Pregnancy, Parturition and Foetal Development Flashcards
What type of nutrition is used during early embryo development and how is it obtained?
Histiotrophic: this is nutrition originating from the degradation of local endometrial tissue and uterine gland secretion - uterine milk
Syncytiotrophoblasts invade maternal endometrium and break down tissue
There’s also some breakdown of maternal capillaries so syncytiotrophoblast bathed in maternal blood for nutrients
Glands within endometrium supplying uterine milk
When does the switch to haemotrophic nutrition in the fetus occur? Why does this switch happen?
At the start of the second trimester
Fetal growth accelerates, can’t be maintained with histiotrophic nutrition
What is haemotrophic nutrition and how is it used?
Fetus derives nutrients from maternal blood
Nutrition derived through a haemochorial-type placenta (maternal blood directly in contact with chorion) where maternal blood directly contacts the fetal membranes
What is the amnion derived from?
The epiblast
What does the amnion form?
The amniotic cavity and then becomes part of the amniotic sac
What does the connecting stalk link?
It links the developing embryo unit to the chorion
What are trophoblastic lacunae?
large spaces filled with maternal blood formed by the breakdown of maternal capillaries and unterine glands
What do the trophoblastic lacunae become?
Intervillous spaces aka maternal blood spaces
What is the chorion?
The second fetal membrane
What are the names of the inner and outer fetal membranes?
Inner= amnion Outer= chorion
When does the amnion begin to secrete amniotic fluid?
Week 5
What does the chorion originate from?
From yolk sac derivative and the trophoblast
What is the blood supply to the chorion like?
Highly vascularised
What does the chorion give rise to?
The chorionic villi
What is the allantois?
An outgrowth of the yolk sac, it grows along the connecting stalk from the embryo to the chorion
What does the allantois become and how?
It becomes coated with mesoderm and vascularises to become the umbilical chord
What does the umbilical chord arise from?
The allantois
What are the 2 layers of the amniotic sac?
The amnion and the chorion
How does the chorion give rise to chorionic villi?
The cytotrophoblast forms finger like projections through the syncitiotrophoblast layer into the maternal endometrium
What is the role of the chorionic villi?
They provide extra surface area for exchange of gases and nutrients
What are the 3 phases of chorionic villi development?
Primary: outgrowth of the cytotrophoblast and branching of these extensions
Secondary: growth of the fetal mesoderm into the primary villi
Tertiary: growth of the umbilical artery and umbilical vein into the villus mesoderm, providing vasculature.
Describe the terminal villus microstructure
Capillary network in villi is a convoluted knot of vessels
Vessel dilation + knotting to slow blood flow and allow efficient exchange
Whole structure is coated in trophoblast
Describe the maternal blood supply to the endometrium
Uterine arteries branch into arcuate arteries, these branch into radial arteries, these branch into basal arteries, these form spiral arteries
What happens to the maternal blood supply to the endometrium if implantation doesn’t occur?
The spiral arteries regress and the endometrium is lost
What arteries provide blood supply to the endometrium?
Spiral arteries
What is the name of the process by which blood supply to the maternal endometrium is established?
Conversion
Describe the process of conversion (in establishing blood supply to the endometrium)
Turns the spiral artery into a low pressure, high capacity conduit for maternal blood flow
By what process is oxygen exchanged across the placenta?
Via diffusional gradient (maternal oxygen tension is high and fetal is low)
By what process is glucose exchanged across the placenta?
Facilitated diffusion by transporters on maternal side and fetal trophoblast cells
By what process is water exchanged across the placenta?
Mainly diffusion but also some local hydrostatic gradient
By what process are electrolytes exchanged across the placenta?
Diffusion and active co-transport
By what process is calcium exchanged across the placenta?
Actively transported against a concentration gradient by magnesium ATPase calcium pump
By what process are amino acids exchanged across the placenta?
Active transport
How does maternal cardiac output change in pregnancy?
Increases 30% in the fist trimester
How does maternal peripheral resistance change in pregnancy?
Decreases up to 30%
How does maternal blood volume change in pregnancy?
Increases to 40%
How does maternal pulmonary ventilation change in pregnancy?
Increases 40%
Describe fetal oxygen tension, content and saturation in comparision to maternal blood
Oxygen tension is lower
Oxygen content and saturation are similar to maternal blood
How much oxygen and glucose that is supplied does the placenta consume?
40-60%
How does the fetal circulatory system work?
Placenta acts as site of gas exchange for fetus
Ventricles act in parallel rather than series
Vascular shunts bypass pulmonary & hepatic circulation but close at birth
When do fetal movements begin?
Late first trimester
When does the fetal endocrine pancreas function and insulin production start?
Start of second trimester
Define labour
The safe expulsion of the fetus at the correct time
How many phases of labour are there and what are they called?
Phase 1= quiescence (prelude to parturition)
Phase 2= activation (preparation for labour)
Phase 3= stimulation (process of labour)
Phase 4= involution (parturient recovery)
What is the difference between the stages and phases of labour?
There are 4 phases of labour and then in phase 3 there are 3 stages of labour itself
What are the 3 stages of labour?
Stage 1= contractions start and cervix dilates
Stage 2= delivery of fetus at full dilation of the cervix with maximal myometrial contractions
Stage 3= delivery of placenta and fetal membranes and post partum replair
In stage 1 of labour what are the 2 phases of cervix dilation?
Latent phase= slow dilation of the cervix to 2-3cm
Active Phase= rapid dilation of the cervix to 10cm
What length is a fully dilated cervix?
10 cm
What characteristics of a pro inflammatory reaction is seen in labour?
Infiltration by immune cells
Inflammatory cytokines and prostaglandin secretion
What structure is important in containing the fetus in the uterus?
The cervix, it does this by the high connective tissue content
When does cervical softening occur?
Begins in the first trimester
When does cervical ripening occur?
Weeks and days before birth
What happens in ripening of the cervix?
Monocyte infiltration and IL-6 and IL-8 secretion
Hyaluron deposition
What are the 4 stages of cervix remodelling in pregnancy?
Softening
Ripening
Dilation
Post partum repair
What hormone is important in initiating labour?
Corticotrophin releasing hormone (CRH)
What are the functions of CRH (corticotrophin releasing hormone) in labour?
Promotes fetal ACTH and cortisol release
Stimulates DHEAS production by the fetal adrenal cortex, this is a substrate for estrogen production
How does CRH bioavailability increase near labour?
Theres a decline in CRH binding protein levels
What are progesterone levels during pregnancy and why?
High to maintain uterine relaxation
What happens to progesterone receptor isoforms near term?
Switch from PR-A isoforms of progesterone receptor (activating) to PR-B and PR-C (repressive) isoforms expressed in the uterus
What happens to estrogen receptor expression near term in pregnancy?
There is a rise in estrogen receptor alpha expression
What happen to the receptivity of the uterus to estrogen and progesterone near term?
Uterus becomes ‘blinded’ to progesterone action and sensitized to estrogen action but levels of both hormones are high
Where is oxytocin synthesised?
Synthesized mainly in the utero-placental tissues and pituitary
When does oxytocin production increase?
Increases sharply at the onset of labour
What drives a rise in oxytocin levels?
Estrogen levels, as they rise more oxytocin receptors are expressed
What reflex causes a sharp rise in oxytocin levels?
Ferguson reflex
Describe the Ferguson reflex
As foetus bears down onto the cervix stretch receptors they signal to the hypothalamus
This triggers eventual release of oxytocin from the posterior pituitary into maternal circulation
It acts on the uterus/ myometrium
Before labour what inhibits oxytocin receptor expression?
Progesterone
What are the functions of oxytocin?
Increases connectivity of myocytes in myometrium (syncytium)- promotes formation of gap junctions
Destabilises membrane potentials to lower threshold for contraction
Enhances liberation of intracellular Ca2+ ion stores
What are the main prostaglandins synthesised in labbour?
PGE2
PGF2alpha
PGI2
How does estrogen drive prostaglandin levels?
Rising estrogen activates phospholipase A2 enzyme, generating more arachidonic acid for PG synthesis
Estrogen stimulation of oxytocin receptor expression promotes PG release
What enzyme is involved in prostaglandin synthesis?
Phospholipase A2
What is the role of PGE2?
Cervix re-modelling
It promotes leukocyte infiltration into the cervix, IL-8 release and collagen bundle re-modelling
What is the role of PGF2 alpha?
Myometrial contractions
Destabilises membrane potentials and promotes connectivity of myocytes (with oxytocin)
What is the role of PGI2?
Promotes myometrial smooth muscle relaxation and relaxation of lower uterine segment
Describe the positive feedback loop involving CRH and cortisol
Foetus pituitary produces CRH
CRH acts of foetal adrenal
Foetal adrenal produces cortisol
Cortisol encourages placental production of CRH
What is the role of DHEAs produced by the fetus?
DHEAS are converted to oestrogen in the placenta
Oestrogen acts on myometrium promoting expression of oxytocin receptor so uterus becomes sensitive to oxytocin
Oxytocin triggers contractions
What pattern do myometrial contractions follow?
Contractions start from the fundus and spread downwards
This causes lower segment and cervix to be pulled up forming birth canal
What are the type of muscle contractions in labour?
Brachystatic, fibres do not return to full length on relaxation
How does the position of the fetus in the womb change before and during delivery?
There is pressure on the fetus’ chin so it presses against the chest (flexion)
The fetus then rotates (belly to mother’s spine) so the head is expelled first when the cervix dilates
What helps expel the placenta and fetal membranes after the fetus is deilvered?
There is rapid shrinkage of the uterus causing the area of contact with the placenta with the endometrium to shrink and causing the fetal membranes to shrink
What state does the uterus stay in after delivery and why?
It remains contracted to facilitate uterine vessel thrombosis
