Parturition

Question 1

What is parturition?

Answer 1

Process by which conceptus (foetus, placenta and foetal membranes) is expelled from the uterus

Question 2

What does parturition require?

Answer 2

Signal from foetus
Cervical softening
Co-ordinated myometrial contractions

Question 3

What is the 1st stage of parturition?

Answer 3

2-6 hrs

• regular uterine contractions
• cervical shortening and dilation occurs
• • latent phase: cervix slowly dilates
• • active phase: rapid dilation of cervix

Question 4

What is the 2nd stage of parturition?

Answer 4

30-120 mins

- complete delivery of foetus, rupture of membranes and abdominal contractions

Question 5

What is the 3rd stage of parturition?

Answer 5

5-8 hrs

- delivery of placenta

Question 6

How do myometrial contractions occur?

Answer 6

Consists of smooth muscle
Oestrogen stimulates muscle cell size from 50-500µm (hypertrophy)
Muscle cell syncytium electrically coupled via gap junctions (or nexuses)
Co-ordinated uterine contraction requires simultaneous activation of all muscle cells

Question 7

How the myometrium contracts?

Answer 7

Excitation of smooth muscle dependant on alterations in membrane potential
Myometrium must be depolarised (-50mV), spontaneous de-polarising pacemaker potentials occur
If the magnitude of these potentials > critical threshold, a burst of action potentials is superimposed on the pacemaker potentials
Increased intracellular Ca2+ occurs, which binds to regulatory sites on actin and myosin allowing expression of ATPase activity causing contraction

Question 8

What is brachystasis?

Answer 8

Contractions retract the lower uterine segment at the cervix - creating a birth canal
Myometrial cells undergo brachystasis - muscle cells contract and shorten but do not regain original length at relaxation
- uterine muscles retract
- uterine wall thickens
- uterine volume reduces

Question 9

What is the retraction ring?

Answer 9

Junction between upper segments (contractions) and lower segment (passive, no contractions)

Question 10

Which two hormones directly regulate myometrial contraction? (parturition cascade)

Answer 10

Oxytocin - lowers excitation threshold of muscle cells

Prostaglandins - stimulates liberation of Ca2+ from intracellular stores

Question 11

How is oxytocin regulated?

Answer 11

Synthesised in the hypothalamus - transported axonally to the posterior pituitary
Released in response to stimulation of the cervix by the foetus - Fergusson reflex

Question 12

What is the parturition cascade?

Answer 12

Prostaglandins 
                       ↓
increase myometrial contractions 
                       ↓ 
         increases pressure 
                       ↓
increases cervical stimulation 
                       ↓
                 oxytocin -feedback to increase myometrial contraction

Question 13

What is the clinical significance of oxytocin?

Answer 13

Uterine inertia
Primary uterine inertia
- failure to initiate contractions at start of parturition
- lack of oxytocin/receptors
- obesity in mother
- lack of induction of contractions by a small uterus
Secondary uterine inerita
- Good uterine contractions at the start of parturition
- Large litter (size or number) and prolonged labour
- dystocia due to abnormal foetus presentation
- uterus fatigued and over-stretched contractions stop

Question 14

How does the cervix dilate?

Answer 14

Cervix has a high connective tissue content
- collagen fibre bundles
- proteoglycan matrix
This connective tissue resists stretch, allowing distention of the uterus whilst maintaining cervix in a closed state
Cervical softening involves:
- reduction of collagen fibres
- increase in proteoglycan matrix fibres
- under endocrine control

Question 15

What is the role of hormones in cervical softening?

Answer 15

Prostaglandins soften the cervix
Cervix produces:
- prostaglandin E2 (PGE2)
- prostacyclin (PGI2)
- prostaglandin F2𝛂
Production increases at term and during parturition
Application of PGE2 induces cervical softening

Question 16

What are prostaglandins roles?

Answer 16

Break down collagen

Stimulate uterine contraction

Question 17

How are prostaglandins regulated?

Answer 17

During pregnancy, progesterone, inhibits prostaglandin production stopping the release of phospholipase A2 (PL-A2) from lysosomes
Parturition requires an increased prostaglandin synthesis
Decrease in progesterone at term removes block on PL-A2
Increase in oestrogen at term liberates PL-A2
Increase oestrogen/progesterone ratio at term we increase prostaglandin production

Question 18

What is ring womb?

Answer 18

Failure of the cervix to soften and dilate but uterine contractions are normal - unable to birth foetus

• caused by: lack of prostaglandin production and prostaglandin receptors, linked to selenium deficiency, genetic component
• treatment: prostaglandin gel to cervix, c-section, in farm animals cull

Question 19

What is false ring womb?

Answer 19

Cervical dilation doesn’t proceed normally and uterine contractions are present
Contraction of muscles around the cervix, palpate a tight ring at cervix
- caused by: premature intervention by the sheperd, lamb in wrong place so not pressing on the cervix
- treatment: gentle stretching of the cervix with fingers - takes up to 45 mins, dont pull lamb through a partly open cervix

Question 20

What is a vaginal/cervical prolapse linked to?

Answer 20

Excess body condition
Multiple foetus’ or a large foetus
High fibre diets
Limited exercise

Question 21

What are complications of vaginal/cervical prolapses?

Answer 21

Abortions
Ring womb at parturition
Second vaginal prolapse at parturition
Uterine prolapse at parturition

Question 22

What are treatments of vaginal/cervical prolapses?

Answer 22

Replace the prolapse
Vulva suture - anchored to haired skin prevents tearing
Retention spoon
Prolapse harness
Cull after parturition - offspring not used for breeding

Question 23

How is parturition initiated in sheep?

Answer 23

Placenta secretes progesterone at the end of pregnancy so sheep must remove placental progesterone to give birth
Sheep metabolise away progesterone in favour of oestrogen - increased oestrogen:progesterone ratio
↓ progesterone + ↑ oestrogen = ↑ prostaglandin = ↑ contraction + ↑ cervical softening

Question 24

How is the onset of parturition achieved in sheep?

Answer 24

Foetus controls onset of parturition and must increase oestrogen:progesterone ratio in maternal circulation to start parturition
- foetal pituitary - adrenal axis
Foetus becomes stressed, foetal ACTH is released and cortisol increases
Increase in foetal cortisol leads to decrease in maternal progesterone

Question 25

How is parturition initiated in goats?

Answer 25

The CL secretes progesterone at the end of pregnancy and goats must remove the CL progesterone to give birth
- CL can be shut down without problems using luteolytic hormone - prostaglandin

Question 26

How is the onset of parturition achieved in goats?

Answer 26

Foetus controls onset of parturition so must stop progesterone production by CL to start parturition
- foetal pituitary - adrenal axis
Foetus becomes stressed, foetal ACTH is released and cortisol increases
Increase in foetal cortisol leads to an increase in oestrogen
↑ oestrogen = ↑ prostaglandin (PGF2𝛂) = ↑ luteolysis = ↓ progesterone
↑ prostaglandin = contraction + ↑ cervical softening

Question 27

How is human parturition achieved?

Answer 27

Human placenta lacks the enzymes 17𝛂 hydroxylase and C17-C20-lyase
Maternal progesterone levels do not fall at parturition
Neither cortisol or placental progesterone initiate parturition
- removal of foetal adrenals (ACTH and cortisol) does not lengthen gestation
- infusion of ACTH or cortisol does not induce parturition
However, prostaglandins act to soften the cervix and increase uterine contractions (as in goat and sheep)