Session 9 - Fetal Physiology and growth and development

Question 1

What is the definition of haemochorial?

Answer 1

The barrier between maternal and fetal blood is at an absolute minimum.

Question 2

Describe how the foetus obtains oxygen?

Answer 2

There must be a gradient of pressure across the placenta to ensure oxygen moves from an area of high concentration to low concentration. This is established by an increased maternal pO2.

Question 3

What is the partial pressures of oxygen in arterial blood of an adult and a foetus?

Answer 3

13.3kPa

4kPa

Question 4

How does the foetus maximise oxygen transfer to increase content in foetal blood?

Answer 4

Increased haematocrit
Foetal haemoglobin has gamma instead of beta chains which has a lower affinity for 2-3 DPG making it have a higher affinity for oxygen.

Question 5

How does the foetus ensure that CO2 is removed efficiently?

Answer 5

CO2 is also removed by passive diffusion, the foetus cannot tolerate higher levels of CO2 than the mother so to generate a gradient the mother has to have lower CO2 levels. This is achieved by physiological hyperventilation to lower the levels of CO2 in the mothers blood.

Question 6

What problems would exist if the fetal circulation remained the same as the adult circulation?

Answer 6

The liver would use up all of the oxygen in the blood before the blood reached the rest of the body. Deoxygenated blood would mix with oxygenated blood in IVC. Which would mix with deoxygenated blood from head and oxygen would be lost in lungs. This means there would be no oxygen left for the brain which is the most important organ to supply.

Question 7

What shunts exist in the fetal circulation?

Answer 7

Ductus venosus
Foramen ovale
Ducts arteriosus

Question 8

Describe the circuit of blood in the fetal circulation from the umbilical vein.

Answer 8

1. Blood enters umbilical vein 70%
2. Blood bypasses the liver via the ductus venous and enters IVC where is becomes 65% as it mixes with blood returning from bottom half of body.
3. Blood enters the Right atrium via the IVC and is directed to the foramen oval by the crista dividens and enters the LA.
4. Blood returning from the SVC goes to the lungs via the pulmonary arteries.
5. Blood in the LA is diluted by returning blood from the pulmonary veins to around 60% oxygenated.
6. LA to LV
7. LV to Aorta
8. From the aorta to the carotids (and other places) supply to the brain is at around 7mmol.l
9. Returns via SVC to RA
10. RA to RV
11. Exit via pulmonary artery and then to aorta (past point of branching) via the ductus arteriosus
12. Blood returns to the placenta via the umbilical arteries.

Question 9

What function do the lungs have in the foetus?

Answer 9

Not for gas exchange,
Every 1-4hr each day they make breathing movements and flush the lungs with amniotic fluids.
During T2/3 they make breathing movements, conditioning the respiratory muscles in order to prepare for birth.

Question 10

What is the function of the amniotic fluid?

Answer 10

Mechanic protection

Provides moist environment for baby to grow

Question 11

How is amniotic fluid formed?

Answer 11

Initially it is made from maternal fluids and from diffusion of fluid across the skin of the foetus. In later pregnancy the foetus produces urine and swallows urine which leads to the constant turnover of the amniotic fluid.

Question 12

What volume of amniotic fluid is present in the amnion?

Answer 12

8 weeks - 10ml
38 weeks - 1L
42 weeks - 300ml

Question 13

How much urine does the foetus produce?

Answer 13

25 weeks - 100ml hypotonic urine

at term - 500ml

Question 14

What does the amniotic fluid contain?

Answer 14

Water
Electrolytes
Dead epithelium

Question 15

What is meconium?

Answer 15

Dead epithelium and debris that the foetus has swallowed from the amniotic fluid.

Question 16

How is bilirubin handled by the placenta?

Answer 16

Foetus is incapable of conjugating bilirubin so unconjuagted bilirubin crosses the placenta and travel to the liver where the mother can conjugate it and excrete in the bile. Neonatal jaundice is not pathological as it can take a while for conjugation to occur in new-borns, exposing them to light can jump-start the process.

Question 17

At what weeks are the pre-embryonic, embryonic and fetal stages?

Answer 17

Pre-embryonic stage - 2 weeks 1-2
Embryonic - 6 weeks 3-8
Fetal - 30 - 9-38
(+2 as calculated from date of last menstrual period)

Question 18

What is the embryonic period defined by?

Answer 18

Organogenic period - morphogenesis and differentiation
Absolute growth small
Placental growth

Question 19

What is the early fetal period defined by?

Answer 19

Accelerated absolute growth - CRL
Weight gain which increases in rate as time goes on
Protein deposition - msucles development

Question 20

What is the late fetal period defined by?

Answer 20

Weight gain

Fat deposition

Question 21

How can we assess fetal well-being?

Answer 21

Ask mother - if it is their first pregnancy by 20 weeks the mother would be able to feel the foetus moving. Earlier if they have already had a child.
Measurements of uterine expansion - symphysis-fundal height
Ultrasound scan

Question 22

What can an ultrasound scan be used for?

Answer 22

Calculate age 
Look for ectopic pregnancies 
No. of foetuses 
Assess fetal growth 
Look for abnormalities 
Sex foetus

Question 23

In what ways can we estimate fetal age?

Answer 23

Last menstrual period - prone to inaccuracies as implantation bleeding can occur
Crown rump length - measure between 7 o 13 weeks and date pregnancy and give estimated delivery date.
Combination of bi-parietal diameter, abdominal circumference and femur length. Information from scan. They can also be used to asses growth and detect anomalies.

Question 24

What is the average birth weight?

Answer 24

3. 5kg

4. 5kg - microsomal baby - maternal diabetes indicated.

Question 25

Describe the development of the respiratory system:

Answer 25

Lungs develop late.
During the embryonic period only the bronchopulmonary tree is developed.
During the fetal period specialisation occurs.
Weeks 8-16 pseudo-glandular stage - duct system begins to form tubes - bronchioles developed
Weeks 16-26 canicular stage - formation of respiratory bronchioles
Weeks 26 to term terminal sac stage - terminal sacs bud from respiratory bronchioles. Also differentiation of Type I and II pneumocytes?

Question 26

What problems can occur if a baby is born prematurely?

Answer 26

If born before 26 weeks the lungs are not fully developed and no type II pneumocytes are producing surfactant which leads to respiratory distress syndrome in that the baby cannot expand its lungs as surface tension is holding them shut.

Question 27

How can respiratory distress syndrome be avoided?

Answer 27

If pre-term delivery is unavoidable or inevitable then the mother can be treated with glucocorticoids to try to increase surfactant production in the foetus.

Question 28

When does kidney function begin in the foetus?

Answer 28

Week 10

Question 29

What is polyhydramnios and oligohydramnios?

Answer 29

Poly - too much amniotic fluid - fetal inability to swallow

Oligo - not enough amniotic fluid - reduced or no urine production - kidney problem

Question 30

Describe the development of the CNS?

Answer 30

1st to begin development and last to finish
Coticospinal tracts required for coordinated movement form in the first 4 months.
Myelination occurs at 9 months
No movement before 8th week, after which foetus practices movement for after birth

Question 31

What is quickening?

Answer 31

When the mother can feel the foetuses movements. From around week 17 onwards. Can be used to monitor foetus wellbeing.