L17: labour & delivery

Question 1

List the processes which occur in the first stage of labour

Answer 1

Creation of a birth canal
Release of the structures which normally retain the fetus in utero
The enlargement and realignment of the cervix and vagina

Question 2

List the process which occurs in the second stage of labour

Answer 2

Expulsion of the fetus

Question 3

List the process which occurs in the third stage of labour

Answer 3

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

Question 4

Describe the ‘lie’ and ‘presentation’ of the fetus in the first stage of labour

Answer 4

Lie – describes the relationship of the long axis of the fetus to the long axis of the uterus
Commonest lie is longitudinal, fetus normally has a flexed attitude
Presentation – describes which part of the fetus is adjacent to the pelvic inlet
If baby lies longitudinally -> presenting part may be head or breech; presenting part may be in a variety of positions which affects the diameter of presentation
Birth canal needs to have a diameter of about 10cm for fetus to pass through, required diameter may change with different positions

Question 5

What are the borders of the pelvic inlet?

Answer 5

Posterior – sacral promontory
Lateral – ilio-pectineal line
Anterior – superior pubic rami & upper margin of the pubic symphysis

Question 6

Describe the creation of the birth canal

Answer 6

Birth canal cannot extend beyond the limits determined by the pelvis – softening of the pelvic ligaments may allow some expansion to occur
To create a birth canal -> cervix must dilate & be retracted anteriorly
During this process, the fetal membranes rupture, releasing amniotic fluid
Cervical dilation is facilitated by cervical ripening, but produced by forceful contractions of uterine smooth muscle -> first thin (effacement) the cervix and then dilate it

Question 7

Describe cervical ripening

Answer 7

Cervix = high connective tissue content made up of collagen fibres embedded in a proteoglycan matrix
Ripening – marked reduction in collagen & marked increase in GAGs -> decrease the aggregation of collagen fibres
Therefore, collagen bundles ‘loosen’
Also, influx of inflammatory cells & increase in NO output – these changes are triggered by prostaglandins

Question 8

Describe the uterine smooth muscle in contractions

Answer 8

APs spread from cell to cell via specialised gap junctions, allowing co-ordinated contractions to spread over the myometrium – some smooth muscle cells are capable of spontaneous depolarisation so can act as pacemakers
Myometrium = always spontaneously motile
In early pregnancy -> contractions occur every 30 mins, but low force
As pregnancy continues -> frequency falls, but force increases, producing noticeable ‘Braxton-Hicks’ contractions (none of these contractions are forceful enough to have any effect on the fetus)

Question 9

Describe the two hormones involved in the onset of labour

Answer 9

Onset of labour is a relatively sudden increase in the frequency and force of contractions
Prostaglandins – act by enhancing the release of calcium from intracellular stores
Oxytocin – peptide hormone is secreted from the posterior pituitary gland under the control of neurones in the hypothalamus, acts by lowering the threshold for triggering action potentials

Question 10

Describe the Ferguson reflex

Answer 10

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
Positive feedback = makes contraction more forceful and frequent

Question 11

What is brachystasis?

Answer 11

Property of uterine smooth muscle
At each contraction muscle fibres shorten, but do not relax fully
The uterus, particularly the fundal region, shortens progressively
Pushes the presenting part into the birth canal and stretches the cervix over it
Descent of the presenting part therefore occurs progressively during labour, until it engages in the pelvis

Question 12

Describe the initiation of labour in humans

Answer 12

Unclear – no clear evidence of pre-labour increases in plasma prostaglandins/changes in oestrogen progesterone ratio (progesterone will however induce labour when given medically)
As labour progresses -> increasingly forceful uterine contractions may temporarily reduce placental blood flow & so reduce oxygen supply to the fetus
May lead to brief reductions in fetal heart rate that may be monitored (if reductions in flow are greater than usual -> fetal becomes distressed)

Question 13

Describe when the second stage of labour begins

Answer 13

First stage ends when cervical dilatation reaches 10cm
Fetus is normally then delivered relatively quickly
Second stage normally lasts up to 1 hour in the multiparous women and up to 2 hours in primigravida (first pregnancies)

Question 14

Describe what happens in the second stage of labour

Answer 14

1) Descended head flexes as it touches pelvic floor, reducing the diameter of presentation
2) Internal rotation
3) Sharply flexed head descends to the vulva, so stretching the vagina and peritoneum
4) Head is then delivered (crowning) & as it emerges it rotates back to its original position and extends
5) Shoulders then rotate followed by the head, and the shoulders deliver, followed rapidly by the rest of the fetus

Question 15

What is shoulder dystocia?

Answer 15

Can occur if the fetal shoulder does not deliver without medical intervention
Can lead to complications for the mother and the fetus
Fetus can get brachial plexus injuries

Question 16

Describe the third stage of labour

Answer 16

With fetus removed, there is a powerful uterine contraction -> separates the placenta, positioning it into the upper part of the vagina/lower uterine segment
The placenta and membranes are then expelled, normally within about 10 minutes – completes the third stage of labour
Contraction of the uterus – compresses blood vessels and reduces bleeding -> normally enhanced by administration of an oxytocic drug

Question 17

Describe the establishment of independent life

Answer 17

Within a short time, fetus takes its first breath
Causes a dramatic fall in pulmonary vascular resistance, so reducing the pulmonary arterial pressure & increasing left atrial pressure relative to right atrial pressure
The atrial pressure change shuts the foramen ovale & rising pO2 causes the ductus arteriosus to constrict, so establishing the adult form of circulation
Sphincter in the ductus venosus constricts, so that all blood entering the liver passes through the hepatic sinusoids

Question 18

What is the APGAR score?

Answer 18

Appearance, pulse, grimace, activity & respiration
Condition of the neonate following delivery is scored by the APGAR score
Assess fetal wellbeing soon after delivery & then five minutes later
Generates a score from 1-10, the higher the number the healthier the baby

Question 19

What can the elements of labour be classified into clinically?

Answer 19

Powers
Passage
Passenger
Failure to progress in labour may be due to inadequate power, inadequate passage and/or abnormalities of the passenger

Question 20

Describe ‘the powers’ in labour

Answer 20

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 shortening, particularly in the fundus
Uterine contractions can be assessed in terms of frequency, amplitude & duration

Question 21

Describe ‘the passage’ in labour

Answer 21

Formed by the bony pelvis and soft tissue
The pelvis is shorter in the anterior-posterior plane
Between the pelvic inlet and outlet, the mid-cavity is circular
Pelvic outlet is narrowest usually mediolaterally
Fetus flexes, extends & rotates as it passes through the birth canal

Question 22

Describe the ‘passenger’ in labour

Answer 22

Size and presentation of the fetus is critical in labour
Orientation of the head of the fetus when entering the pelvis is variable & as such the head diameter of the fetus is different in differing positions
Moulding of the fetal cranium may occur since cranial sutures have not yet fused