11. Physiology of Parturition Flashcards
What are some pro-labour factors (theories)?
The exact mechanism of the onset of labour is unclear. What could it possibly be due to?
1. The placental clock: placenta has predefined lifespan, usually up to 42w.
- Foetus can trigger its adrenal glands -> oestrogen which contracts progesterone -> prod of prostaglandins and oxytocin -> contractions
3. Endothelin: from placenta, triggers contractions
4. Inflammation: of chorionic/placental membrane -> prostaglandin release
Actual/functional withdrawal of progesterone at term, foetal adrenals produce more DHEAS which is converted to oestrogen by placenta which promotes release of prostaglandins from decidua and foetal membranes
What cervical changes happen at term?
Prostaglandins: increased in myometrium, decidua and foetal membranes, promote cervical ripening and stimulate contractions by acting directly on myometrium and indirectly by increasing the expression of oxytocin receptors. (PG types: vowels = dilatory, consonants = contractors)
Cervical ripening: distensibility index increases, collagen concentration decreases (collagenolysis) - replaced by amorphous substance. PG E2 acts on granulocytes which release collagenase, oestrogen increases sensitivity to E2, fibroblasts release MMPs
What myometrial changes happen at term?
What stimulates myometrial activity?
What is Ferguson’s reflex?
Myometrial ripening: prostaglandins (develop gap junctions, increased oxytocin receptors)
Oxytocin (from post pituitary), prostaglandins
Neuroendocrine reflex comprising the self-sustaining cycle of uterine contractions initiated by pressure at the cervix or vaginal walls. Baby drops into saucer -> impulses to pituitary to release oxytocin -> arrives at fundus of uterus where PGs have already caused expression of oxytocin receptors. Triggers involuntary contractions, cranial -> caudal.
What are the 3 signs of labour?
What are the 3 stages of labour? (briefly)
The ‘show’: release of mucus plug (operculum) which seals cervix opening
Rupture of membranes (as tube becomes saucer it stretches membrane and there’s a tearing of BV - blood stained amiotic discharge = waters break)
Painful contractions: Braxton Hicks become reglar, when every couple of mins and last longer than 40s = labour.
1) 0-10cm cervical dilation (latent and active phases)
2) full dilation to expulsion of foetus (propulsive and expulsive phases)
3) expulsion of foetus to expulsion of placenta
Describe the 1st stage of labour.
Describe the 2nd stage of labour.
Latent phase: 0 - 3cm cervical dilation. Active phase: 3 - 10cm cervical dilation. Braxton Hicks contractions lead to painful rhythmic contractions every 2-3m. Myometrial fibres contract and retract. Upper segment of uterus heaped up while lower thins and stretches over presenting part.
Propulsive phase: full dilation - presenting part reaching pelvic floor). Expulsive phase: reaching pelvic floor - delivery of baby. Uterus and vagina form continuous tube. Uterine contractions supplemented by voluntary contraction of abdominal muscles and fixing the diaphragm to increase intra-abdominal pressure. Pelvic floor muscles stretched backwards.
Describe the 3rd stage of labour.
Normal labour depends on what 3 key factors?
Delivery to baby -> placenta expulsion. Uterine muscles contract tonically to constrict BV passing between interlacing fibres. Placenta seperates as placental bed constricted down to half its size, and is expelled by uterine contractions moving caudally. If retained >30m anticipate severe post partum haemorrhage.
The passenger, the passages and the powers.
How is ‘the passenger’ adapted for birth?
What is the curve of Carus?
How is the female pelvis adapted for childbirth?
What issues arise with the African pelvis?
Baby’s skull bones held together by membrane: anterior and posterior fontanelle - skull bones start to override each other if mother’s pelvis is too tight for the baby’s head.
An arc corresponding to the pelvic axis. Path for the baby to traverse.
Inlet wider. Pubic arch wider - obtuse rather than acute. Flared pelvis (not funnel). Gynaecoid pelvis - all diameters >10cm.
Shallow and kidney shaped, babies stuck entering pelvis -> impacted labours that cause holes in bladder and rectum -> fistula.
What direction does the baby’s head usually enter the pelvis?
How do the hammock-shaped levator muscles help childbirth?
What are the functions of the pelvic floor?
Head over side, take up largest availble diameter.
As woman contracts, hammock seperates and contracts backwards to propel baby forwards to lie outside pubic symphysis.
Support pelvic viscera, help continence, build effective intra-abdominal pressure by contracting with diaphragm and abdominal wall muscles, helps rotation of presenting part (obstetrics) -> as baby hits PF, it turns.
What are the major stages in birth?
Delivery of anterior shoulder, then posterior shoulder.
What is an episiotomy?
Describe how oestrogen, oxytocin and prostaglandins contribute to contractions.
Surgical cut made at the opening of the vagina during childbirth, to aid a difficult delivery and prevent rupture of tissues. Can be midline (US) or mediolateral (UK). Vaginal epithelium and perineal skin, bulbocavernosus and superifical and deep perineal muscles cut, sometimes EAS.
