Pregnancy, Parturition & Late Fetal Development Flashcards
Which trimesters of pregnancy have the most rapid growth?
2 and 3
Term used to describe early embryo nutrition?
Histiotrophic
Define histiotrophic nutrition
Nutrition of fetus is reliant on uterine gland secretions and breakdown of endometrial tissues
What is the issue with depending on histiotrophic nutrition?
It cannot support the exponential growth of the second and third trimesters
How is nutrition achieved in the second and third trimesters?
There is a switch to haemotrophic nutrition (around wk 12)
Define haemotrophic nutrition and how is this achieved?
Fetus derives nutrients from the maternal blood
Achieved via haemochorial-type placenta - maternal blood directly contacts fetal membrane (chorion)
What is the chorion?
The outer fetal membrane
What is the connecting stalk?
Structure linking the developing embryo to the chorion
What are trophoblastic lacunae filled with & how?
They are spaces that are filled with maternal blood due to the breakdown of maternal capillaries and uterine glands by the syncitiotrophoblast
What do trophoblastic lacunae become?
intervillous spaces - maternal blood spaces
What is the inner fetal membrane?
Amnion
Arises from the epiblast and forms a closed avascular sac
Secretes amniotic fluid which encapsulates and protects the fetus
What is the outer fetal membrane?
Chorion
Arises from derivatives of the yolk sac and trophoblast
Highly vascularised membrane
Gives rise to CHORIONIC villi - outgrowths from cytotrophoblast forming the basis of the fetal side of placenta
Describe the vascularisation of the chorion and amnion
Amnion = Avascular Chorion = highly vascular
What is the allantois
The outgrowth of the yolk sac that grows alongside the connecting stalk
Becomes coated in mesoderm and vascularises to form the umbilical cord
Has a role in the removal of toxins in the bladder
Describe the formation of the amniotic sac
There is accumulation of amniotic fluid causing the amnion and chorion to fuse
Breif description the formation of primary chorionic villi
Projections from the cytotrophoblast which form through the syncitiotrophoblast into the maternal endometrium
Role of primary chorionic villi
Provide a substantial SA for exchange
Outline the 3 phases of chorionic villi development
Primary - projection and branching of cytotrophoblast
Secondary - mesoderm grows into primary villi
Tertiary - growth of umbilical artery and vein into villus mesoderm
Describe the microstructure of terminal villus
It’s a CONVOLUTED KNOT of vessels and vessel dilation. Slows blood flow thus allowing exchange between maternal and fetal blood
How does villus diameter change as pregnancy progresses?
Reduces so that the distance for exchange is reduced
Outline the arterial blood supply to the endometrium
Ovarian Uterine Arcuate Radial Basal Spiral
How is endovascular extra-villus trophoblast (EVT) formed?
EVT cells invade into spiral arteries
What is conversion?
Process by which the spiral artery is turned into a low pressure, high capacity conduit for maternal blood flow
Outline the pathophysiology of pre-eclampsia
Women w/ pre-eclampsia have v little spiral artery re-modelling thus the placenta gets stressed and there are defects in the mothers circulation as there is a reduced blood volume reaching the legunae
Symptoms include protein in urine and seizure
Describe the transport of oxygen, glucose, water, electrolytes, calcium and amino acids
Oxygen - diffusional gradient to fetus
Glucose - facilitated diffusion by transporters on maternal side and fetal trophoblast cells
Water - placenta main site & diffusion via hydrostatic gradients
Electrolytes - diffusion and co-transport
Ca2+ - actively transported by Mg2+ ATPase Ca2+ pump
Amino acids - reduced maternal urea excretion & active transport of amino acids to fetus
Describe some of the maternal changes during pregnancy
Cardiac output, pulmonary ventilation & blood volume increase
Peripheral resistance decreases
How is fetal/embryonic haemoglobin adapted for oxygen transport?
Hb has a higher affinity for oxygen than maternal Hb
Which group of molecules have a role in organ maturation?
Fetal corticosteroids
When does a surge of fetal corticosteroids occur?
Late stage of gestation for the final maturation
Where does gas exchange occur in the fetus?
Placenta rather than lungs
Describe the structure of the fetal ventricles
They act in parallel rather than series and thus vascular shunts bypass pulmonary and hepatic circulation
Outline the development of the respiratory system in fetus’
Primitive air sacs and surfactant production begin around week 20
Air sacs vascularise around week 28
Fetus spends 1-4 hrs a day making rapid respiratory movements during REM sleep
Describe the development of the pancreas
Pancreas is functional at the start of trimester 2 and insulin production begins mid trimester 2
What does the fetus swallow in large amounts?
Amniotic fluid is swallowed and the debris and bile acids form meconium
What is the meconium?
The first stool delivered by the fetus after birth
When do fetal movements begin and when are they detected by the mother?
They begin in the late first trimester and are detectable around 14 weeks
Does the fetus show conscious wakefulness?
No, it’s mostly in slow-wave or REM sleep
What three things are required for labour?
Expulsion of the fetus at the correct time
Expulsion of the placenta and fetal membranes
Healing to permit future reproductive events
What molecules are released during labour, causing it to resemble a pro-inflammatory reaction?
Inflammatory cytokines and prostaglandin secretion
Infiltration of immune cells also occurs
Briefly outline the 4 phases of labour
- Quiescence - cervical softening
- Activation - cervical ripening
- Stimulation- cervical dilation and uterine contraction
- Involution - breastfeeding, cervical repair and uterine involution
Outline the 3 stages within labour
- Latent phase: 2-3cm dilation THEN active phase: rapid dilation to 10cm. Contractions begin here
- Maximal myometrial contractions as the cervix is fully dilated
- Placenta and fetal membrane expulsion. Post-partum repair
What is the role of the cervix and how does it do this?
The cervix has a role in retaining the fetus in the uterus until labour.
It is highly STRETCH RESISTANT and RIGID due to high connective tissue content
Name 2 other factors with a role in cervix re-modelling
Nitric oxide
Relaxin
What occurs during cervical re-modelling?
Weeks before birth there is monocyte infiltration and hyaluron deposition
There are matrix metalloproteases which breakdown collagen in ECM thus increasing the cervical elasticity
During labour hyaluronidases break down hyaluron
Describe the changes that occur in corticotrophin releasing hormone (CRH) in labour initiation
There is an exponential increase in the amount of bioavailable CRH.
There is also a decrease in CRH-binding protein levels thus there is more free CRH.
What is the role of CRH in labour?
Increases the amount of CRH and cortisol released from fetal adrenal gland
Increased dehydroepiandrosterone (DHEAS) release from adrenal cortex
There is a +ve feedback effect, more cortisol drives placental production of CRH
What is DHEAS?
Dehydroepiandrosterone
It is a substrate for oestrogen production
What is the role of progesterone during pregnancy?
Maintains uterine relaxation
What causes a shift in oestreogen:progesterone ratio?
As term approaches, the progesterone receptor (PR) shifts from an active isoform (PR-A) to inactive isoforms (PR-B/C).
There is progesterone withdrawal and the uterus become unresponsive to progesterone and more sensitised to oestrogen due to increased oestrogen receptor
What kind of peptide is oxytocin and where is it released from in response to what?
Nonapeptide (9 amino acids)
Produced in the posterior pituitary and uterus in response to oestrogen as well as cervical and vaginal stretch
Effects of progesterone and oestrogen on the oxytocin receptor?
Before labour: progesterone reduces OXTR expression as the uterus is relaxed
Oestrogen causes an increase in OXTR expression in the uterus
What are 3 functions of oxytocin in labour?
Increases connectivity of myocytes in myometrium through gap junction formation
Destabilises membrane potentials by reducing the threshold for contraction
Enhances the liberation of intracellular Ca2+ stores
What are the 3 primary prostaglandins synthesised during labour?
PGE2
PGF2alpha
PGI2
Which 2 ways does oestrogen stimulate prostaglandin synthesis?
Activates phospholipase A2 enzyme which produces arachidonic acid for PG synthesis
Increases OXTR expression which promotes PG release
Outline the roles of the 3 PGs
PGE2 - cervix remodelling; leukocyte infiltration, IL-8 release and collagen remodelling
PGF2alpha - myometrial contractions; destabilises membrane potential and promotes myocyte connectivity
PGI2 - myometrial and lower uterine relaxation so that blood-flow to fetus can be maintained
Where do myometrial contractions begin?
At the fundus
Which parts of the uterus participate in contraction and which don’t?
The upper segment is the only part involved; the lower segment is not involved
Define brachystatic contractions
The fibres do not return to full length on relaxation
What are the implications of brachystatic contraction on formation of the brith canal?
The lower segment and cervix are pulled up - forms a large birth canal
What happens during fetal expulsion?
Fetus chin presses on chest - FLEXION; due to pressure
Fetus rotates - belly to mothers spine
Head expelled first
Upper then lower shoulder expelled followed by torso
What causes villi to collapse?
The clamping of the umbilical cord - no blood flow to placenta
Where does a hematoma form?
Between the decidua and placenta
What occurs to the uterus to cause vessel thrombosis?
The uterus contracts, thus allowing thrombosis and healing, preventing interuterine bleeding
How is a non-pregnant state restored following placenta and fetal expulsion?
Uterine involution and cervix repair
Shields uterus from bacteria and restores endometrial cyclicity in response to hormones
