Onset and management of parturition Flashcards
What is parturition
The process of giving birth
What are the 4 stages of parturition
0 = latent phase. irregular contractions, effacement + dilation of cervix 1 = onset of regular uterine contractions. effacement + dilation of cervix to 10cm 2 = full dilation to birth of baby 3 = birth of baby to expulsion of placenta + membranes
What are the key pathways identified in onset and maintenance of labour
Activation of myometrium through maternal.fetal HPA axis + proliferation of stretch + oxytocin receptors
Action of placental steroids
Action of prostaglandins + inflammatory uterocervical processes
Action of oxytoci
Positive biofeedback- ferguson reflex
What is myometrial quiescence
Proliferation + hypertrophy of smooth muscle in uterus throughout pregnancy
Capacity for contractility decreased
hCG inhibits myometrial gap junction formation
Progesterone inhibits oxytocin
Relaxin acts on smooth muscle
Oxytocin stimulates synthesis of relaxatory prostaglandins (until hCG decreases at labour)
Melanin suppresses myometrial oxytocin recpetors
Corticotrophin releasing hormone?
How is myometrial contractivity enhanced at term
Uterine stretch + activation
Increase in oxytocin receptors in endometrium + myometrium, increased myometrial gap junctions
Endocrine pathway involving fetal HPA axis + increased output of cortisol
Describe uterine stretch + activation
Rapid fetal growth at term causes accelerated uterine distension, associated with increase in oxytocin receptors (50x), highest in fundus + corpus
What are myometrial gap junctions
Formed from 32 weeks
Symmetrical portions of plasma membrane from adjacent cells
Form intracellular channels allowing rapid propagation of action potential between cells
Formation stimulated by oestrogen, prostaglandin + melatonin, inhibited by progesterone, hCG + relaxin
Fundal dominance
Synchronise myometrial responsiveness to neuroendocrine + intrauterine oxytocin systems
What is the function of the maternal/fetal HPA axis
Known feature of increasing myometrial contractility
Primary function- control response to stress
Key component- corticotrophin releasing hormone
What corticotrophin releasing hormone
Synthesised by placental tissues and secreted into maternal circulation during pregnancy, increasing
Precise function unknown
Series of molecular events mediated by autocrine, endocrine + paracrine action of CRH activated at term
Duration of pregnancy related to levels of CRH
What are the hypotheses for the role of corticotrophin releasing hormone
Direct quiescent/contractile effect on myometrial muscle
Indirect effect by stimulating prostaglanding production by fetal membranes + placenta
Augments myometrial contractile response to PGE2
How is the fetal HPA function activated
Fetus triggers onset of labour through activation of HPA axis
Increased output of cortisol from fetal adrenal gland
Fetal cortisol acts on placenta, resulting in decrease in placental output of progesterone + increased output of oestrogen
Overall increased output of stimulatory prostaglandins from uterus between layers of fetal membranes
What is the role of prostaglandins in parturition
Concentration increased in blood, urine + amniotic fluid during labour
Synthesised by uterine tissues + fetal membranes, increased during labour
Administration can induce labour, inhibitors can delay
Concentration increase before myometrial contractions
What is the role of inflammatory uterocervical processes in labour
Begin at later part of pregnancy
Progressive release of inflammatory mediators (cytokines), stimulate synthesis of prostaglandins
Interleukins increased collagenolytic activity of cervix
Nitric oxide stimulates release of PGE2 from fetal membranes
Cervical ripening + dilation associated with major alterations of extracellular matric + cellular composition of cervical tissue
What is the role of oxytocin in labour
Maternal oxytocin produced by the supraoptic + paraventicular nucleus
Pwerful uterotonic with significant role in parturition
Modulated by progesterone
Under certain conditions indirectly inhibits release of adrenocorticotrophin releasing hormone + cortisol
Increase in nocturnal release from 32 weeks, coinciding with decrease in progesterone:oestrogen ratio + rise of oxytocin receptors
Positive feedback from uterus, cervix + vagina
Regulated by oxytocinase, rapidly degrades + prevents receptor desensitisation
May act directly on myometrium or indirectly on prostaglandin production
Describe oxytocin pathway
Stretching pathway transmits signals via spinal cord + brainstem to oxytocin neurons in hypothalamus causeing release of oxytocin via hypothalamo-hypophyseal tract into maternal circulation. Sensory afferent nerve causes +ve feedback
