S7 Fetal physiology Flashcards
describe oxygen transport in foetal blood
Gas exchange : diffusion barrier decreased during pregnancy, maternal P02 only slightly increases so foetal p02 must be lower than maternal.
what is foetal 02 content increased by ?
foetal haemoglobin : predominant form from week 12, 2 alpha and 2 beta subunits, greater 02 affinity as doesnt bind to 2,3 - DPG as effectively as HbA
Higher Hb(18g per dl)
Foetal haemtocrit
More maternal production of 2,3 -DPG
- get a double bohr effect : increased speed of 02 transfer : when C02 enters intervillous blood get lower pH so reduced affinity of Hb for 02 ; as CO2 is lost, pH increases so Bohr effect increases affinity of Hb for 02
describe C02 transfer in fetal physiology
maternal physiology adaptation to pregnancy
progesterone- driven hyperventilation
hence lower pCO2 in maternal blood
double haldane effect - feotus gives up and accepts 02/C02 so no alterations in local pH
describe foetal circulation
receive oxygenated blood from mother via placenta in umbilical vein by pass the non-functional lungs return to placenta via the umbilical arteries. Small amount of blood still goes from RA to RV so muscles dont atrophy and also to lungs for their growth and development
what is the ductus venosus ?
shunt needed to bypasses liver into IVC —> RA, to maintain saturation
what is ductus arteriosus
Pulmonary trunk —> aorta , minimises drop on 02 saturations ( as collapsed lungs have high resistance to flow )
what does the aorta do in foetal circulation
oxygenated blood to brain
what the left atrium do in foetal circulation
small amount of pulmonary venous return (deoxygenated), blood reached here pumped from LV to aorta, ensures heart and brain get most 02
what is the foramen ovale ?
RA pressure > LA pressure so blood goes into LA (after birth LA pressure higher so FA shuts) , free border of septum secundum forms a crest - crista dividens (which prevents mixing of venous blood from brain and directs blood to foramen ovale). Creates two streams of blood flow mainly to LA but small amount to RV, mixes with deoxygenated blood from SVC to allow the growth of the ventricular muscle
what is foetal response to hypoxia
more Hbf and Hb, redistribution of flow to protect heart and brain (so less to GI, kidneys)
slowing of HR to decrease 02 demand ( low P02 or high pC02 stimulates foetal chemoreceptors ; vagus stimulation activates bradycardia - unlike in adults when low 02 causes tachycardia). Chronic hypoxaemia in pregnancy can impact growth and development
what are the hormones necessary for foetal growth
insulin, IGF1 and II, leptin (placental production), TGF- alpha
describe the effects on the foetus of poor nutrition in pregnancy
growth restricitons : can be symmetrical (generalised, proportional) or asymmetrical (abdomen growth lags but heads normal, more common caused by malnutrition)
describe amniotic fluid composition
amniotic sac encloses the foetus in fluid, protects and aids lung development
10 ml at volume at week 8, 1 litre at 38 weeks
pre w 8 is made by the transudation of fluid across the amnion and foetal skin, inhalation of AF develops lungs, while GI absorbs essential water + electrolytes. Later is mostly urine which begins to be made in W9
compostion : 98% water, also has urea, electrolytes, creatinine, glucose, vernix caseosa (wax-type thing to protect skin)
what is meconium
debris accumulates in gut : from AF and intestinal secretions —> leads to meconium
meconium in AF signigies foetal distress, shouldnt be passed until after delivery as can cause foetal RDS as it is swallowed
describe bilirubin metabolism
during gestation clearance of fetal bilirubin is handled by the placenta. The fetus cannot conjugate bilirubin due to the immaturity of the liver and intestinal processes so physiological jaundice is common
describe the foetal period
end of W8 - term, growth and physiological maturation of the structures created during the embryonic period. Preparation for the transition to independent life after birth
pre embryonic period is 1-3 weeks, embryonic 3-8 weeks and foetal 8-38 weeks
describe foetal growth in the foetal period
dominant cell growth mechanism changes. 0-20 w = hyperplasia, 20-28w = mix of hyperplasia and hypertrophy, 28 w - term - hypertrophy
foetus relies upon relatively high maternal glucose
foetal insulin secretion begins around W10
embryonic period has intense activity but growth is small. Growth and weight gain accelerates in the foetal period
what is the crown-rump length
rapidly increases in pre-embryonic, embryonic and early foetal periods
Crown-rump length (CRL) is the measurement of the length of human embryos and fetuses from the top of the head (crown) to the bottom of the buttocks (rump)
at W9, the head is half of the CRL but after this, the body lengthens and limbs grow more.
describe weight gain in a fetus
weight gain is slow at first but increases rapidly in mid-late foetal period
embryo: morphogenesis and differentiation, little weight gain, placental growth is the main thing
early foetus: protein deposition
late foetus: adipose deposition
what is amniocentesis
sample amniotic fluid for foetal cells to diagnose conditions, can use karyotyping
what are the techniques used to assess foetal development
amniocentesis
foetal movements
Symphysis-fundal height
Ultrasound scan
describe how the symphysis - fundal height is used to assess foetal development
distance in cm from pubic symphysis to fundus of uterus e.g 20 cm at 20W. 36cm at 36W or in relation to other structures. Lag of >4cm indicates foetal growth restriction
how is an USS used to assess foetal development
safe, can predict weight, appearance, number, age, rule out ectopics
routinely carried out at approx 20 weeks as this is when everything in the foetus has developed large enough to see
measurements include CRL (between 7-13 weeks for dating of pregnancy), scan in T1 also used to check location, number, viability. Biparietal diameter (distance between the parietal bones, used to date in T2 and T3), abdominal circumference and femur length, 3D/4D USS
why is dating important during pregnancy
as babies can have low birth weight as they are premature or they are constitutionally small (small mother will have a small baby) or they have suffered growth restriction (associated with neonatal morbidity and mortality)
late menstrual period is used to estimate foetal age but can be inaccurate due to irregular periods
