pregnancy, parturition and late fetal development Flashcards
why is embryo fetal growth during the first trimester relatively limited
nutrition is histiotrophic
what is meant by histiotrophic nutrition
derivation of nutrients from breakdown of surrounding tissues
reliant on uterine gland secretions and breakdown of endometrial tissues (and maternal capillaries)
what switch in fetal growth takes place from the first to second trimester
switch to haemotrophic support at start of 2nd trimester
as foetal growth cannot. be maintained/supported by histiotrophic nutrition
what is haemotrophic support
foetus will start to derive its nutrients from maternal blood
what is meant by the haemochorial-type placenta
maternal blood is directly in contact with chrorion/one of the foetal membranes
activation is around 12 weeks
what is amniotic sac
comes from amnion/amniotic cavity
surrounds and cushions foetus for its development through second and third trimesters
what is the connecting stalk
links developing embryo unit to chorion
what is the trophoblastic lacunae
large spaces filled with maternal blood formed by breakdown of maternal capillaries and uterine glands
become intervillous spaces aka maternal blood spaces
what is the consequence of of breakdown of maternal capillaries and uterine glands
creates a continuous space through which maternal blood can flow
what are the fetal membranes
extraembryonic tissues that form a tough but flexible sac encapsulates fetus and forms basis of maternal fetal interface
predominantly the amnion fetus
what is the amnion
inner fetal membrane-
- arises from epiblast
- forms a closed, avascular sac with developing embryo at one end
- begins to secrete amniotic fluid from 5th week - froms fluid filled sac that encapsulates and protects fetus
what is the chorion
outer fetal membrane -
- formed from yolk sac derivatives and trophoblast
- highly vascularised
- gives rise to chorionic villi
what are chorionic villi
outgrowths of cytotrophoblast from chorion that forms basis of fetal side of placenta
what happens when accumulation of amniotic fluid causes amnion to be in contact with chorion
both form forming amniotic sac which has 2 layers; amnion on inside and chorion on outside
what is the allantois
outgrowth of yolk sac
grows along connecting stalk from embryo to chorion
becomes coated in mesoderm and vascularises to form umbilical cord
what are the primary chorionic villi
cytotrophoblasts form finger like projections through syncitiotrophoblast layer into maternal endometrium
what are chorionic villi
provide substantial surface area for exchange
finger like extensions of chorionic cytotrophoblast which grow into syncitiotrophoblast and undergo branching
what happens in secondary phases of chorionic villi development
growth of fetal mesoderm into primary villi
what happens in tertiary phase of chorionic villi development
growth of umbilical artery and umbilical vein into villus mesoderm, providing vasculature
describe terminal villus microstructure
convoluted knot of vessels and blood vessel dilation-
slows blood flow enabling exchange between maternal and fetal blood
whole structure coated with trophoblasts
what happens to diameter of villi as we move through pregnancy
diameter decreases from around 150-200 ym in early pregnancy to 40ym later on
distance from trophoblast to maternal blood also decreases
describe maternal blood supply to endometrium
uterine artery branches give rise to network of arcuate arteries
radial arteries branch from arcuate arteries which branch further to form basal arteries
basal arteries form spiral arteries during menstrual cycle endometrial thickening
what is role of spiral arteries
provide maternal blood supply to endometrium
what are the extra-villus trophoblast(evt) cells
coat the villi and invade down into maternal spiral arteries forming endovascular evt
endothelium and smooth muscle is broken down - evt coats inside of vessels
what is conversion in spiral artery remodelling
turns spiral artery into low pressure high capacity conduit for maternal blood flow
what are the nutrients exchanged across placenta
oxygen via diffusion gradient
glucose via facilitated diffusion by transporter
water - majority via diffusion
electrolytes - diffusion and active transport
calcium. -active transport
amino acids - active transport
what happens to maternal cardiac output during the first trimester
increases by 30%
peripheral resistance decreases
maternal blood vol increases to 40% - increase in erythrocyes and plasma
pulmonary ventilation increases 40%
describe consumption by fetus/placenta
consumes 40-60% glucose and o2 supplied
embryonic and fetal hbs have greater affinity for o2 than maternal hb therefore bind at higher capacity
describe circulatory system in late fetal development
placenta acts as site of gas exchange
ventricles act in parallel rather than series
vascular shunts bypass pulmonary and hepatic circulation - close at birth
describe resp system in late fetal development
primitve air sacs form in lungs around 20wks, vascularisation from 28wks
surfactant production begins around 20 wks,
fetal spends 1-4hrs a day making rapid respiratory movements during rem sleep
describe gastrointestinal system in late fetal development
endocrine pancreas is 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
describe nervous system development during late fetal development
fetal movements begin in late 1t, detectable by mother from 14 weeks
stress responses from 18wks
does not show conscious wakefulness
what is labour
safe expulsion of fetus at correct time
expulsion of placenta and fetal membranes
resolution/healing to permit future reproductive events
what pre inflammatory reactions do you expect to see in labour
immune cell infiltration
inflammatory cytokine and prostaglandin secretion
what happens in first stage of labour
contractions start, cervix dilation
latent phase - slow dilation of cervix to 2-3cm
active phase - rapid dilation of cervix to 10cm
what happens during second stage of labour
delivery of foetus
- commences at full dilation of cervix > 10cm
maximal myometrial contractions
what happens during third stage of labour
delivery of placenta and fetal membranes
post partum repair
why does cervix have high connective tissue content
provides rigidity
stretch resistant
describe other properties of cervix
bundles of collagen fibres embedded in proteoglycan matrix
- changes to collagen bundle structure underlie softening, but mechanism unclear
when does cervical softening occur
first trimester
chnages in compliance but retains cervical competence
when does cervical ripening occur
weeks and days before birth
- monocyte infiltration and il6,il8 secretion
hylauron depositin
what happens during cervical dilation
increased elasticity
increased hylauronidase expression - ha breakdown
mmps decrease collagen content
what happens to cervix during post partum repair
recovery of tissue integrity and competency
theories of how labour is initiated
fetus determines timing of partruition through chages in fetal hpa axis
cpa levels rise exponentially towards end of pregnancy
- decline in crh binding protein levels so crh bioavailabilty increases
what is crh functions in labour
promotes fetal acth and cortisol release
increasing cortisol drives placental production of crh
stimulates dheas production by fetal adrenal cortex - substrate for oestrogen production
what does estrogen and progesterone levels look like during pregnancy
high progesterone maintains uterine relaxation
serum estrogen:progesterone ratio may shift in favour of estrogen
as term approaches switch from PR-A isoforms to PR-B AND PR-C isoforms - functional prog withdrawal
uterus becomes blinded to progesterone action and sensitised to estrogen action
local changes in e:p ratio in uterine tissue
what does oxytocin look like in pregancy
uterine production increases sharply driven by increase in oestrogen release promoted by stretch receptors - ferguson reflex signal through otr/oxr g coupled protein prog inhibits oxtr - uterus is relaxed
what is function of oxytocin
increases connectivity of myocytes in myometrium
destablises membrane potentials to lower threshold for contraction
enhances liberation of intracellular ca2+ ion stores
what is role of prostaglandins in labour
key effector of labour
PGE2,PGF2 alpha and PGI2 synthesised during labour
1.rising estrogen activates phospholipase a2 enzymes, generating more arachidonic acid for pg synthesis
2. estrogen stimulation of oxytocin receptor expression promotes pg release
what is role of pge2 in cervix remodelling
leukocyte infiltration into cervix, il8 release and collagen bundle remodelling
what is role of pgf2 alpha
myometrial contractions -
destabilises membrane potentials and promotes connectivity of myocytes with oxytocin
what is role of pgi2
in myometrium
promotes myometrial smooth muscles relaxation and relaxation of lower uterine segment
what are some other factors involved in cervix remodelling
peptide hormone relaxin and nitric oxide
how do myometrial contractions work
contractions start from fundus and spread down upper segment
muscle contractions are brachystatic
causes lower segment and cervix to be pulled up forming birth canal
what occurs during fetal expulsion
head engages with pelvic space within 34-38 wks
pressure on fetus causes chin to press again chest - flexion
fetus rotates
head expelled first
what happens during placental expulsion
rapid shrinkage of uterus
shrinkage causes folding of foetal membranes
clamping of umbilical cord after birth stops fetal blood flow to placenta
haematoma between decidua and placenta
contractions expel placenta
what happens during repair
uterus remains contracted to facilitate uterine vessel thrombosis
uterine involution and cervix repair shields uterus from commensal bacteria
endometrial cyclicity is restored in response to hormones
