Physiological Labour

Question 1

When is birth classed as ‘pre-term’ labour?

Answer 1

Labour that occurs before the 37th week of gestation

Question 2

When is expulsion of the products of conception classed as sponaneous abortion (miscarriage)?

Answer 2

If this occurs before 24 weeks

Question 3

What processes are required to expel a fetus?

Answer 3

• Creation of a birth canal
• Release of structures which retain the fetus in utero
• Elargement and realignment of the cervix and vagina
• Expulsion of fetus
• Expulsion of placenta and chages to minimise blood loss

Question 4

What are the 3 stages of labour?

Answer 4

1. Stage 1- onset of regular contractions until the cervix is fully dilated
2. Stage 2- When the cervix is fully dilated untl the birth of the baby
3. Stage 3- From birth of the baby to the delivery of the placenta and membranes and bleeding is controlled

Question 5

What is the ‘lie’ of a fetus? What is the most common lie?

Answer 5

The relationship of the long axis of the fetus to the long axis of the uterus

The most common lie is longitudinal

Question 6

What is the ‘presentation’ of the fetus?

Answer 6

Describes which part of the fetus is adjacent to the pelvic inlet

Presenting part may be the head- cephalic or breeched (podalic) which is feet 1st

Question 7

What diameter does the cervix need to be in order for the fetus to pass through?

Answer 7

10cm

Question 8

What are the boundaries of the pelvic inlet?

Answer 8

• Posterior - the sacral promontory
• Lateral- the ilio-pectinal line
• Anterior - the superior pubic rami and upper margin of the pubic symphysis

Question 9

What is the true diameter of the pelvic inlet?

Answer 9

~11 cm

Question 10

What happens to the pelvic ligaments before birth?

Answer 10

Pelvic ligaments relax due to high progesterone, allows expansion to occur

Question 11

What is ‘cervical ripening’ / ‘softening’

Answer 11

A marked reduction in collagen and an increase in glycosaminoglycans which decrease the aggregation of collagen fibres so collagen bundles loosen

There is an influx of inflammatory cellls and increased NO output triggered by prostoglandins E2 and F2 alpha

Question 12

What 2 changes happen to the cervix during labour?

Answer 12

1. Dilation
2. Effacement - thinning of the cervix

Question 13

What happens to the uterine smooth muscle during pregnancy?

Answer 13

• Myometrium gets thicker due to an increased cell size and glycogen depositon

Question 14

How is contraction of the myometrium co-ordinated?

Answer 14

• Action potentials spread by gap juctions cell to cell for co-ordinated contraction over the myometrium
• Some cells can spontaneously depolarise and act as pacemakers

Question 15

What are Braxton- Hicks contractions?

Answer 15

Contractions in the late stage of pregnancy that aren’t foreceful enough to effect the fetus

Generates as amplitude of contractions increases

Question 16

Which 2 hormones are involved in increasing the frequency and force of the contractions and how?

Answer 16

Prostaglandins: enhances the release of Calcium from intracellular stores

Oxytocin: from posterior pituitary lowers the threshold for triggering action potentials. Myometrium becomes more sensitive to oxytocin as receptors are upregulated

Question 17

Explain the Ferguson Reflex during labour

Answer 17

An example of positive feedback

1. Fetal head pushes on the cervix
2. Oxytocin release from posterior pituitary
3. Causes more contractions
4. Causes fetal head to push onto cervix making contractions more forceful & frequent
5. Will continue until the baby is delivered

Question 18

Explain the principle of brachystasis

Answer 18

During labour, uterine smooth muscle fibres will shorten with each contraction but do not fully relax

Causes the uterus to progressively shorten pushing the presenting part into the birth canal

Question 19

How is pregnancy initiated in humans?

Answer 19

Unclear situation

However, medically prescribed prostaglandins will induce labour

Question 20

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

Answer 20

• Foreful uterine contractions can temporarily reduce placental blood flow → reducing O₂ supply to the fetus
• May cause brief drops in fetal heart rate
• If reductions are larger greater dips in HR occur showing the fetus is distressed

Question 21

How long does the second stage of labour last?

Answer 21

1 hour in multiparous women

Up to 2 hours in primigravida (1st pregnancy)

Question 22

Give the steps of the 2nd stage of labour

Answer 22

1. Descended head flexes as it meets the pelvix floor, reducing the diameter of presentation
2. Internal rotation of fetus
3. Sharply flexed head descends to the vulva, stretching vagina and perineum
4. Head is delivered ‘crowing’, as it emerges it rotates back to it’s orginal postion and extends
5. Shoulders rotate followed by the head, shoulders are delivered rapidly followed by the rest of the fetus

Question 23

What is the typical legnth of the 3rd stage of labour?

Answer 23

10 minutes

Question 24

How is uterine bleeding stopped after placental delivery?

Answer 24

The uterus contracts and compresses blood vessels to reduce bleeding

Oxytocin can be given to help

Question 25

How is the health of a neonate assessed after delivery?

Answer 25

APGAR score

Assesses baby on Appearance, Pulse, Grimace, Activity and Respiration at delivery and 5 mins after

The higher the score the healthier the baby

Question 26

What are the Power, Passage and Passenger?

Answer 26

The Power = myometrium contraction and progressive shortening

The Passage = getting through the bony pelvis and soft tissues

The Passenger = the size and presentation of the fetus in labour

Failure to progress into labour can come from any of these factors