fetal and placental physiology Flashcards
what does fetal growth depend on?
adequate transfer of nutrients and oxygen across the placenta
adequate maternal nutrition and uterine perfusion
hormones that affect metabolic rate, growth of tissues and maturation of individual organs
what do insulin growth factors do?
co-ordinate a precise and orderly increase in growth through late gestation
what do insulin and thyroxine do?
required through late gestation to ensure appropriate growth in normal and adverse nutritional circumstances
what is fetal growth determined by?
fetal genome - IGFs are an important mediator
what is fetal growth rate like?
slow up to week 20
accelerates to peak at week 30-36 then slows again
postnatal growth peak at week 8
what does amniotic fluid volume do during pregnancy?
increase until week 34 then declines
where does red cell production take place in the fetus?
yolk sac then liver then finally bone marrow
what is a normal fetal HR?
120-140 bpm
what does a lack of thyroid hormone produce?
skeletal and cerebral immaturation = cretinism
delayed surfactant production
what is cortisol essential for?
lung compliance and surfactant release
in the liver it induces beta-receptor and glycogen deposition to maintain a glucose supply to the neonate after delivery
in the gut it is responsible for villus proliferation and induction of digestive enzymes which enables the neonate to switch to enteral feeding after birth
what factors are used to predict a foetuses optimal growth potential?
pre-pregnancy weight and maternal booking weight
maternal height
maternal age and parity increased with mother >para2
ethnic group
fetal sex
paternal height
what are the 3 differences in fetal circulation?
oxygenation takes place in the placenta not in the lungs
right to left ventricles work in parallel rather than series
heart brain and upper body receives blood from the left V, placenta and lower body receive blood from both right and left V
what shunts are present in the fetus?
ductus venous - shunts blood away from liver
foramen ovale - shunts blood from right to left atrium
ductus arteriosus - shunts blood from pulmonary artery to aorta
what is the path of oxygenated blood from the placenta?
down the umbilical vein -> either through portal vein to liver OR the ductus venosus into the IVC -> right atrium -> foramen ovale -> left atrium -> left ventricle-> aorta -> 50% to head and arms remainder mixes with blood from DA
how does the DA stay open in utero?
production of prostaglandin E2 and prostacyclin which act as local vasodilator
what causes the DA to close?
cycle-oxygenase inhibitors (COX) = NSAIDS - ibuprofen
what causes the shunts to close?
cessation of umbilical blood flow causes cessation of ductus venosus
fall in right atrium pressure and closure of foramen ovale
when does persistent fetal circulation occur?
when there is a delayed closure of the DA after birth because the pulmonary vascular resistance fails to fall despite adequate breathing, resulting in left to right shunt of blood from aorta through DA to the lungs, baby remains cyanosed and can suffer from life threatening hypoxia, occur mostly in premature infants, results in congestion in pulmonary circulation and reduction in blood flow to GI tract and brain that lead to NEC and IV haemorrhage
what happens to the fluid in the lungs at birth?
production of fluid ceases and the present fluid is absorbed, adrenaline plays a major role in this process
what does surfactant do?
it is a group of phospholipids that prevent collapse of small alveoli during expiration by lowering surface tension, it is produced by type 2 alveolar cell, max production will be after 28 weeks
what is lecithin?
the predominant phospholipid (phosphatidylcholine, its production is enhanced by cortisol, growth retardation and prolonged rupture of membranes. its production is delayed in diabetes.
it is present in amniotic fluid and can be predictive of RDS
how can the incidence and severity of RDS be reduced?
giving steroids antenatally to mother
when and where are the first fetal blood cells formed?
surface of the yolk sac from 14-19 days after conception
haemopoiesis from yolk sac continues until the 3rd post-conceptional month
when does haemopoiesis begin in the liver?
5th week
when does the bone marrow start producing RBCs?
7-8 weeks but is the predominant source from 26 weeks
what haemoglobin is in the fetus?
fetal haemoglobin has 2 gamma chains and 2 alpha chains
what is the composition of adult Hb?
HbA = 2 alpha and 2 beta chains HbA2 = 2 alpha and 2 delta chains
when does HbF switch to HbA occur?
28-34 weeks, ratio of HbF:HbA = 80:20 by 6th month of age only 1% of Hb is HbF
what is different about HbF?
HbF has a higher affinity for oxygen than HbA, as in utero the fetus is exposed to lower oxygen concentrations, to enhance transfer of oxygen across the placenta, and will dissociate more easily than adult
what does beta major thalassaemia without treatment result in?
severe anaemia
FGR
poor musculoskeletal development
skin pigmentation due to increased iron absorption
what does alpha thalassaemia major result in?
severe fetal anaemia with cardiac failure
hepatosplenomegaly and oedema
infants are stillborn or shortly die after birth
at what gestation do lymphocytes appear?
8th week
by the middle of he second trimester all phagotic cell, T B cells and complement are available
what Ig’s are indicative of a fetal infection?
IgA and IgM
what are some general immunological defences of the fetus?
amniotic fluid (lysosomes, IgG)
placenta (lymphoid cells, phagocytes, barrier)
granulocytes from liver and bone marrow
interferon from lymphocytes
what is vernix?
consisting of desequemated skin cells, cholesterol and glycogen covering the skin of fetus in last wks
preterm infants have no vernix and thin skin, allows proportionately large amounts of insensible water loss
when does the gut reenter the abdomen cavity?
12th week
failure to re-enter results in defects like omphalocele or gastroschisis
(primitive forgot and hind gut are present by end of 4th week as straight tube
what can a failure in a foetuses swallowing mechanism cause?
polyhydraminos
what causes defects in swallowing mechanisms?
neurological abnormalities = anencephaly
obstruction of gut = atresia of oesophagus
what does the primitive liver appear out of?
diverticulum arising from the duodenum
how does the fetal liver differ from adult?
reduced ability to conjugate bilirubin because of enzyme deficiencies = glucuronyl transferase (placenta is performing the normal metabolic function of liver)
what does the metanephros form?
renal collecting system = uerters, pelvis, calyces, collecting duct
when is nephrogeneiss complete?
week 36
what does renal agencies result in?
severe oligohydraminos
when can fetal movements be 1st perceived?
18-20weeks
what are the 4 defined behavioural states in he fetus?
1F = quit sleep, absence of eye and body movements
2F = periodic body and eye movements are present
3F = quiet wakefulness when there are eye but no body movements
4F = body in active ongoing body and eye movement
>80% time in 1F and 2F state
what is amniotic fluid initially secreted by?
amnion
10th week = transudate of fetal serum via skin and umbilical cord
16th week = kidney and lung fluids removed by swallowing
how does amniotic fluid increase?
10wk = 30ml 20wk = 300ml 30wk = 600ml 38wk = 1000ml 40wk = 800ml 42wk = 350ml
what is the function of amniotic fluid?
protect fetus from mechanical injury
permit fetal movement and preventing limbs contracture
prevents adhesions between the fetus and amnion
permits fetal lung development, if there is absence of the fluid especially in the 2nd trimester this will lead to pulmonary hypoplasia
what is the function of the placenta?
protection nutrition respiration excretion hormone production
what are the two parts of the placenta?
fetal component from chorion
maternal component derived from modifications of uterine endometrium
what does the placenta transport?
gases water minerals and vitamins glucose and AA proteins lipids large peptide hormones = TSH, ACTH, GH, insulin, glucagon smaller molecular weight hormones = T3 and T4 and catecholamines toxic substances bacteria and viruses
how does water cross the placenta?
simple diffusion across the amnion and chorion - hydrostatic pressure from maternal blood
what electrolytes are transported across the placenta?
sodium is pumped from fetus to mother making fetus electronegative,
potassium simply diffuses down an electrochemical gradient
chloride - active transport
iodide - trapped in placenta
what waste products cross the placenta?
bilirubin, unconjugated and crosses by diffusion to mother to excrete.
gut and urinary tract open into amniotic fluid
what energy sources does the fetus use?
carbohydrates
glucose
amino acids
lactate
how is glucose transported?
facilitated diffusion - fetal glucose is directly related to the maternal glucose as fetal mechanisms for regulating glucose are immature
why do pregnant mothers not need to eat more protein?
urea excretion falls so the amino acids are used more efficiently due to progesterone
AA use active transport into the fetus
how is iron transported?
actively across placenta
maternal intestinal absorption is enhanced
what vitamins are needed in pregnancy?
folic acid and vitamins B12
calcium
KADE
how does gas exchange take place?
across the placenta
oxygen diffuses readily across the placenta and CO2 diffuses even more readily fall in pH of maternal blood as it passes through the placenta causes release of O2
rise in pH of fetal blood increases uptake of O2
flow is not the limiting factor - level of fetal oxygenation is regulated by feta requirement for oxygen
what are the 2 main types of hormones produced by the placenta?
protein hormones
steroid hormones
what do the placenta hormones affect?
stimulate ovarian function maintain pregnancy influence fetal growth stimulate mammary function assist in parturition
when does the placenta start producing progesterone?
2 and half months gestation
previously produced by corpus luteum
what action does progesterone have on the myometrium?
inhibits contractions
what type of oestrogen does the placenta produce?
oestriol - E3
what are the 2 major effects of lactogen/somatomammotropin?
promoting growth of uterus
stimulating the mammary glands
what are the effects of relaxin?
softening of connective tissue in the cervix
promotes elasticity of pelvic ligaments (can cause pelvic girdle pain or symphysis pubis dysfunction)
what is the physiology of parturition?
limited space for fetus -> fetus hypothalamus -> fetus ACTH -> fetus cortisol -> placental estrogen -> uterus PGF2alpha -> progesterone -> induction of parturition
