7. Foetal physiology

Question 1

what is the difference between maternal and foetal pO2 and why is this important

Answer 1

foetal pO2 (4 kPa) must be lower than maternal pO2 (11-13 kPa) to maintain a gradient of partial pressures

Question 2

name 2 FOETAL factors that increase foetal blood O2 content

Answer 2

1. foetal Hb variant - HbF (2 alpha and 2 gamma) - is predominant from wk12 to term. Has greater O2 affinity as doesn’t bind 2,3-BPG as effectively.
2. high foetal haematocrit

Question 3

name 2 MATERNAL factors that increase foetal blood O2 content

Answer 3

1. increased production of 2,3-BPG (secondary to physiological resp. alkalosis of pregnancy)
2. low blood pCO2 due to progesterone-driven hyperventilation blowing off extra CO2 - creates conc. gradient

Question 4

what are the double Bohr and double Haldane effects

Answer 4

Double Bohr effect: CO2 diffuses from foetal blood to maternal blood causing:

• increased CO2 in intervillous blood on maternal side… decreased pH… decreased maternal Hb O2 affinity = Bohr effect
• decreased CO2 in foetal blood… increased pH… increased foetal Hb O2 affinity = Bohr effect

Double Haldane effect:

• maternal Hb gives up O2… can accept increasing amounts of CO2
• foetal Hb gives up CO2… can accept increasing amounts of O2

Question 5

name 3 foetal adaptations to manage transient decreases in oxygenation

Answer 5

1. HbF and increased haematocrit
2. redistribution of flow to protect supply to heart and brain (reduce supply to GIT, kidneys and limbs)
3. bradychardia to decrease O2 demand: foetal chemoRs detect decreased pO2 or increased pCO2 causing vagal stimulation to decrease HR

Question 6

what is the most common caused of chronic hypoxaemia and how can this impact dev.

Answer 6

• most commonly caused by maternal smoking
• can impact dev. by causing: growth restriction and behavioural changes (suggested by changes in foetal mobility and sleep patterns)

Question 7

suggest 3 general causes of oligohydramnios

Answer 7

1. foetal kidney malfunction
2. foetal bladder outlet obstruction
3. premature leakage of amniotic fluid

Question 8

suggest 2 general causes of polyhydramnios

Answer 8

1. maternal hypertensive disorders

2. foetal GI defects preventing swallowing, e.g. tracheoesophageal septa, oesophageal atresia, facial clefts

Question 9

describe the composition of amniotic fluid

Answer 9

• 98% water
• electrolytes, creatinine, urea, bile pigments, renin, glucose, hormones, foetal cells, lanugo (fine downy hair covering foetus) and vernix caseosa (waxy skin covering)

Question 10

what is amniocentesis

Answer 10

sampling of amniotic fluid allowing for collection of foetal cells. useful diagnostic test, e.g. foetal karyotyping

Question 11

how is amniotic fluid produced in early and later pregnancy

Answer 11

< 9 wks: maternal plasma moved across foetal membranes by osmotic and hydrostatic forces

> 9 wks:

• produced by foetal urinary tract
• recycled by the:
  i) foetal GI tract - swallowing is main pathway for recycling.
  ii) foetal lungs - inhaled during breathing movements, important for normal lung dev.
  iii) placental and foetal membranes (intramembranous pathway)

Question 12

how is meconium formed

Answer 12

swallowing of amniotic fluid:

• water and electrolytes are absorbed
• debris accumulates in gut forming meconium - AF debris + intestinal secretions inc. bile

Question 13

why is physiological jaundice common in the neonate

Answer 13

during gestation, clearance of foetal bilirubin is handled efficiently by the placenta as the foetus cannot conjugate bilirubin

immaturity of liver and intestinal processes for metabolism, conjugation and excretion can cause jaundice