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regub 19: pregnancy (ii) (factors that trigger parturition onset…
regub 19: pregnancy (ii)
secretion of oestrogen by placenta
placenta extracts DHEA (made in foetal adrenal cortex) from foetal blood + converts it to oestrogen
this oestrogen gets secreted into maternal blood
primary oestrogen made = estriol (E3)
measured in urine to asses viability of foetus
amount of hormone secreted is proportional to placental weight
stages of labour
1) cervical dilation
10cm
baby's head helps
longest stage (esp in 1st pregnancy)
up to 24hrs
2) delivery of baby
when dilation is complete
30-90 mins
3) delivery of placenta
2nd series of contractions
placenta separates from myometrium
shortest stage (15-30 mis after baby)
involution (uterus shrinks to pregestational size)
aided by breastfeeding
factors that trigger parturition onset
mechanical factors
distension of uterine muscle
softening of cervix
progesterone decreases
due to increased progesterone binding protein + decreased no. of Rs
oestrogen soars
causes synthesis of connexions between uterine smooth muscle cells
so uterus contracts as coordinated unit
increases no. of oxytocin Rs
causes production of local PGs
increases responsiveness to oxytoxin
cervical ripening
oxytocin
levels don't increase, uterus just becomes more responsive (x100)
critical threshold -> labour
used clinically to induce labour
relaxin
large polypeptide hormone made by corpus luteum + decidua
softens cervix, allows passage of foetus
increases oxytocin Rs
the ferguson reflex
+ve feedback to maintain labour
foetal head stretches cervix
cervical stretch causes oxytocin release
oxytocin stimulates PG production to enhance contractions
continues until cervical pressure relief