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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
Q

What are Braxton hicks?

A

False labour pains when the uterus contracts and relaxes - 2nd/3rd trimester

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26
Q

Describe what happens to the foetal head as it descends towards the pelvic brim

A

Moves into either left or right occipito-transverse - occiput facing either left or right side of mother’s pelvis

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27
Q

What is engagement?

A

When the largest diameter of the foetal head successfully descends deep into the maternal pelvis

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28
Q

What is engagement identified by?

A

<3/5ths the foetal head palpable

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29
Q

Describe how cervical flexion occurs

A

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

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30
Q

When does cervical flexion occur?

A

When the foetal head comes into contact with the pelvic floor

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31
Q

How is the presenting part described after neck flexion?

A

Sub-occipitobregmatic

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32
Q

What is the purpose of the head being in a sub-occipitobregmatic position?

A

Allows the head to move through the smallest diameter of the pelvis

33
Q

Describe the shape of the pelvis

A

Gutter shape - forward and downward slope

34
Q

Describe the position of the foetal head under the suprapubic arch

A

Rotayes from left or right occipito-transverse to an occipital-anterior position

35
Q

Describe how internal rotation of the foetal head occurs

A

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
Q

When does internal rotation occur?

A

During established labour and is completed by the start of the second stage

37
Q

What does further descent after internal rotation lead to?

A

Foetus moves into the vaginal canal and eventually with each contraction the vertex becomes increasingly visible at the vulva

38
Q

What is crowning?

A

When the widest diameter of the foetal head successfully negotiates through the narrowest part of the maternal bony pelvis

39
Q

When is crowning clinically evident?

A

When the head is visible at the vulva and is no longer retreating between contractions

40
Q

What are women encouraged to do during crowning?

A

Pant so the head is born with control

41
Q

Describe extension of the presenting part?

A

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
Q

What is restitution and how does it occur?

A

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
Q

Describe how the shoulders and body are delivered

A

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
Q

What needs to happen for labour to commence?

A

Cervical ripening and increased myometrium excitability

45
Q

What is cervical ripening?

A

Softening of the cervix

46
Q

When is a woman said to be in labour?

A

When regular, painful contractions lead to effacement and dilation of the cervix

47
Q

What does ripening involve?

A

Reduction in collagen

Reduced aggregation of collagen fibres

Increase in glycosaminoglycans

Increase in hyaluronic acid

48
Q

What is the purpose of cervical ripening?

A

Less resistance to the presenting part during labour

49
Q

What causes cervical ripening

A

Oestrogen
Relaxin
Prostaglandins

50
Q

What are prostaglandins produced by?

A

Placenta
Uterine decidua
Myometrium
Membranes

51
Q

What happens to prostaglandin synthesis in the third trimester?

A

Increases as a result of oestrogen: progesterone ratio

52
Q

What causes an increase in myometrial excitability?

A

Relative decrease in progesterone in relation to oestrogen

53
Q

What does progesterone do to contractions?

A

Inhibits them

54
Q

What does oestrogen do to contractions?

A

Increases them by increasing the number of gap junctions between smooth muscle cells

55
Q

What does mechanical stretching of the uterus as a result of foetal growth do to contractions?

A

Increases contractions

56
Q

What is oxytocin responsible for?

A

Initiating uterine contractions

57
Q

What is oxytocin inhibited by?

A

Relaxin

Progesterone

58
Q

Describe the Ferguson reflex

A

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
Q

What happens at 36 weeks to the number of oxytocin receptors?

A

Increase in the myometrium

60
Q

What is the release of oxytocin described as?

61
Q

Where is oxytocin released from?

A

Posterior pituitary

62
Q

What is the first stage of labour?

A

Creation of the birth canal - lasts from beginning of labour until the cervix is fully dilated (10cm)

63
Q

How often will contractions occur in the 1st stage of labour?

64
Q

What are the two phases of the first stage of labour?

A

Latent phase

Active phase

65
Q

Describe the latent phase of the 1st stage of labour

A

Slow cervical dilation over several hours which lasts till the cervix has reached 4cm dilation

66
Q

Describe the active phase of the 1st stage of labour

A

Faster rate of cervical dilation until 10cm dilation reached, rate of 1cm/hr in nulliparous women and 2cm/hr in multiparous women

67
Q

What is the maximum time the active phase of the 1st stage of labour should last?

A

No more than 16 hrs

68
Q

What is the second stage of labour?

A

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
Q

What are the two stages of the second stage of labour?

A

Passive stage

Active stage

70
Q

Describe the passive stage of the second stage of labour

A

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
Q

Describe the active stage of the second stage of labour

A

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
Q

Describe how the contractions get stronger as labour progresses

A

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
Q

How long after initiation of active stage does the foetus deliver?

A

40mins in nulliparous

20 mins in multiparous woman

74
Q

What is the 3rd stage of labour?

A

From delivery of foetus to delivery of placenta

75
Q

Describe the 3rd stage of labour

A

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
Q

How long does the 3rd stage last?

77
Q

How much blood loss is normal for the 3rd stage?

78
Q

How is bleeding controlled during the 3rd stage?

A

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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