Fetal Membranes and Fluid Balance

Question 1

What are the fluid compartments in adults/newborns vs the fetus?

Answer 1

Adult/Newborn:

1. Intracellular fluid
2. Extracellular fluid
   - Blood
   - Intersitial
   - Lymph etc.

Fetus:

1. Intracellular fluid
2. Extracellular fluid
   - Blood
   - Intersitial
   - Lymph etc.
3. Amniotic sac (amniotic fluid)
4. Allantoic sac/fluid (not in humans)
5. Lung liquid in lungs

Question 2

Describe the fetal membranes:

Answer 2

1. Amniotic membrane:
   - Completely surrounds fetus
   - Has some vascularity
2. Chorionic membrane:
   - Surrounds both amniotic membrane and allantoic membrane- is the membrane closest to uterine wall
   - Highly vascularised
3. Allantoic membrane:
   - regresses during embyronic human development and is left has a remnant
   - In most species it is avascular

Question 3

What fluids contribute to amniotic fluid?

Answer 3

• There is a 10 fold increase in amniotic fluid during gestation
• During the first half of gestation amniotic fluid is isotonic to fetal plasma but during the second half is it hypotonic

1. Fetal urine:
   - 800-1200mL per day near term
   - Very dilute
2. Fetal lung liquid:
   - Leaves the trachea by fetal breathing movements
   - 200-400mL per day
   - Some is immediately swallowed so does not enter amniotic sac (half in sac and half swallowed)
   - Very high in chloride
3. Fetal saliva and nasal secretions:
   - Mainly mucous
   - 30ml per day
   - High in K+ and viscous

Question 4

How is amniotic fluid volume lost?

Answer 4

1. Fetal swallowing:
   - In sheep 300-1000mL per day
   - Contibutes to 50-100% of amniotic fluid removal per day
2. Trans-membranous reabsorption:
   - Transfer of fluid from amnion across the amniotic and chorionic membranes into uterine capillaries
   - 10mL per day
   - Due to osmotic gradient
3. Inta-membranous reabsorption:
   - Transfer of fluid from the amniotic sac into fetal circulation
   - 200-500mL per day
   - Due to action of osmotic gradient

Question 5

What fluids contribute to allantoic fluid?

Answer 5

1. Fetal urine:
   - The only contributor to allantoic fluid is fetal urine via the urachus
   - Contibutes 500mL per day

• No mucous = less viscous than amniotic fluid

Question 6

How is allantoic fluid volume lost?

Answer 6

1. Trans-membranous reabsorption:
   - Transfer of fluid from the allantoic sac into uterine capillaries (across the allantoic and chorionic membranes)
   - Approx 10mL per day
   - Due to action of osmotic gradient
2. Intra-membranous re-absorption:
   - Transfer of fluid from allantoic sac into fetal circulation
   - Approximately 200-500 ml per day
   - Due to action of osmotic gradient

Question 7

What are the important functions of amniotic fluid?

Answer 7

1. Protection of the fetus- buffering against physical trauma and temperature regulation
2. GI tract development
   - via amniotic swallowing
3. Space to exercise muscles
   - allows fetus to not be compressed by uterus
4. Lung development:
   - allows fetus to not be compressed by uterus
5. Lubrication at delivery

Question 8

What is oligohydramnios?

Answer 8

• Low levels of amniotic fluid (<300mL)
• Occurs in 0.5-5.5% of pregnancies
• Leads to lung hypoplasia, facial abnormalities, limb and joint deformities and GI tract abnormalities

Causes:

• Loss of fluid from PPROM
• Reduced fluid production due to:
  1. Congenital defects in kidney, bladder or urethra
  2. IUGR (hypoxic fetus will not produce sufficient fetal urine
  3. Certain drugs

Question 9

What is polyhydramnios?

Answer 9

• Very high levels of amniotic fluid, defined as >2000mL of amniotic fluid
• Occurs in 0.1-3.2% of pregnancies
• May require drainage which can increase risk of infection and also can increase risk of preterm birth
• Causes are 60% unknown and 40% increased production of amniotic fluid or impaired removal

Question 10

What is twin-twin transfusion syndrome?

Answer 10

• Common cause of oligohydramnios and polyhydramnios
• Result of anastomoses between the blood vessels of two twins- one twin becomes a donor and the other twin becomes a recipient
• The recipient fetus has a greater blood supply will grow more efficiently, has more plasma to process through its kidneys, will produce more fetal urine and thus will produce more amniotic fluid- this inverse occurs in the donor twin
• The recipient twin develops polyhydramnios and the donor twin develops oligohydramnios and growth restriction
• Occurs in 10-15% of all monozygotic twins