
• Head rotates and extends
• Shoulders rotate and deliver
• Rapidly followed by the rest of the baby

The second stage is normally an efficient way of expelling the baby.

Question 4

Describe the Third Stage of Labour

Answer 4

Expulsion of placenta

• Sheared off by strong contractions of the uterus
• Effect of uterine contractions dramatically increased by expulsion of fetus

Contraction of uterus (to prevent excessive blood loss)

• Compresses blood vessels to reduce haemorrhage
• Enhanced by giving oxytcic drug

Lasts between 5 and 15 minutes but any time up to an hour may be considered within normal limits.

Question 5

Describe what needs to be assessed at the end of the pregnancy.

Answer 5

[*] The uterus first becomes palpable at around 12 weeks of gestation, by 20 weeks it has reached the level of the umbilicus, by 36 weeks it reaches the xiphisternum.

[*] The girth at the umbilicus remains at about 60cm average until 24 weeks gestation, then increases by about 2.5cm per week till term, when it should be about 100cm.

[*] Towards the end of pregnancy, the lie and presentation of the baby need to be assessed as it determines delivery.

[*] The lie of the fetus describes the relationship of its long axis to the long axis of the uterus (relationship of foetus’s vertebral column to maternal vertebral column)

• The commonest lie is longitudinal, with the head or buttocks posterior.
• The fetus normally has a flexed attitude.

[*] The presentation of the fetus describes which part is adjacent to the pelvic inlet (which part is going to come out first)

• If the baby lies longitudinally the presenting part may be the head (cephalic) or the breech (podalic).
• The presenting part may be in a variety of positions, which affects the diameter of presentation.
• The clinical management of labour depends crucially on the lie, presentation and position of the fetus.
• Most commonly the baby lies longitudinally, in a cephalic presentation, well flexed so that the vertex presents to the pelvic inlet at minimum diameter. In this case the diameter of presentation is typically 9.5cm.
• The birth canal therefore needs to have a diameter of about 10cm for the fetus to pass through.
• This required diameter may change with different positions.

Question 6

Describe the Second Stage of Labour in more depth

Answer 6

[*] The first stage of labour ends when cervical dilatation reaches 10 cm. The fetus is then expelled relatively quickly.

[*] The second stage of labour normally lasts up to 1 hour in the multiparous woman and up to 2 hours in primigravida (first birth)

[*] The descended head flexes as it meets the pelvic floor, reducing the diameter of presentation.

[*] There is then internal rotation.

[*] The sharply flexed head descends to the vulva, so stretching the vagina and perineum.

[*] The head is then delivered (‘crowning’ – baby’s head becomes visible), 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 fetus.

[*] The second stage of labour ends with delivery of the fetus.

Question 7

What happens after the fetus has been born during the third stage?

Answer 7

With the fetus removed, there is a powerful uterine contraction, which separates the placenta, positioning it into the upper part of the vagina or lower uterine segment.

After the neonate has been born the uterus continues to contract, though it is now contracting on nothing. The uterus contracts down hard, with its fibres shortening much faster. This shears off and expels the placenta, normally 10 minutes after birth.

This leaves the maternal blood vessels which ran into the placenta exposed, giving the risk of haemorrhage. Normal blood flow through this site is 500-800ml/minute (10-15% of cardiac output), so bleeding can be very severe.

The continued uterine contraction compresses the blood vessels (which run through the myometrium), closing them off and reducing haemorrhage.

Question 8

What are the normal physiological processes critical in minimising blood loss and protecting the woman?

Answer 8

• Powerful contraction/retraction of uterus especially action of interlacing muscle fibres “living ligature” which constrict blood vessels running through the myometrium.
• Pressure exerted on placental site by walls of contracted uterus (apposition – once placenta and membranes delivered).
• Blood clotting mechanism (sinuses and torn vessels)

This effect can be enhanced by giving an oxytocic drug.

After the baby is born, there is a marked reduction in size of uterus due to powerful contraction and retraction (ongoing). Size of placental site therefore reduced (can be up to ½ before separation begins).

• Inelastic placenta is squeezed by contraction.
• You need to ensure placenta is separated from the intrauterine wall otherwise there is a risk of involuted uterus and shock.

[*] Blood in intervillous space forced back into veins of spongy layer of decidua basilis.

[*] Veins become tense and congested and kept under pressure by underlying muscle layer of uterus.

[*] Blood can’t drain back into maternal bloodstream because uterus has retracted and doesn’t allow it.

Question 9

What processes are necessary to create a birth canal?

Answer 9

Expansion of Soft Tissues

• Cervix
  
  • At some point in dilation of the cervix the fetal membranes rupture, releasing amniotic fluid
• Vagina
• Perineum

The expansion is to ~10cm, and requires both structural changes and a lot of force (see next LO)

Size of Birth Canal

In normal presentation the head is the biggest part

• Diameter of 9.5cm
• Maximum size of birth canal is determined by the pelvis
• Pelvic inlet typically 11cm
• Softening of ligaments may increase this

Question 10

What also happens during the creation of a birth canal?

Answer 10

• The fetus is normally retained in the uterus by the cervix (the cervix supports the developing foetus from underneath) and relative quiescence of the myometrium.
• To create a birth canal the cervix must dilate and be retracted anteriorly.
• At some time during this process the fetal membranes rupture, releasing amniotic fluid. ‘breaking of the waters’
• Cervical dilatation is facilitated by structural changes known as cervical ripening, but produced by forceful contractions of uterine smooth muscle. These contractions first thin and flatten the cervix (‘effacement’) and then dilate it.

Question 11

What is Cervical Ripening?

Answer 11

Throughout pregnancy the cervix retains the fetus. It is made of tough, thick collagen, which is coiled to give greater structural strength.

The cervix needs to soften for birth to occur. This is called Cervical Ripening and is an essential part of labour. If the uterus applies force to an un-ripened cervix a great deal of damage can be done.

Question 12

What changes occur in Cervical Ripening?

Answer 12

Cervix collagen in a proteoglycan matrix
Ripening involves:

• Reduction in collagen production (Turnover altered)
• Increase in glycosaminoglycans (Disrupts the matrix)
• Reduces aggregation of collagen fibres (Uncoils)

Triggered by Prostaglandins

• PG E2 and F2x
• Locally diffused from the uterus

Once released the cervix is ready to be stretched. Cervical stretching elicits uterine contractions.

Myometrium (generating force)

• Smooth muscle, greatly increased (thickened) in pregnancy
• Force generated when intracellular [Ca2+] rises
• Due to action potentials (Bursts generating a sustained contraction)
• Triggered spontaneously (Pacemakers. Body of the uterus near tubes)

The cervix has a high connective tissue content made up of collagen fibres embedded in a proteoglycan matrix.
Ripening involves a marked reduction in collagen and marked increase in glycoasminoglycans (GAG’s), which decrease the aggregation of collagen fibres.
Keratin sulphate increases at the expense of dermatan sulphate.
In consequence collagen bundles ‘loosen’ - uncoil
There is also influx of inflammatory cells, and increase in nitric oxide output.
All of these changes are triggered by prostaglandins, namely E2 and F2x.

Question 13

Describe Uterine Contractions in early and late pregnancy

Answer 13

The uterus contracts throughout pregnancy. Progesterone suppresses the myometrium’s contractions, preventing labour from occurring until the proper time.

Early

• Low amplitude, every 30 minutes
• Generally speaking, mother is not aware (doesn’t normally generate any sensation)

Late

• Higher amplitude, less frequent
• ‘Braxton-Hicks’ contractions. Pain from these contractions can be experienced as early as 32 weeks.

Question 14

What makes the contractions more forceful and frequent?

Answer 14

Prostaglandins ( as well as ripening the cervix)

• Increased intracellular [Ca2+] per action potential

Oxytocin

• More action potentials
• Threshold lowered for triggering action potentials (increases excitability)

Question 15

Describe Prostaglandins

Answer 15

Biologically active lipids

• Local hormones
• Produced mainly in the endometrium
• Production is controlled by the Oestrogen : Progesterone ratio
• Progesterone > Oestrogen
  
  • Low Prostaglandins
  • Throughout pregnancy
• Oestrogen > Progesterone
  
  • Increased Prostaglandins
• End of pregnancy – a relative fall in progesterone increases prostaglandins (can be administered) => ripen cervix, promote uterine crontractions
• Prostaglandins are biologically active lipids synthesised in most body tissues. They act as local hormones, acting near their site of production.*
• [*] The endometrium is a major producer.*
• [*] Prostaglandin synthesis is controlled by changing the release of phospholipase from liposomes; the major factor influencing this in the endometrium is the oestrogen/progesterone ratio. Mechanical damage also stimulates prostaglandin synthesis.*
• [*] Placenta, decidua, myometrium and membranes can all synthesise prostaglandins.*
• [*] If progesterone is high relative to oestrogen, prostaglandin synthesis is lower.*
• [*] A fall in progesterone or rise in oestrogen increases prostaglandin synthesis*
• [*] Prostaglandin release may also be stimulated by the action of oxytocin.*
• [*] Increased synthesis of prostaglandins by amnion in third trimester.*
• [*] Levels of prostaglandins in amniotic fluid rise very early in labour.*
• [*] Cervical ripening is due to oestrogen, relaxin and prostaglandins breaking down connective tissue.*

Question 16

Describe Oxytocin

Answer 16

Secreted by the Posterior Pituitary

• Controlled by the hypothalamus (Neurosecretion)

Secretion increased by afferent impulses from the cervix and vagina

• Ferguson Reflex (positive feedback loop)
• Mechanical stimulation of the cervix due to prostaglandin contractions increases oxytocin secretions, strengthening contractions, stimulating the cervix more, releasing more oxytocin etc. etc….

Acts on smooth muscle receptors

• More receptors if Oestrogen > Progesterone

Fetal oxytocin is also produced.

[*] Oxytocin acts by binding to receptors on uterine smooth muscle cells, making them contract harder. It initiates uterine contractions.

• [*] Action of oxytocin is at first inhibited in pregnancy by progesterone, relaxin and low number of oxytocin receptors.*
• [*] Pregnancy = increased number of gap junctions to aid communication between muscle cells (coordinates effective uterine activity).*
• [*] A fall in progesterone levels relative to oestrogen increases the receptor population making cells more sensitive to low circulating levels of the progesterone.*
• • At 36 weeks = increased number of oxytocin receptors in myometrium – therefore uterus can respond to pulsatile release of oxytocin from posterior pituitary gland.*

Question 17

What is meant by Brachystasis?

Answer 17

Uterine smooth muscle relaxes less than it contracts.

Unique feature of uterine smooth muscle
Fibres shorten in the body of the uterus
Drives the presenting part of the fetus to the cervix

Question 18

Summarize the myometrium in pregnancy

Answer 18

• The myometrium is made up of bundles of smooth muscle cells. During pregnancy, the myometrium gets much thicker primarily due to increased cell size (10 fold) and glycogen deposition.
• An intracellular apparatus containing actin and myosin, triggered by a rise in intracellular calcium concentration, generates force. The rise in calcium concentrations is produced by action potentials in the cell membrane.
• Action potentials spread from cell to cell via specialised gap functions, allowing co-ordinated contractions to spread over the myometrium.
• Some smooth muscle cells are capable of spontaneous depolarization and action potential generation, and so can act as ‘pacemaker’s.
• The myometrium is therefore always spontaneously motile.
• In early pregnancy contractions may occur every 30 minutes or so, but are of low amplitude. As pregnancy continues, the frequency falls, with some increase in amplitude, producing noticeable ‘Braxton-Hicks’ contractions. None of these contractions are normally forceful enough to have any effect on the fetus.
• Uterine smooth muscle has a crucial property – brachystasis. At each contraction muscle fibres shorten, but do not relax fully. The uterus, particularly the fundal region therefore shortens progressively. This pushes the presenting part into the birth canal and stretches the cervix over it.
• [*] Descent of the presenting part (commonly the fetal head) therefore occurs progressively during labour, until it engages in the pelvis.

Question 19

What hormones are implicated re the onset of labour?

Answer 19

The onset of labour is a relatively sudden increase in the frequency and force of contractions. Two hormones are implicated in this change:

[*] Prostaglandins: these act by enhancing the release of calcium from intracellular stores

[*] Oxytocin: this peptide hormone is secreted from the posterior pituitary gland under the control of neurons in the hypothalamus. It acts by lowering the threshold for triggering action potentials.

[*] The onset of labour is therefore associated with increased prostaglandin synthesis and release stimulating more forceful contractions in conjunction with increased sensitivity to oxytocin.

Question 20

What is meant by the Ferguson Reflex?

Answer 20

As contractions increase, the ‘Ferguson Reflex’ increases oxytocin secretion massively. 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.

Question 21

Describe how the initiation of labour in humans is not very clear

Answer 21

[*] All the evidence from animals suggests labour is initiated by an increase in prostaglandin production and myometrial sensitivity to oxytocin triggered by a fall in progesterone levels relative to oestrogen.

[*] Oestrogen and progesterone are produced in the placenta. In some species, it is well established that a fall in placental progesterone production and rise in oestrogen production is triggered by rising production of cortisol by the fetus as it matures.

[*] The situation in humans is much less clear.

[*] Fetal cortisol does rise prior to birth but foetuses that do not produce cortisol (no adrenals) are born. There is no consistent evidence of pre-labour increases in plasma prostaglandins or changes in oestrogen progesterone ratio.

Prostaglandins will, however, induce labour.

Question 22

Why may brief reductions in fetal heart rate be observed during labour?

Answer 22

As labour progresses increasingly forceful uterine contractions may temporarily reduce placental blood flow, and so reduce oxygen supply to the fetus. This may lead to brief reductions in fetal heart rate that may be monitored. If the reductions in flow are greater than usual, larger ‘dips’ occur, as the fetus becomes ‘distressed’.

Question 23

Describe the immediate physiological changes in the neonate, which enable independent life

Answer 23

• Within a short time of delivery, the fetus takes its first breath triggered by multiple stimuli – delivery trauma, temperature change and others. This is key to reorganising circulation and getting oxygen into lungs.
• This causes a dramatic fall in pulmonary vascular resistance, so reducing pulmonary arterial pressure and increasing left atrial pressure relative to right atrial pressure (blood fills lungs increasing return to LA)
• The atrial pressure change shuts the foramen ovale and rising arterial pO2 causes the ductus arteriosus to contract and constrict, so establishing the adult form of circulation.
• The sphincter in the ductus venosus constricts so that all blood entering the liver passes through the hepatic sinusoids.

Question 24

What is meant by the APGAR Score

Answer 24

The condition of the neonate is scored by the Apgar Score, which is assessed as a number from 0-10, normally soon after delivery, and a few minutes later.

Question 25

Describe in outline the most common fetal presentation

Answer 25

Most commonly the baby lies longitudinally, in a cephalic presentation, well flexed so that the vertex presents to the pelvic inlet. In this case the diameter of presentation is typically 9.5cm.

Question 26

Clinically, elements of labour can be classified into “The Powers, the Passage and the Passenger”. What is meant by the Powers?

Answer 26

[*] Delivery of the fetus is dependent upon contraction of the myometrium, which has undergone considerable hypertrophy and hyperplasia during pregnancy.

[*] Contraction and retraction of the multidirectional smooth muscle fibres causes progressive, permanent partial shortening, particularly in the fundus of the uterus.

[*] Symmetry and polarity: the contractions create from two poles of uterus, then go to the fundus and upper part of the uterus, then go down to the lower segment. The forces of the upper segment are more powerful than that of lower segment of uterus.

[*] Retraction: after each contraction the length of each myometrium muscle of the uterus can not return to the former length – it becomes shorter and shorter. The uterine capacity is progressively reduced so the pressure inside the uterus becomes stronger and stronger.

[*] Uterine contractions can be assessed in terms of frequency, amplitude and duration. Measured at 10 minute intervals, ideally 3-4 per 10 minutes, lasting 60 seconds of moderate amplitude

Question 27

Describe the Cervical Changes

Answer 27

• Increasing levels of relaxin during pregnancy
• Causes changes in cervix – collagen: ground substance ratio (enzymes degrade collagen)
• Occurs over a period of weeks – evident from 36/40
• Labour cervix offers less resistance to presenting part
• Known as effacement and dilatation
• The effacement takes longer and dilatation cannot occur without effacement

Question 28

Describe the Passage

Answer 28

[*] The passage is formed by the bony pelvis and soft tissues.

[*] The pelvic inlet is shorter in the anterior-posterior plane (obstetric conjugate, 10.5cm)

[*] Between the pelvic inlet and outlet, the mid-cavity is circular (12 cm diameter).

[*] The pelvic outlet is narrowest, usually mediolaterally (11 cm). The fetus flexes, extends and rotates as it passes through the birth canal.

[*] Resistance of the soft tissues (cervix, vagina, perineum) can slow labour.

[*] The stretching of the fibres of the levator ani and the thinning of the central portion of the perineum transforms to almost transparent membranous structures.

Question 29

Describe the Passenger

Answer 29

[*] The size and presentation of the fetus is critical in labour. Also need to consider number of passengers.

[*] The transverse lie is more common in multiples and women who have had previous pregnancies. The vast majority tend to be in longitudinal lie. Need to consider cephalic or breech, back up or back down.

[*] Complete extension => face presentation => diameter 9.5cm

[*] The orientation of the head of the fetus when entering the pelvis (in a cephalic delivery) is variable and as such the head diameter of the fetus is different in differing positions. However, moulding of the fetal cranium may occur since, cranial sutures have not yet fused.

[*] In a single footling breech, the cord could slip out therefore common to perform a C-section.

Question 30

What could failure to progress in labour be due to?

Answer 30

Failure to progress in labour may be due to inadequate power (insufficient uterine contractions), inadequate passage (abnormal bony pelvis, rigid perineum) and/or abnormalities of the passenger (fetus too big, fetal presentation). Progress in labour is plotted graphically on a partogram.

Perhaps it is possible to rotate the baby manually if there is a problem with the passenger of there is a problem with the passage, it might be possible to do an episiotomy to increase space.

Question 31

How may labour be induced?

Answer 31

Labour can be induced by giving the mother Prostaglandins and Oxytocic drugs. These drugs will cause the cervix to begin to ripen and the uterus to begin its contractions.

Question 32

How can fetal heart rate be assessed during labour?

Answer 32

During labour fetal heart rate can be assessed using a Fetal Scalp Electrode.

Question 33

Describe a Cesarean Section

Answer 33

Subrapubic Incision

• Linea alba and anterior layers of the rectus sheaths are transected and resected superiorly,
• Rectus muscles are retracted laterally or divided through their tendinous parts allowing reattachment without muscle fibre injury.

Risks: anaesthetics, bleeding, injury to other organs, infection, cut to the baby

Question 34

How may an operative delivery be facilitated?

Answer 34

Forceps
Vacuum Extraction