16 - Fetal Physiology

Question 1

The mother has physiological respiratory alkalosis due to progesterone induced hyperventilation, how does the fetus overcome this and allow oxygen to be transferred to itself, as this alkalosis would cause the Hb curve of the mother to shift to the left, so less oxygen for itself?

Answer 1

- Increased 2,3 DPG production by mother

- Fetal Hb with greater oxygen affinity and does not bind 2,3DPG. 2 alpha 2 gamma at week 12

- Double Bohr effect

Question 2

Why is there a double bohr effect at the placenta?

Answer 2

Mother: pH decreases as CO₂ passes into intervillous blood so decreased affinity for oxygen

Fetus: pH increases as CO₂ passing out so increased affinity for oxygen

Question 3

How does CO₂ transfer across the placenta from fetus to mother?

Answer 3

- Progesterone driven hyperventilation lowers pCO₂ in maternal blood so bigger gradient for CO₂ exchange

- Double Haldane effect: as mother gives up oxygen she can accept more CO₂ and as fetus accepts oxygen it can give up more CO₂

Question 4

What is the fetal response to hypoxia?

Answer 4

- Bradycardia due to vagal stimulation to reduce oxygen demand

• Redistribution of flow to supply heart and brain
• Chronic hypoxaemia, e.g from smoking, can lead to growth restriction and

Question 5

What issue can arise with chronic hypoxaemia in pregnancy?

Answer 5

Intrauterine growth restriction, e.g from smoking

Question 6

Where are the different shunts in the fetal circulation and why are they needed?

Answer 6

• Lungs are not functional in utero so need to bypass them
• All the shunts allow majority of oxygen to get to brain and the heart

Question 7

What growth factors cause growth in the fetus and at different stages in pregnancy, what type of cell growth is occurring?

Answer 7

• IGF II which is nutrient independent is dominant in first trimester
• IGF I which is nutrient depent in second and third trimesters
• Maternal malnutrition can lead to symmetrical or asymetrical growth restriciton and can lead to poor health of fetus in adult life

Question 8

What is symmetrical and asymmetrical growth restriction?

Answer 8

Symmetrical: all parts of fetus are small

Asymmetrical: head sparing

Question 9

What is the role of amniotic fluid?

Answer 9

• Mechanical protection
• Lung development
• Approximately 1 litre at 38 weeks

Question 10

How can we use amniotic fluid for diagnostic purposes?

Answer 10

• Aminocentesis
• Fluid contains fetal cells that can be used for karyotyping
• It is invasive so there is a risk of miscarriage

Question 11

What is meconium staining?

Answer 11

• Meconium released prematurely from GI tract and the fetus can inhale it so meconium aspiration
• Sign of fetal distress

Question 12

How is amniotic fluid produced?

Answer 12

• Composed mainly of fetal urine and starts production at week 9
• Contains water, electrolytes, urea, renin, glucose, lanugo, fetal skin cells
• Fetus swallows it, absorbs the water and electrolytes and the debris accumulates in the gut as meconium which is passed as first stool after delivery

Question 13

Why is amniotic fluid important for lung development?

Answer 13

• Allows baby to practice breathing movements when recycling amniotic fluid
• Helps with surfactant production

Question 14

Why does physiological jaundice occur after birth?

Answer 14

• Delay in newborn’s ability to conjugate and excrete bilirubin as normally placenta deals with this
• If jaundice appears within 24 hours indicative of more serious pathology