Physiology of Childbirth

Question 1

How is the position of the fetus assessed in pregnancy?

Answer 1

Position is assessed through palpitation and ultrasound scans
- important to determine the position of the fetus before onset of labour

Question 2

What is the lie of the fetus?

Answer 2

Describes orientation of the baby’s long axis

- orientation of the back

Question 3

What is the longitudinal lie of a fetus?

Answer 3

this is the normal position with the back along the uterus

Question 4

What is the oblique lie?

Answer 4

the back is at an angle across the uterus

Question 5

What is the transverse lie?

Answer 5

the back lies across the uterus

Question 6

What is the presentation of the fetus?:

Answer 6

the part of the fetus that is nearest the cervix

Question 7

What are the 3 main presentations of the fetus?

Answer 7

1. Cephalic: head first, this is normal
2. Breech: the bottom or feet first
3. Shoulder: associated with a transverse lie

Question 8

What is labour?

Answer 8

a sequence of actions leading to childbirth (parturition) including the expulsion of the placenta
- Labour begins with regular, painful uterine contractions accompanied by cervical dilatation

Question 9

In normal pregnancies, childbirth occurs after?

Answer 9

37 - 42 weeks

Question 10

Labour is preceeded by?

Answer 10

several weeks of false labour with irregular, painful contractions (Braxton Hicks contractions) and no cervical dilatations

Question 11

Describe the first stage of labour?

Answer 11

• From the onset of true labour until full cervical dilatation (10cm)
• The length varies between women; however the cervix should dilate at a rate of 1 cm/hr
• As labour progresses the uterine contractions become stronger and more frequent
• Contractions push the fetal head into the pelvis towards the cervix
• The amniotic membrane ruptures at this stage

Question 12

How long does the first stage of labour last?

Answer 12

• 8-10 hrs in first labour (nulliparous women, known as “primips”)
• 2-6 hrs in subsequent labours (multiparous women or “multips)

Question 13

The pain during labour is because of?

Answer 13

due to the hypoxia of the uterus caused by occlusion of blood vessels during the muscular contractions

Question 14

What 2 actions does the fetus perform before or during the first stage of labour?

Answer 14

1. Engagement of the head into the pelvis

2. Descent of the head through the pelvis

Question 15

The first stage of labour is divided into what 2 phases?

Answer 15

1. Latent phase: the cervix dilates slowly from 0-4cm

2. Active phase: the cervix dilates more rapidly from 4-10cm

Question 16

Describe the second stage of labour?

Answer 16

• From full cervical dilatation until the birth of the baby
• The baby perform 8 actions for normal birth to occur
• Uterine contractions continue and are assisted by voluntary pushing by the mother
• Once the head has crowned the mother must stop pushing so the head can pass the vaginal opening smoothly to prevent tearing
• Pain is severe during second stage due to stretching of the cervix and vagina

Question 17

How long does the second stage of labour last?

Answer 17

• 40-60 min in primips

- 10-15 min in multips

Question 18

Describe the 8 actions the fetus performs for normal birth to occur?

Answer 18

1. Flexion of the neck so its chin is on its chest
2. Internal rotation of the head so that it faces the sacrum
3. Crowning of the occiput (back of the head)
4. Extension of the neck as the head is born (support the head and neck for the umbilical cord at the back of the neck)
5. Restitution as the head rotates back to the normal position outside the mother
6. External rotation of the head towards the mother’s thigh as the shoulders rotate
7. Birth of the anterior (top) shoulder (push the baby’s head down)
8. Birth of the posterior (bottom) shoulder and body (pull the baby’s head up and support the body)

Question 19

Describe the third stage of labour?

Answer 19

• From birth until the delivery of the placenta and membranes
• Lasts between 10 and 45 min
• The uterus shrinks to the 20wk size and contractions continue
• The placenta detaches from the uterus and expelled causing haemorrhage from ruptured vessels

Question 20

State the factors responsible for the myometrium of uterus becomes more excitable towards the end of gestation causing false labor?

Answer 20

1. estrogen:progesterone ratio
2. uterine distension
3. cervical distension
4. fetal hypothalamus maturation
5. fetal adrenal activity

Question 21

Describe how the oestrogen : progesterone ratio causes false labour?

Answer 21

• Progesterone inhibits contractions and promotes uterine quiescence during pregnancy but its secretion stabilizes or drops towards the end
• Oestrogens promote excitation and stimulate contractions by stimulating increased gap junctions, prostaglandin synthesis oxytocin receptors and oxytocin
• The balance of these hormones moves in favour of oestrogen, causing increased excitability

Question 22

Describe how uterine distension causes false labour?

Answer 22

Stretching the muscle of the uterus increases contractility, so fetal growth and movement may have a stimulatory effect

Question 23

Describe how cervical distension causes false labour?

Answer 23

Irritation and stretching of the cervix causes oxytocin release, which stimulates contractions
- The fetal head stimulates this release by pressing against the cervix

Question 24

Describe how fetal hypothalamus maturation causes false labour?

Answer 24

• At full term, the fetal hypothalamus and pituitary secret more ACTH and oxytocin
• This oxytocin may cross the placenta to act on the uterus

Question 25

Describe how fetal adrenal activity causes false labour?

Answer 25

• Cortisol secretion from the fetal adrenal glands increases as fetal ACTH rises
• This stimulates placental oestrogen secretion and prostaglandin synthesis in the uterine muscle, which raises myometrial contractility

Question 26

Which hormones control parturition?

Answer 26

1. oxytocin
2. prostaglandins
3. relaxin

Question 27

Describe the hormonal control of parturition by oxytocin?

Answer 27

During labour its levels rise due to cervical stimulation by the head
Stimulates uterine contractions that pushes the fetus against the cervix, stimulating further oxytocin release
A positive feedback mechanism develops called the Ferguson reflex
Oxytocin receptors in the uterus are increased during late pregnancy by action of oestrogen
Oxytocin binding stimulates prostaglandin production which causes the increased contractility (especially PGE2)

Question 28

Describe the hormonal control of parturition by prostaglandins?

Answer 28

Are locally acting eicosanoids that regulate many processes throughout the body
During labour the prostaglandin PGE2 is synthesised in the uterine muscle cells in response to oxytocin
It increases gap junctions and stimulates the release of calcium ions, which cause muscle contraction
PGE2 is also synthesized in the cervix, where it stimulates cervical softening and dilatation known as ripening

Question 29

Describe the hormonal control of parturition by relaxin?

Answer 29

Promotes the relaxation of the pelvic ligaments and softens the cervix prior to the purturition
This allows both structures to stretch so that the fetus can pass through the pelvis

Question 30

Describe 3 ways labour can be induced

Answer 30

1. Prostaglandins (PGE2) by vaginal pessary or gel, which acts within a few hours
2. Intravenous oxytocin, which takes about 12 hours to act
3. Rupture of the amniotic membrane (amniotomy), which can only be performed if the cervix is more than 4cm dilated