Parturition

Question 1

What is the function of a closed cervical canal during pregnancy?

Answer 1

1. Supports fetus and prevents prolapse of amnion
2. Prevents infections

Question 2

What is the structure of cervical tissue during pregnancy?

Answer 2

1. Low water content
2. Firm
3. Many collagen bundles
4. Low hyaluronic acid (6%)
5. High dermatan
6. High chondroitin sulfate

Question 3

What is the structure of cervical tissue after remodelling?

Answer 3

• Soft, plastic
• High water content
• Few collagen fibres
• High (33%) hyaluronic acid
• Low chondroitin
• Low dermatan

Question 4

What are the stages of cervial remodelling?

Answer 4

1. Softening: Gradual process leading up to birth from 1^st trimester and involves rearrangement of collagen in ECM of cervical tissue.
2. Ripening: Occurs closer to term and involves the transition from firm cervical tissue to soft cervical tissue.
3. Dilation: Breakdown of hyaluronic acid by hyaluronidase and breakdown of collagen by metalloproteases increases elasticity of cervical tissue, allowing for dilation and shortening of the cervical canal during birth.
4. Post-partum repair

Question 5

What factors mediate cervical remodelling?

Answer 5

1. PGE₂: Increases elasticity of cervical tissue possibly by influencing tissue collagen
2. Oestrogen: Increases collagenase activity
3. Progensterone: Also mediates collagen breakdown
4. Relaxin
5. NO

Question 6

How is the uterus prevented from contracting pre-term?

Answer 6

1. Progesterone makes resting potential more –ve and thus inhibits contraction
2. Progesterone pharmacologically decouples myometrium from uterine nerves
3. There is mechanical and electrical uncoupling of the myocytes in myometrium

Question 7

How is uterine contractility promoted during parturition?

Answer 7

1. More +ve resting potential caused by decrease in progesterone and increase in oestrogen
2. Increased pacemaker potential and spontaneous discharge rate of pacemaker cells caused by oxytocin and PGF_2α
3. Increased calcium release from myocytes during stimulation caused by oxytocin and PGF_2α
4. Increased number of gap junctions and better electrical conductivity caused by oxytocin and PGF_2α

Question 8

What are Braxton-Hicks contractions?

Answer 8

Painless uterine contractions that occur before term signifying increased uterine contractility.

Question 9

What is the Ferguson reflex?

Answer 9

Positive feedback system during labour whereby distension of the cervix stimulates more oxytocin release that subsequently increases uterine contractions, which causes more distension of the cervix.

Question 10

What types of drugs affect uterine contractions?

Answer 10

1. Uterotonins: Drugs that promote uterine contractility by increasing intracellular [Ca²⁺].
2. Tocolytics: Drugs that inhibit uterine contractility by reducing intracellular [Ca²⁺] or inhibiting MLCK activity.

Question 11

What is the proposed sequence of events that triggers human parturition?

Answer 11

1. Increased levels of maternal and placental CRH near parturition.
2. Stimulates release of cortisol from fetal adrenals.
3. Cortisol promotes further placental CRH production and sets up +ve feedback loop.
4. CRH also promotes secretion of DHEAS from fetal adrenals, which is converted to oestrogen in placenta. Causing rise in oestrogen.
5. Oestrogen stimulates production of PGF_2α and oxytocin.
6. All 3 mediate events that cause initiation of parturition.

Question 12

What may be the cause of raised CRH levels preceeding labour?

Answer 12

Response to stress due to placenta no longer adequate to supply fetus with adequate nutrition.

Question 13

How is the lower uterine segment formed?

Answer 13

During birth, as the head of the baby passes the pelvic inlet and becomes ‘engaged’, the isthmus is retracted to form part of the uterine sac, forming the lower uterine segment.

Question 14

What is the clinical significane of the lower uterine segment?

Answer 14

It is a prime route for access during C-sections (suprapubic C sections). This is because:

1. Retroperitoneal
2. More fibrous than muscular so scars heal better

Question 15

What are the stages of labour and their durations?

Answer 15

1. Stage 1: 10-12 hours
2. Stage 2: 1-2 hours
3. Stage 3: 10 minutes

Question 16

What are the events that occur during stage 1 of labour?

Answer 16

• Begins with onset of regular painful contractions.
• The cervix begins to dilate and blood/mucous is visible around it.
• Dilation of cervix can be divided into the latent (slow dilation up to 3cm) stage followed by the active (rapid dilation up to 10cm) stage.
• The first stage ends with full dilation of cervix up to 10cm.

Question 17

What are the events that occur during stage 2 of labour?

Answer 17

• Consists of the process of delivering the baby.
• This is mainly mediated by uterine, abdominal and diaphragmatic contractions pushing the baby through birth canal.
• The head of the baby enters the pelvic inlet with sagittal suture in coronal plane of mother, but rotates so that its sagittal suture is in the sagittal plane of mother as it exits the pelvic outlet.

Question 18

What are the events that occur during stage 3 of labour?

Answer 18

• Consists of delivery of the placenta.
• The placenta becomes detached from the uterine walls as a consequence of uterine contractions.

Question 19

How is post-partum haemorrhaging reduced?

Answer 19

• Strong contraction of the uterus induced by oxytocin

Question 20

What are the changes to maternal physiology during parturition?

Answer 20

• There is substantial increase in CO and MAP during labour.
• This is mainly a result of stress induced by pain and anxiety.
• This transient hypertension may rupture berry aneurysms in cerebral vasculature.

Question 21

What are the changes to fetal physiology during parturition?

Answer 21

• Uterine contractions reduce placental blood supply to fetus, causing hypoxia.
• Reflex bradycardia during contractions is caused as response to hypoxia and increased ICP.

Question 22

What are the factors that may cause obstructive labour in mother?

Answer 22

1. Abnormal fetal lie (orientation in uterus)
2. Malpresentation
3. Cephalo-pelvic disproportion
4. Pelvic obstruction