L15: fetal physiology

Question 1

Explain oxygen transport in fetal blood

Answer 1

Fetal blood has a much lower partial pressure of oxygen
Progesterone causes a physiological hyperventilation -> mother developing a physiological respiratory alkalosis
Increased pH would normally increase maternal affinity for oxygen, therefore more 2,3-BPG is produced = pushes curve back to right to reduce Hb affinity for oxygen -> promotes release of oxygen to the fetus

Question 2

Describe fetal haemoglobin

Answer 2

Has adapted Hb for survival in relatively hypoxic environment
HbF – predominant by week 12 & is comprised of 2 alpha and 2 gamma subunits
Has a greater affinity for oxygen because it does not bind 2,3-BPG

Question 3

Describe the Bohr effect

Answer 3

Double Bohr effect
Mother – as CO2 passes into the intervillous blood, the pH will decrease, causing a decreased affinity for oxygen for the mother
Fetus – will be giving up CO2, due to gradient of transfer, causing increased pH, which results in increased affinity for oxygen

Question 4

Describe the Haldane effect

Answer 4

Also, a double Haldane effect
Mother – maternal Hb gives up oxygen, so it can accept increasing amounts of CO2
Fetus – fetal Hb gives up more CO2 as oxygen is accepted

Question 5

Describe a fetal response to hypoxia

Answer 5

Bradycardia via vagal stimulation – try and reduce oxygen demand required by the heart -> can be included in ‘fetal distress’ & important to pick up on during intrapartum monitoring
(NB: smoking can cause chronic hypoxaemia, leading to conditions such as intrauterine growth restriction)

Question 6

Describe the ductus venosus in the fetal circulation

Answer 6

Liver could potentially engulf the whole fetal circulation – ductus venosus allows blood from the umbilical vein to shunt directly to the IVC
Shunting blood away from the liver -> maintains a high level of oxygen in the circulation that will be arriving to the brain & rest of body

Question 7

Describe the foramen ovale in the fetal circulation

Answer 7

Allows most of the blood to travel from the right atrium to the left atrium (due to higher pressure in the right atrium)
At birth – pressures reverse, which will force the foramen ovale to close
Only a small amount of deoxygenated blood is returning to the left atrium – allows the saturation of oxygen to remain quite high -> important as first organs to receive blood are heart & brain

Question 8

Describe the ductus arteriosus in the fetal circulation

Answer 8

Small amount of blood enters the right ventricle due to the adaptation of the crista dividens – ensures the right ventricle does not atrophy through disuse & allows a small amount of blood to travel to the lungs to promote its development
Between the pulmonary trunk & the aorta = ductus arteriosus

Question 9

Describe fetal growth

Answer 9

Insulin-like growth factors are key
Weeks 0-20 (first trimester) = hyperplasia of cells
Weeks 20-28 (second trimester) = mix of hyperplasia & hypertrophy
Weeks 28-term (third trimester) = predominantly hypertrophy

Question 10

What effects does maternal malnutrition have on growth?

Answer 10

Can cause symmetrical or asymmetrical growth restriction
Symmetrical growth restriction = all parts of the fetus are small
Asymmetrical growth restriction = restriction on the abdomen but it is disproportionally smaller than the head

Question 11

Describe amniotic fluid

Answer 11

Fills the amniotic sac to surround the fetus during pregnancy
Offers an element of mechanical protection, but also contains substances that are critical for lung development
Volume is proportional to the size of the fetus (as fetus grows, the volume of amniotic fluid will increase)

Question 12

Describe amniocentesis

Answer 12

Useful investigation technique as it contains fetal cells
Aspirating amniotic fluid = amniocentesis
Can be used for fetal karyotyping eg. Down’s syndrome
Invasive & carries an associated risk of miscarriage

Question 13

Describe meconium staining

Answer 13

If fetus is in distress, one clinical sign that this is happening is meconium staining – meconium is released prematurely from the GI tract
If fetus inhales it, this can lead to meconium aspiration
Sometimes meconium-stained liquor can also occur in pregnancies lasting longer than 40 weeks -> can cause fetal distress

Question 14

Describe the production, recycling and composition of amniotic fluid

Answer 14

Production – composed of fetal urine, which starts to be produced at around 9 weeks
Recycling – fetus inhales the amniotic fluid (practising breathing movements – helps with production of surfactant), fetus swallows the fluid & therefore enters GI tract – debris from the GI tract accumulates as meconium, which is passed after delivery as the baby’s first stool
Composition – water, electrolytes, fetal skin & substances typically found in urine

Question 15

Describe physiological jaundice in the neonate

Answer 15

Common because there is a delay in the newborn’s ability to conjugate and excrete bilirubin
However, if jaundice appears within 24 hours, this is often indicative of a more serious pathology

Question 16

How does the mother smoking affect the foetus’ growth?

Answer 16

Foetal hypoxia can cause bradycardia via vagal stimulation to try and reduce the oxygen demand required by the heart
Smoking can cause chronic hypoxia = intrauterine growth restriction