Parturition Flashcards
What does the pre natal circulatory system look like?
- Placenta acts as site of gas exchange for fetus
- Ventricles act in parallel rather than series (pump together)
- vascular shunts bypass pulmonary & hepatic circulation -> close at birth
What does prenatal resp system look like
- Primitive air sacs form in lungs around 20 weeks, vascularization from 28 weeks
- Surfactant production begins around week 20, upregulated towards term
- Fetus spends 1-4h/day making rapid respiratory movements during REM sleep* important for diapragm development and practice for air
What does Gastrointestinal system look like prenatally?
- Endocrine pancreas functional from start of 2T, insulin from mid-2T
- Liver glycogen progressively deposited – accelerates towards term
- Large amounts of amniotic fluid swallowed –debris and bile acids form MECONIUM
What does the nervous system look like prenatally?
- Fetal movements begin late 1T, detectable by mother from ~14 weeks
- Stress responses from 18 weeks, thalamus-cortex connections form by 24 weeks
- Fetus does not show conscious wakefulness – mostly in slow-wave or REM sleep
What is labour?
Safe expulsion of fetus at the right time
expulasion of the placenta and fetal membranes too so it is empty for future
resolution/healing for future events
What increases in the fetal blood near the end of pregnancy?
Corticosteroids increase
Causes increase in liver glycogen and lecithin - surfactant. This spurs organ maturation ready for birth
How is labour a pro-inflammatory reaction?
Immune cell infiltration
Inflammatory cytokine and prostaglandin secretion
What is phase 1 of parturition?
Quiescence:
contractile, unresponsive, cervical softening
from late T1 onwards
What is phase 2 of parturition?
Activation:
uterine preparedness for labour, cervical ripening
What is phase 3 of parturition?
Stimulation:
uterine contraction, cervical dilation, fetal and placenta expulsion
What is phase 4 of parturition?
Involution:
recovery, uterine involution, cervical repair, breast feeding
What are the 3 stages of labour
different organisation method to the 4 phases
First stage: latent phase and active phase, contractions start
Second stage - commenced at full cervical dilation, maximal myometrial contractions
Third stage - explusion of placenta and membranes, post partum repair
What is the latent phase?
Slow dilation of the cervix to 2-3cm
What is the active phase?
Rapid dilation of cervix to 10cm
Which stage of labour is the longest?
Latent and active phase: 0-14 hours with active being longer
fetal descent and delivery in second phase taking 1-2 hours
placenta delivery taking 1 hour
How long is a first delivery?
8-18 hours
How long is subsequent deliveries?
5-12 hours
Why does the cervix need to have high connective tissue content?
provides rigidity
stretch resistant
connective tissue: bundles of collagen fibres embedded in proteo-glycan matrix
changes to the collagen bundle causes softening
How is the cervix remodelled?
Softening
Ripening:
Dilation
post partum repair
What is cervix softening?
Softening: in first trimester, changes in compliance but retains cervical COMPETENCE - keeps fetus in uterus
What is cervical ripening?
Weeks and days before birth the cervix will be:
- infiltrated by monocyte and IL-6 and IL-8 secretion
And Hylaluron deposition
What is cervical dilation?
increased elasticity
- increased hyluronidase expression –> HA breakdown
Matrix MatelloProteases decrease collagen content
What is Post partum cervix repair?
Recovery of tissue integrity and competency
How is labour initiated by corticotrophin released hormone (CRH)?
Fetus determines timing of parturition through changes in fetal HPA axis. And it releases CRH
CRH levels rise exponentially towards the end of pregnancy
There is a decline in CRH binding protein levels so CRH bioavailability increases
What are the functions of CRH?
promotes fetal ACTH and cortisol release
increasing cortisol drives placental production of CRH –> positive feed back
stimulated DHEAS produced by the fetal adrenal cortex -> substrate for estrogen production
What are DHEAS?
Dehydroepiandrosterone Sulphate - substrate to increase estrogen by the placenta
What is progesterone important for?
maintains uterine relaxation throughout pregnancy
Why may the estrogen progesterone ratio increase?
as serum estrogen:progesterone ratio may shift in favour of estrogen
As term approaches how does progesterone decrease?
Switch from PR-A isoforms to PR-B and PR-C which are repressive isoforms
This causes Progesterone withdrawel * levels may be high but receptors have changed
What happens to estrogen receptor Alpha expression during labour?
Rises
uterus becomes ‘blinded’ to progesterone action and sensitised to estrogen action.
Where is oxytocin made?
a Nonapeptide hormone synthesised mainly in the utero-placental tissues and pituitary
What drives Oxytocin expression increase in uterus?
Rise in estrogen levels
- levels increase sharply at the onset of labour
What promotes pituitary oxytocin release?
The Ferguson reflex:
promoted by stretch receptors
As fetus pushes down onto cervix, stretch receptors signal to hypothalamus trigers oxytocin release
How does oxytocin signal?
Through G-coupled receptor (OTR/OXTR)
What inhibits OXTR expression pre-labour?
Progesterone inhibits OXTR expression to keep the uterus relaxed
- in comparison to estrogen which promotes large increase in uterine OXTR expression
What are the functions of oxytocin?
increase connectivity of myocytes in myometrium
destablise membrane potentials to lower threshold for contraction
enhances liberation of intracellular Ca2+ ion stores to aid contraction
What are the primary prostaglandins which are synthesized during labour?
PGE2, PGF2alpha and PGI2.
How does estrogen levels drive prostaglandin action?
Rising estrogen activates phospholipase A2 enzyme, generating more arachidonic acid for PG synthesis
- Estrogen stimulation of oxytocin receptor expression promotes PG release.
What does PGE2?
Cervix remodelling:
leukocyte infiltration - IL-8 release and collagen bundle re-modelling
What does PGF2 Alpha do?
Acts on myometrium, destablise membrane potentials and connectivity of myocytes
What does PGI2 do?
- myometrium
Promotes myometrial smooth muscle relaxation and relaxation of lower uterine segment.
What other factors may be involved in cervix re modelling?
peptide hormone relaxin and nitric oxide (NO)
How does myometrial contractions lead to cervix to be pulled up to form the birth canal?
Myometrial muscle cells form a syncytium (gap junction)
contractions start from the fundas and work down the upper segement
muscle contractions are brachystatic causing lower segment and cervix to be pulled up to form birth canal
What is brachystatis contractions?
fibres do not return to full length on relaxation
How is the fetus expelled? *in what order
head engages with pelvic space
pressure on fetus causes chin to press against chest - flexion
fetus rotates so the belly is towards mothers spine
head expelled first after cervix dilates
shoulders delivered sequentially
What shrinks rapidly after birth?
Uterus –> causea area of contact of placenta with endometrium to shrink
uterine shrinkage also causes folding of fetal membranes which then peel off the endometrium
What does umbilical cord clamping do?
Stops fetal blood flow to placenta –> villi collapse
How are the placenta and fetal tissues expelled?
Contractions
what forms between the decidua and placenta?
Hematoma
why does the uterus remain contracted post delivery?
facilitate uterine vessel thrombosis
Why is it vital for uterus involution and cervix repair to occur?
Go back to non pregnant state
To shield uterus from commensural bacteria
restore endometrial cyclicity in response to hormones
