Pregnancy, Parturition and Late Fetal Development

Question 1

What occurs during the first trimester of pregnancy?

Answer 1

Single cell zygote to highly complex embryo

Body plan established - each of the major organ systems are put in place

Although the actual growth during the first trimester is limited

Question 2

Why is growth limited during the first trimester?

Answer 2

Partly become the embryo is dependent on histiotrophic nutrition in the first trimester

Histiotrophic nutrition - derivation of nutrients from the breakdown of surrounding (endometrial) tissues i.e. when the syncitiotrophoblast invading the maternal endometrium

As it invaded, it broke down the local tissues and used those products of tissue breakdown to fuel the development of the embryo

Question 3

Where else does the syncitiotrophoblast get nutrients from? Other than the breakdown of endometrial tissue

Answer 3

It can also breakdown maternal capillaries so the syncitiotrophoblast bathes in maternal blood - from which is derives nutrients

And glands within the endometrium which supply uterine milk - also a source of nutrients for the developing embryo

Question 4

What happens between first and second trimester?

Answer 4

First trimester = limited growth

Between first and second trimester - rapid increase in rate of growth

Question 5

Can this sudden increase in rate of growth in the second trimester be supported by histiotrophic nutrition?

Answer 5

No

Question 6

How is the increase in rate of growth supported?

Answer 6

Switch to haemotrophic support at the start of the second trimester

Starts to derive nutrients from maternal blood

Achieved through activation of haemochorial-type placenta around the 12th week of gestation, where maternal blood directly contacts the foetal membranes (AKA chorion)

Question 7

What is the the first foetal membrane?

Answer 7

The amnion - derivative of epiblast

But unlike the rest of the epiblast, does not go on to form part of foetus

Forms the amniotic cavity that goes on to become the amniotic sac (closed and avascular) - surrounds and cushions the foetus during the second and third trimester

Question 8

What causes the amniotic sac to expand?

Answer 8

Secretions from the amniotic sac spilling into the amniotic sac space - fluid accumulation = amniotic space expands until the amnion and chorion join to for the amniotic sac = more fluid = more amniotic sac expansion

Question 9

What is the second key foetal membrane?

Answer 9

Chorion - outer membrane surrounding the whole conceptus unit

Question 10

What is the connecting stalk?

Answer 10

Part of extra embryonic tissues which grows from the embryo and connects the conceptus with the chorion

Question 11

How are trophoblastic lacunae formed and what are they?

Answer 11

Formation trophoblastic lacunae =

As the syncitiotrophoblast invades the endometrium and breaks down the maternal capillaries and glands

The lumens of the maternal capillaries and glands as a consequence of breakdown - creating a continuous space through which maternal blood can flow

These spaces are called lacunae - when these large spaces develop and are filled with blood, they become known as intervillous spaces (AKA maternal blood spaces)

So maternal blood flows through those spaces and contacts the syncitiotrophoblasts

Question 12

What are the foetal membranes?

Answer 12

Predominantly the amnion and chorion

Extraembryonic tissues that do not contribute to the foetus ultimately, instead form a tough but flexible sac encapsulates the fetus and forms the basis of the maternal-fetal interface

Question 13

What is the amnion?

Answer 13

Inner of the foetal membranes - arises from the epiblast but does not contribute to fetal tissues

Forms a closed avascular sac with the developing embryo at one end

Question 14

What happens to the amnion cells at the 5th week of gestation?

Answer 14

Amnion cells start to secrete amniotic fluid - amniotic space increases

Forms fluid filled sac that encapsulates and protects the foetus

Question 15

What is the chorion?

Answer 15

Outer foetal membrane
Derived from yolk sac and part of the trophoblast
Highly vascularised - unlike the amnion

Question 16

Why is the chorion important in placental development?

Answer 16

Gives rise to the chorionic villi - outgrowths of cytotrophoblasts from the chorion that forms the basis of the fetal side of the placenta

Question 17

What happens as the amniotic sac expands?

Answer 17

Forces amnion into contact with the chorion

Fusion of the amnion and chorion = formation of the amniotic sac

Question 18

What is the third foetal membrane?

Answer 18

Allantois

Derived from yolk sac - outgrowth of the yolk sac

Grows along the connecting stalk from embryo to chorion

Becomes coated in mesoderm and vascularises to form the umbilical cord

Question 19

Why is the allantois important?

Answer 19

Still unclear

Seems to contribute partly to the embryonic bladder - maybe important in the removal of toxins from the developing embryo

Important role in the development of the umbilical cord - provides circulatory link from the embryo to the fetal side of the placenta

Question 20

What forms the umbilical cord?

Answer 20

Connecting stalk
Allantois
Additional mesoderm

Question 21

What are cytotrophoblast cells?

Answer 21

Sits on the outside of the chorion

Trophectoderm divides into trophoblast - trophoblast divides into the outer syncitiotrophoblast layer and proliferative cytotrophoblast inner layer (divides to give rise to cells that contribute to the syncitiotrophoblast)

Outgrowths of the cytotrophoblast form finger-like projections through syncitiotrophoblast layer into maternal endometrium called primary chorionic villi - begins to form part of the maternal foetal interface

Important in the development of the placenta

Question 22

What are the chorionic villi and why are they important?

Answer 22

Derived from outgrowths of cytotrophoblasts

Provide substantial surface area for exchange (gases and nutrients)

Finger-like extensions of the chorionic cytotrophoblast, which then undergo branching

Question 23

What are the three phases of chorionic villi development?

Answer 23

Primary = outgrowth of the cytotrophoblast and branching of these extensions

Secondary = growth of the fetal mesoderm into the chorionic primary villi

Tertiary = growth of the umbilical artery and umbilical vein into the villus mesoderm, providing vasculature

Question 24

What is the microstructure of terminal chorionic villi?

Answer 24

Capillary cast (trophoblast cast) of the vasculature of one of the chorionic villi - blood vessels come up through into the villus

Villus normally coated in trophoblast - the space surrounding it (outside trophoblast) would be filled with maternal blood (lacunae)

Capillary network in the villus forms a convoluted knot of vessels and vessel dilation

Question 25

What does the convoluted knot structure and dilation of vessels allow for?

Answer 25

Combination of both slows down the blood flow through the terminal villi structures - enabling exchange between maternal and fetal blood

Question 26

How big are the villi in early pregnancy?

Answer 26

150-200µm diameter
With a think layer of trophoblast - approx. 10µm trophoblast thickness over the surface, between capillaries and maternal blood

Question 27

How big are the villi in late pregnancy?

Answer 27

Villi thin to 40µm,
Blood vessels within these villi move within the villi to become closer to the maternal blood supply

So the layer of trophoblasts laying over the villi shrinks to become thinner = 1-2µm trophoblast separation from maternal blood

Diffusion distance reduced between maternal blood and fetal circulation

Question 28

How does the maternal blood supply perfuse the endometrium?

Answer 28

Uterine artery travels up to fuse with the ovarian artery

Uterine artery branches give rise to a network of arcuate arteries - supplies the uterus

Radial arteries branch from arcuate arteries, which go thorugh the myometrium of the unterus into the endometrium

Radial arteries branch further to form basal arteries

Basal arteries form spirals known as spiral arteries - supplies endometrium lining of uterus

Question 29

What happens to the spiral arteries during the menstrual cycle?

Answer 29

Basal arteries begin to spiralise as the menstrual cycle goes on

These spiral arteries grow out (i.e. continue to grow and spiralise as endometrium thickens) during the menstrual cycle - during process of endometrial development / thickening

If implantation does not occur - there is loss of the endometrium and regression of the spiral arteries

If implantation does occur, the spiral arteries are stabilised and provides maternal blood supply to the foetus

Question 30

When does spiral artery remodelling occur?

What happens in spiral artery remodelling?

Answer 30

Extensive remodelling of spiral arteries occurs during implantation and placental development

As the spiral arteries develop, the trophoblast cells on the outside of the villi starts to invade the maternal spiral arteries

Outgrowth of trophoblast down into maternal spiral arteries

Known as extra-villus trophoblast cells

Question 31

What forms the endovascular EVT?

Answer 31

Extra-villus trophoblast (EVT) cells coating the villi invade down into the maternal spiral arteries, forming endovascular EVT

Question 32

What is the role of the endovascular EVT?

Answer 32

Important in replacing maternal endothelium of the spiral blood vessels

As the endovascular EVT grow down, they break down the maternal endothelium and maternal smooth muscle - causes the spiral arteries to de-spiralise

Forms a new endothelial layer where the foetal EVT cells now coat the inside of the vessel

This causes the arteries to go from spiral to non-spiral (straight channels)

Question 33

What is the process of going from spiral to non-spiral arteries called?

Answer 33

Conversion

Question 34

What is the purpose of conversion?

Answer 34

Turns the spiral artery from a highly convoluted, high pressure vessels

Into a low pressure, high capacity conduit for maternal blood flow - to feed the maternal blood spaces

Question 35

What can issues with conversion cause?

Answer 35

Pre-eclampsia

Intra-uterine growth retardation

Question 36

What is the structure of the placenta and how does it contribute to exchange between the maternal and foetal blood?

Answer 36

Maternal unit forms the side closest to the mother:
Blood supply gives rise to spiral arteries
Spiral arteries supply maternal blood spaces with blood
Some of that blood drains away via the venous system

From the foetal side:
Formation of chorionic villi - invasions of trophoblast that branch and become vascularised

Invasion of fetal circulatory system into chorionic villi that provide the large surface area for exchange between mother and fetus

Question 37

How does nutrient exchange across the placenta occur? What does it depend on?

Answer 37

Diffusion - size and morphology
Facilitated diffusion - concentration gradients or transporter abundance
Active transport - nutrient metabolism

Nutrient exchange across the placenta is nutrient specific - i.e. type of exchange depends on the nutrient being exchanged

Question 38

How is oxygen exchanged in the placenta?

Answer 38

Diffusional gradient (high maternal O2 tension, low fetal O2 tension) - leads to diffusion of O2 across the placenta

Question 39

How is glucose exchanged in the placenta?

Answer 39

Facilitated diffusion by transporters on maternal side and on the fetal trophoblast cells

Question 40

How and where is water exchanged?

Answer 40

Placenta = main site of exchange
Though some crosses amnion-chorion meeting pointd

Majority by diffusion, though some local hydrostatic gradients

Question 41

How are electrolytes exchanged?

Answer 41

Large traffic of sodium and other electrolytes across the placenta – combination of diffusion and active energy-dependent co-transport

Question 42

How is calcium exchanged?

Answer 42

Actively transported against a concentration gradient by magnesium ATPase calcium pump
Important for the development of the foetal skeleton

Question 43

How are amino acids exchanged?

Answer 43

Reduced maternal urea excretion and the amino acids that make up the urea are more efficiently utilised by being transported actively across the placenta to the foetus

Question 44

What are the changes observed in the mother’s cardiac system?

Answer 44

Maternal cardiac output increases 30% during first trimester (increase in stroke vol & rate)

Maternal peripheral resistance decreases up to 30% (about a 1/3)

Maternal blood volume increases by 40% (near term - 20-30% erythrocytes, 30-60% plasma)

Question 45

What are the changes observed in the mother’s respiratory system?

Answer 45

Pulmonary ventilation increases 40%

Question 46

How much glucose and oxygen does the placenta consume?

Answer 46

Placenta consumes 40-60% glucose and O2 supplied by maternal circulation

Question 47

What are the features of fetal blood O2?

Answer 47

But although fetal O2 tension is low, O2 content and saturation are similar to maternal blood

This is achieved because during the embryonic and foetal stages of development, there are specific developmental haemoglobins that are used (different to maternal or adult Hb)

Embryonic and fetal hemoglobins = greater affinity for O2 than maternal hemoglobin = binds ot O2 at a higher capacity

Question 48

What happens to the organ systems in the first trimester VS the second and third trimesters?

Answer 48

First trimester = establishing the different organ systems

Second and third trimester = maturation and growth of the organ systems

Question 49

How did the circulatory system begin?

Answer 49

Tube of mesoderm that was pumping blood by day 22 of embryo development

By the beginning of the 2nd trimester - relatively complete circulatory system

Question 50

What are some differences between the circulatory system of the foetus compared to a neonate (newborn)?

Answer 50

Placenta acts as site of gas exchange for fetus (compared to lung in a newborn)

Ventricles act in parallel rather than series

Vascular shunts bypass pulmonary and hepatic circulation to permit the heart to drive oxygenated blood from the placenta around the body with greater efficiency -> these vascular shunts close at birth to give the circulatory system we know in neonatals and adulthood

Question 51

How did the respiratory system begin?

Answer 51

Lungs began as a bud around the foregut, and that went through a period of branching in the first trimester to give off different lung structures

Question 52

What are the stages of maturation for the respiratory system?

Answer 52

Primitive air sacs form in lungs around 20 weeks, vascularisation from 28 weeks

Surfactant production begins around week 20, upregulated towards term

Fetus spends 1-4hr/day making rapid respiratory movements during REM sleep even though it is within the amniotic sac and the lungs are not actually the site of gas exchange
This is practice for the breathing reflex for once it leaves the uterus and also important in development of diaphragm

Question 53

How did the GI system begin?

Answer 53

Gut tube forms in the early embryo from the endoderm with some contribution from the yolk sac

Liver cells form around 23 days of development

Question 54

What are the stages of maturation for the GI system?

Answer 54

Second trimester (2T) = developing pancreas

Endocrine pancreas functional from start of 2T, insulin secreted from mid-2T

Liver glycogen progressively deposited – accelerates towards term

Large amounts of amniotic fluid inhaled and swallowed by foetus – this fluid contains a large amount of debris, and that together with bile acids and cells from the developing intestine form the meconium (first stool)

Meconium delivered just after birth

Question 55

How did the nervous system begin?

Answer 55

x

Question 56

What are the stages of maturation for the nervous system?

Answer 56

Fetal movements begin late 1T, detectable by mother from ~14 weeks (early in 2T)

Foetus can make responses to stress from 18 weeks, but thalamus-cortex connections form around 24 weeks onwards (so sensory inputs can only be processed from around mid-pregnancy onwards)

Fetus does not show conscious wakefulness – mostly in slow-wave or REM sleep

Question 57

What initiates the madevelopmental changes and maturation process of the organ systems?

Answer 57

Orchestrated by increase in foetal corticosteroids towards the end of pregnancy

Exponnential increase in level of foetal corticosteroids in foetal blood

Question 58

What are the aims of labour?

Answer 58

Safe expulsion of the fetus at the correct time

Expulsion of the placenta and fetal membranes - so uterus is empty for future reproductive events

Resolution/healing to permit future reproductive events

Question 59

What are the characteristics of labour?

Answer 59

Labour has the characteristics of a pro-inflammatory reaction

Extensive immune cell infiltration into the tissues of the female reproductive tract

Extensive inflammatory cytokine and prostaglandin secretion - important in orchestrating the timing and sequence of events of labour

Question 60

What are the 4 phases of parturition (giving birth) and what occurs at each phase?

Answer 60

Phase 1: Quiescence (prelude to parturition)
Uterus is quiet, no contractions yet (contractile unresponsiveness), but there are some changes in the cervix - cervical softening
Occurs from late first trimester onwards

Phase 2: Activation (preparation for labour)
Some activity of the uterus (uterine preparedness)
Further developments in the cervix (cervical ripening) - getting it ready to dilate to allow for delivery

Phase 3: Stimulation (processes of labour
Uterine contractions, cervical dilations, fetal and placental expulsion (achieved via the 3 stages of labour)

Phase 4: Involution (paturient recovery(
Restoration of uterus to original size - uterine involution, cervical repair, onset of lactation for breastfeeding

Question 61

What are the 3 stages of labour (comes in phase 3 of parturition)?

Answer 61

1. First stage = contractions start, cervix dilation
   - Latent phase = slow dilation of the cervix to 2-3cm
   - Active phase = rapid dilation of the cervix to 10 cm
2. Second stage = delivery of the foetus
   - Commences at full dilation of the cervix (10cm)
   Maximal myometrial contractions
3. Third stage = delivery of the placenta
   - Expulsion of the placenta and foetal membranes
   - Onset of post partum repair

Question 62

What is the timings for labour?

Answer 62

Prolonged first stage - the latent (slow increase in dilation) and active (accelerated increase in dilation) phases are roughly equal in duration

Second and third - each takes around an hour or two

Between 8 and 18 hours for first delivery

Between 5 and 12 for subsequent

Question 63

What is the role of the cervix?

Answer 63

Cervix has a critical role in retaining the fetus in the uterus

Question 64

Why is remodelling the cervix important tolabour?

Answer 64

Cervical softening = allows for cervix to dilate = opens birth canal to let foetus through

Question 65

What features of the cervix allow it to function to retain the foetus in the uterus?

Answer 65

High connective tissue content:
Provides rigidity and stretch resistance

This is achieved through bundles of collagen fibres within the cervical tissue embedded in a proteoglycan matrix

As time of delivery is approached, there are changes to collagen bundle structures - remodelling of collagen underlines cervical softening for dilation, but mechanism unclear

Question 66

When does cervical softening begin?

Answer 66

Begins in first trimester
Measurable changes in compliance (in terms of stretch resistance) but retains cervical competence (i.e. cervix remains closed and capable of keeping the foetus inside the uterus)

Question 67

When does cervical ripening occur?

Answer 67

As you get close to birth – weeks and days before birth
Characterised by extensive immune cell infiltration of the cervix
Monocyte infiltration and IL-6 and IL-8 secretion
Hylaluron deposition

Question 68

When does cervical dilation occur?

What is cervical dilation?

Answer 68

Occurs with the onset of labour - opening up the cervix to allow the transit of the foetus

Dilation = increased elasticity
Characterised by increased hyaluronidase expression -> leads to HA breakdown
MMPs (MatrixMetalloProteinases) - breaks down collagen and so decreased collagen content = increased elasticity of tissue

Question 69

When does post-partum repair begin?

What occurs during post-partum repair?

Answer 69

After the foetus is born

Recovery of tissue integrity and competency - ensures if the woman gets pregnant again, the cervix is capable of keeping the foetus within the uterus

Question 70

What do we currently think are the triggers for the initiation of labour?

Answer 70

Fetus determines timing of delivery through changes in fetal HPA axis - so through increasing amounts of corticosteroids

CRH (corticotrophin releasing hormone) production and release rises exponentially towards the end of pregnancy

Decline in CRH-BP (corticotrophin releasing hormone binding protein) levels, so CRH bioavailability increases

Question 71

What is the role of Corticotrophin-releasing hormone (CRH) in labour?

Answer 71

Increased level of CRH acts on foetal adrenal cortex = promotes fetal ACTH and cortisol release

Cortisol travels via foetal circulation to placenta

Increasing cortisol drives placental production of CRH -> Positive feedback loop (as placental CRH travels back to foetus to drive more foetal CRH production)

CRH also stimulates DHEAS production by the fetal adrenal cortex -> substrate for estrogen production by the placenta

Question 72

What are levels of oestrogen and progesterone through pregnancy?

Answer 72

Oestrogen and progesterone conitnue to increase throughout the pregnancy

This high progesterone level is produced by the placenta, and is maintained through pregnancy to maintain uterine relaxation

Question 73

What happens to the oestrogen:progesterone ratio around the time of birth?

Answer 73

Serum oestrogen:progesterone ratio may shift in favour of oestrogen and reduce the ratio overall

And this may be associated with the onset of labour – this is unclear in humans but is true in other mammals e.g. sheep

Question 74

What are the changes in oestrogen and progesterone signalling as delivery approaches?

Answer 74

As term approaches, we start ot approach delivery = switch from PR-A (progesterone receptor A) isoforms (activating functions of progesterone) to PR-B and PR-C (repressive functions of progesterone) isoforms expressed in uterus

Leads to functional progesterone withdrawal - so although levels of progesterone remain high, the change in the type of receptor = uterus no longer reacts to progesterone

Rise in Oestrogen Receptor Alpha expression within the uterus = uterus sensitised to the action of oestrogen

So overall, although levels of progesterone and oestrogen stay high, it becomes blinded to the action of progesterone whilst being sensitised to the action of oestrogen

Question 75

What is driving the change in the oestrogen:progesterone ratio?

Answer 75

Unknown what is driving this change

But locally, in the uterine tissues, oestrogen:progesterone ratio changes where oestrogen = high, progesterone = low

Question 76

What is oxytocin and what promotes its release?

Answer 76

Nonapeptide (9aa) hormone synthesized mainly in the utero-placental tissues and pituitary

Change in oestrogen:progesterone ratio = production of oxytocin

Question 77

What happens to uterine oxytocin during labour?

Answer 77

Uterine oxytocin production increases sharply at onset of labour - partly driven by oestrogen so increasing levels of oestrogen production by the placenta promotes local uterine production of oxytocin

Expression increase is driven by increase in oestrogen levels

But predominantly, oxytocin production comes from maternal pituitary - release promoted by stretch receptors as the foetus bares down on the cervix, stretching it -> Ferguson reflex (i.e. the neuroendocrine stretch reflex)

Stretch receptors send signals to hypothalamus, firing of hypothalamus onto posterior pituitary triggers release of oxytocin from posterior pituitary into maternal circulation where it acts back onto the uterus and myometrium

Question 78

How does Oxytocin signal?

Answer 78

Signals through G-protein coupled receptor known as the oxytocin reception = OTR / OXTR

Question 79

How do the levels of oxytocin receptors (OXR / OXTR) change from pre-labour to labour?

Answer 79

Pre-labour =
Through most of pregnancy = progesterone dominance
Progesterone inhibits OXTR expression in the myometrium of the uterus = uterus relaxed

Labour =
Rise in oestrogen promotes large increase in number of uterine OXTR expression
This sensitises the myometrium of the uterus to oxytocin

Question 80

What are the functions of oxytocin?

Answer 80

Increases connectivity of myocytes in myometrium (myocytes form syncytium in the myometrium, allowing myometrium to act as a syncitium - fusion of the cells so there are no cell boundaries)

Destabilise membrane potentials to lower threshold for contraction

Promotes liberation of intracellular Ca2+ ion stores - which additionally aids contraction

Question 81

What does oxytocin work in conjunction with?

Answer 81

Prostaglandins (PGs)

Question 82

What are prostaglandins (PGs) and what are the 3 primary PGs synthesised during labour?

Answer 82

Key effectors of labour

Primary PGs synthesised during labour = PGE2, PGF2alpha and PGI2

Question 83

How do rising oestrogen levels drive PG action in the uterus?

Answer 83

1. Rising oestrogen activates phospholipase A2 enzyme, generating more arachidonic acid for PG synthesis
2. Oestrogen stimulation of oxytocin receptor (OXTR) expression promotes PG release thorugh oxytocin signalling

Question 84

What is the function of PGE2?

Answer 84

Cervix re-modelling

Promotes leukocyte infiltration into the cervix, IL-8 release, and ultimately resulting in collagen fibre rebundle-ing that allow the softening and ripening of the cervix

Question 85

What is the function of PGF2 alpha?

Answer 85

Predominantly acting on the myometrium

Myometrial contractions

Alongside oxytocin, helps destabilise membrane potentials and promotes connectivity of myocytes

Question 86

What is the function of PGI2 (pee-gee-eye-two)?

Answer 86

Acting on the myometrium

Promotes myometrial smooth muscle relaxation and particularly relaxation of lower uterine segment

Important that inbetween myometrial contractions there is some relaxation, because that relaxation allows the blood flow to return into the uterus and placenta to ensure blood flow to foetus

Question 87

What are other factors important for cervix remodelling?

Answer 87

Peptide hormone relaxin - levels increase sharply towards end of pregnancy = important in cervix remodelling

And nitric oxide (NO) implicated in cervix re-modelling

Question 88

What is the integrated hypothesis for the regulation of labour?

Answer 88

Foetal CRH production by foetal pituitary acts on foetal adrenal cortices

Foetal adrenals produce foetal cortisol

Foetal cortisol transfers to placenta to trigger further production of placental CRH

Placental CRH acts back as a positive feedback loop on the foetal CRH production, to ultimately promote more cortisol production

The CRH acting on the foetal adrenal also promotes production of DHEAS, which is converted to oestrogen in the placenta

Oestrogen acts on myometrium promoting the expression of the oxytocin receptor

Uterus becomes sensitive to the pituitary production of maternal oxytocin and that triggers contractions

Oestrogen also promotes local production of oxytocin, which acting via OXTR stimulates contractions and prostaglandin production

PGs (particularly PF2 alpha) mediate myometrial contractions and help to soften and ripen the cervix to ensure dilation can occur

Question 89

What happens in the non-labouring state to stop premature contractions?

Answer 89

Progesterone inhibits the production of these oxytocin receptors that keeps the myometrium quiescent

Stops contractions occurring prematurely

Question 90

How do the myometrial contractions work?

Answer 90

Driven by the top part of the uterus - known as the fundus; but delivery also many other parts of the uterus e.g. dilated cervix, lower segment of uterus = passive (does not contribute to contractions), upper segment of uterus = muscles driving contractions

Myometrial muscle cells form a syncytium (extensive gap junctions so contractions can transmit across muscle of upper segment)

Contractions start from the fundus, spread down upper segment

Muscle contractions are brachystatic –fibres do not return to full length on relaxation between contractions

So each consequent contraction opens the cervix and lower segment until complete dilation is achieved

This also causes lower segment and cervix to be pulled up - forming the birth canal

Question 91

What happens to foetal positioning towards the end of term?

Answer 91

Head engages with pelvic space 34-38wks

Pressure on fetus from contractions causes chin to press against chest (flexion)

Question 92

How is the foetus expelled as delivery progresses?

Answer 92

Fetus rotates (belly to mother’s spine)

Head expelled first after cervix dilates

Shoulders delivered sequentially (upper first) followed by torso

Rest of foetus can follow quickly once shoulders are out

Question 93

What occurs to allow for placental expulsion?

Answer 93

Expulsion of placenta achieved by collapse of uterus after delivery

Rapid shrinkage of the uterus after fetal delivery causes area of contact of placenta with endometrium to shrink

Uterine shrinkage also causes folding of fetal membranes – peel off the endometrium

Clamping of the umbilical cord after birth stops fetal blood flow to placenta - leads to villi collapsing

Triggers haematoma formation between decidua and placenta

That, couples with ongoing uterine contractions expel the placenta and foetal tissues

Question 94

How is the uterus repaired?

Answer 94

Uterus remains contracted after delivery to facilitate uterine vessel thrombosis and healing - prevent intrauterine bleeding

Eventually, restoration of the uterus and repair of the cervix - uterine involution and cervix repair to restore non-pregnant state

Important to stop commensal bacteria that live within the reproductive tract getting up in the uterus - shielding uterus from commensural bacteria

Restore endometrial cyclicity in response to reproductive hormones to ensure the uterus is ready for consequent implantations of embryos if another one comes in the next reproductive cycle