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Obstetrics and gynaecology > Mechanism of normal labour > Flashcards

Mechanism of normal labour Flashcards

1
Q

List the key steps in the mechanism of labour

A 
Descent
Engagement
Neck flexion
Internal rotation
Crowning
Extension of the presenting part
Restitution 
External rotation rotation 
Lateral flexion
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2
Q

Name the most common presentation and lie of the foetus

A

Cephalic with longitudinal lie

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3
Q

What does normal labour involve?

A

The widest diameter of the foetus successfully negotiating the widest diameter of the maternal body pelvis

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4
Q

Name the borders of the pelvic inlet

A

Anterior - pubic symphysis
Lateral - iliopectineal line
Posterior - sacral promontory

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5
Q

Name the borders of the pelvic outlet

A

Anterior - Pubic arch
Lateral - ischial tuberosity
Posterior - tip of the coccyx

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6
Q

What is the transverse diamter of the pelvic inlet?

A

13cm

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7
Q

What is the transverse diameter of the mid pelvis?

A

12cm

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8
Q

What is the transverse diameter of the pelvic outlet?

A

11cm

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9
Q

What is the antero-posterior diameter of the pelvic inlet

A

11cm

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10
Q

What is the antero-posterior diameter of the mid pelvis

A

12cm

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11
Q

What is the antero-posterior diameter of the pelvic outlet?

A

13cm

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12
Q

Describe the size of the transverse diameter compared to the antero-posterior diameter at the pelvic inlet

A

Transverse > AP at pelvic inlet

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13
Q

Describe the size of the transverse diameter compared to the antero-posterior diameter at the pelvic outlet

A

AP > transverse at pelvic outlet

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14
Q

What position is the babys’ head at the pelvic inlet?

A

Transverse

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15
Q

What position is the babys’ head at the pelvic outlet?

A

Antero-posterior

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16
Q

What does the fetal head diameter vary with?

A

Degree of neck flexion

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17
Q

What is the diameter when the head is suboccipitobregmatic (vertex, flexed)

A

9.5cm

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18
Q

What is the diameter of the head when it is occipitofrontal (vertex, neutral flexion) ?

A

11cm

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19
Q

What is the diameter of the head when it is submentobregmatic (face)?

A

9.5cm

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20
Q

What is the diameter of the head when it is verticomental (brow)?

A

13.5 cm

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21
Q

What is descent?

A

When the foetus descends into the pelvis

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22
Q

When is descent likely to occur in a primigravida?

A

38 weeks and onwards

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23
Q

When is descent likely to occur in a multigravida woman?

A

When labour is established

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24
Q

What is descent encouraged by?

A

Increase in abdominal muscle tone - maternal effort

Braxton hicks in late stages of pregnancy

Fundal dominance of the uterine contractions in labour

Increased frequency and strength of contractions in labour

Amniotic fluid pressure

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25
What are Braxton hicks?
False labour pains when the uterus contracts and relaxes - 2nd/3rd trimester
26
Describe what happens to the foetal head as it descends towards the pelvic brim
Moves into either left or right occipito-transverse - occiput facing either left or right side of mother's pelvis
27
What is engagement?
When the largest diameter of the foetal head successfully descends deep into the maternal pelvis
28
What is engagement identified by?
<3/5ths the foetal head palpable
29
Describe how cervical flexion occurs
A foetus descends through the pelvis, fundal dominance of uterine contraction exerts pressure down the foetal spine towards the occiput, forcing the occiput to come into contact with the pelvic floor
30
When does cervical flexion occur?
When the foetal head comes into contact with the pelvic floor
31
How is the presenting part described after neck flexion?
Sub-occipitobregmatic
32
What is the purpose of the head being in a sub-occipitobregmatic position?
Allows the head to move through the smallest diameter of the pelvis
33
Describe the shape of the pelvis
Gutter shape - forward and downward slope
34
Describe the position of the foetal head under the suprapubic arch
Rotayes from left or right occipito-transverse to an occipital-anterior position
35
Describe how internal rotation of the foetal head occurs
With each maternal contraction, the foetal head pushes down on the pelvic floor Following each contraction, a rebound effect supports a small degree of rotation Regular contractions eventually lead to the foetal head completing the 90 degree turn
36
When does internal rotation occur?
During established labour and is completed by the start of the second stage
37
What does further descent after internal rotation lead to?
Foetus moves into the vaginal canal and eventually with each contraction the vertex becomes increasingly visible at the vulva
38
What is crowning?
When the widest diameter of the foetal head successfully negotiates through the narrowest part of the maternal bony pelvis
39
When is crowning clinically evident?
When the head is visible at the vulva and is no longer retreating between contractions
40
What are women encouraged to do during crowning?
Pant so the head is born with control
41
Describe extension of the presenting part?
The occiput slips beneath the suprapubic arch allowing the head to extend The foetal head is now born and will be facing the maternal back with its occiput anterior
42
What is restitution and how does it occur?
The head aligning with the shoulders The shoulders are reaching the pelvic floor at the point of head delivery and at next contraction will turn from transverse position to AP position Head externally rotations to face the left or right medial thigh of the mother
43
Describe how the shoulders and body are delivered
Downward traction by the HCP will assist the delivery of the anterior shoulder below the suprapubic arch Upward traction assisting the delivery of the posterior shoulder The foetal body will be delivered by contractions, the HCPs role is only to assist safe negotiation of this last stage
44
What needs to happen for labour to commence?
Cervical ripening and increased myometrium excitability
45
What is cervical ripening?
Softening of the cervix
46
When is a woman said to be in labour?
When regular, painful contractions lead to effacement and dilation of the cervix
47
What does ripening involve?
Reduction in collagen Reduced aggregation of collagen fibres Increase in glycosaminoglycans Increase in hyaluronic acid
48
What is the purpose of cervical ripening?
Less resistance to the presenting part during labour
49
What causes cervical ripening
Oestrogen Relaxin Prostaglandins
50
What are prostaglandins produced by?
Placenta Uterine decidua Myometrium Membranes
51
What happens to prostaglandin synthesis in the third trimester?
Increases as a result of oestrogen: progesterone ratio
52
What causes an increase in myometrial excitability?
Relative decrease in progesterone in relation to oestrogen
53
What does progesterone do to contractions?
Inhibits them
54
What does oestrogen do to contractions?
Increases them by increasing the number of gap junctions between smooth muscle cells
55
What does mechanical stretching of the uterus as a result of foetal growth do to contractions?
Increases contractions
56
What is oxytocin responsible for?
Initiating uterine contractions
57
What is oxytocin inhibited by?
Relaxin | Progesterone
58
Describe the Ferguson reflex
Oxytocin production is increased by afferent impulses from the cervix and vagina Contractions result in a positive feedback loop to the posterior pituitary gland to release more oxytocin, leading to stronger contractions which then rives the process of labour
59
What happens at 36 weeks to the number of oxytocin receptors?
Increase in the myometrium
60
What is the release of oxytocin described as?
Pulsatile
61
Where is oxytocin released from?
Posterior pituitary
62
What is the first stage of labour?
Creation of the birth canal - lasts from beginning of labour until the cervix is fully dilated (10cm)
63
How often will contractions occur in the 1st stage of labour?
2-3mins
64
What are the two phases of the first stage of labour?
Latent phase | Active phase
65
Describe the latent phase of the 1st stage of labour
Slow cervical dilation over several hours which lasts till the cervix has reached 4cm dilation
66
Describe the active phase of the 1st stage of labour
Faster rate of cervical dilation until 10cm dilation reached, rate of 1cm/hr in nulliparous women and 2cm/hr in multiparous women
67
What is the maximum time the active phase of the 1st stage of labour should last?
No more than 16 hrs
68
What is the second stage of labour?
From full dilation of the cervix until the foetus has been expelled Uterine contractions become expulsive and this pushes foetus through the birth canal
69
What are the two stages of the second stage of labour?
Passive stage | Active stage
70
Describe the passive stage of the second stage of labour
Lasts until the head of the foetus reaches the pelvic floor at which point the woman experiences desire to push Rotation and flexion are completed in this stage and only lasts a few minutes
71
Describe the active stage of the second stage of labour
Pressure of the foetal head on the pelvic floor results in an urge to bear down - woman pushes in conjunction with contractions to expel the foetus
72
Describe how the contractions get stronger as labour progresses
Fibres of myometrium do not fully relax following each contraction - reduces uterine capacity and pressure inside increases slowly Prostaglandins - more intracellular calcium released per action potential increasing the force of contractions Oxytocin - lowers the threshold for action potentials, increasing the frequency of contractions
73
How long after initiation of active stage does the foetus deliver?
40mins in nulliparous | 20 mins in multiparous woman
74
What is the 3rd stage of labour?
From delivery of foetus to delivery of placenta
75
Describe the 3rd stage of labour
Uterine muscle fibres contract to compress the blood vessels supplying the placenta which then shears away from the uterine wall. Contractions continue until placenta and membranes delivered
76
How long does the 3rd stage last?
15 mins
77
How much blood loss is normal for the 3rd stage?
<500ml
78
How is bleeding controlled during the 3rd stage?
Contraction of the uterus constricts blood vessels in the myometrium Pressure is exerted on the placental site once it has been delivered by the walls of the contracted uterus Normal blood clotting mechanism

Obstetrics and gynaecology (67 decks)

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