Amniotic Fluid Dynamics

Introduction to Amniotic Fluid Dynamics

• Amniotic fluid is also known as the 'fluid of life.'
• The Chest expansion on swallowing amniotic fluid supports normal lung development in an unborn fetus.
• The fetus swallows the amniotic fluid and then excreates it. This cycle is repeated quite a few times. Fetal swallowing permits GI tract development.
• Amniotic fluid creates physical space for fetal movement, which is important for neuromusculoskeletal development.
• The amniotic fluid prevents umbilical cord compression, protects the fetus from trauma, and acts as a cushion for the baby. 
• It also shows bacteriostatic properties.

Amniotic Fluid Formation

• In the Initial few weeks of pregnancy, it is Formed by the Mother.
• Transmembranous flow is Across the amnion.
• Intramembranous flow is the flow across fetal vessels on the placental surface.
• Transcutaneous Flow is the flow across fetal skin.
  • After 20 weeks, the skin becomes keratinized, and thus, its contribution decreases instead, fetal urine becomes the major contributor.

Circulation of Amniotic Fluid

• Additive contribution: these increase the circulation of amniotic fluid
  • Urine - 1000 ml (+)
  • Lung Secretion - 350 ml (+)
• Reductive contribution: These decrease the circulation of amniotic fluid
  • Fetal swallowing - 750 ml (-)
  • Intra membranous Fluid transport across fetal blood vessels and the surface of the placenta → 400 ml(-)

 Properties of Amniotic Fluid

• pH: Alkaline [7.4 - 7.6]  
• Specific Gravity: 1.008 - 1.010
• Color: Straw colored  
• Osmolality: 260 mosm/l, which is hypotonic as compared to fetal serum

Types of Colors of Amniotic Fluid & Associated Conditions

• Red: Antepartum
• Green: Hemorrhage (Abruption)
• Yellowish: Meconium stained liquor
• Green: Post-term pregnancy 
• Tobacco Juice: Intra Uterine Death (IUD) 
• Golden Yellow: Rh isoimmunization

Amniotic Fluid Measurement

It is important for the diagnosis of major pathologies in a pregnant female. It can be done by two ways:

1. Dye Dilution Technique
2. Amniotic fluid index (AFI): It is the sum of the deepest vertical pockets of liquor in each of the four quadrants on USG.

For example, Sum of all the liquor present in the 4 pockets.

• Normal Amniotic Fluid Index: 10-15cms (range 5-24); Normal amount in 1000 ml
• Oligohydramnios: < 5cm (< 500 ml); amount < 500 ml
• Polyhydramnios: > 25cm (> 2000 ml); amount > 2000ml

There is a Single Pocket Criteria that is used for the assessment of amniotic fluid.

For this criteria, Cord-free and limb-free pockets are assessed in all quadrants, and the single deepest pocket is measured.

• Oligohydramnios: Any single pocket having < 2 cm
• Polyhydramnios: Any single pocket having > 8 cm

Amniotic Fluid Volume Versus Gestational Age

• Maximum at → 32-34 weeks (1000 ml)
• External cephalic version is possible at 36 weeks because:
  •  AFI is 800ml, which is less than that at 32 weeks
  • Fetus is bigger
  • Uterus is relaxed
• Around 40 weeks: 600 ml
• AFI is maximum at 32-34 weeks.

1. Oligohydramnios:

Let us read about oligohydramnios in detail as this is a very high-yield topic for all the major competitive exams like NEET-PG, FMGE and INICET. The main causes of oligohydramnios are:

• Renal anomalies
• Amnion nodosum
• IUGR- Intrauterine growth retardation
• Post-term pregnancy
• Preeclampsia
• NSAIDS
• Premature rupture of membranes
• Ace inhibitors
• Amniocentesis (Leak)

Management of Oligohydramnios

• Oligohydramnios needs increased fetal Surveillance, with USG and Doppler, to detect fetal compromise early and intervene timely.
• Early delivery may be planned after lung maturity.
• Amnio–infusion can increase amniotic fluid in labour to reduce complications like cord compression. However, it does not solve the main problem.

There are two other concepts worth reading here about oligohydramnios. They are:

Potter’s Triad: This is seen in severe Oligohydramnios, usually when the cause of Oligohydramnios is renal anomalies. The triad consists of : 

• Pulmonary hypoplasia
• Renal anomalies
• Contracted & malfigured limbs & flat / compressed face

And Potter Sequence tells us The cause of Oligohydramnios is other than Renal anomalies. The sequence consists of

• Pulmonary hypoplasia
• Contracted & malfigured limbs & flattening of Facies.

2. Polyhydramnios

It is defined as Liquor:  >25 AFI or > 2 liters of Amniotic fluid. Acute Polyhydramnios is polyhydramnios seen at less than 24 weeks of gestation

Grading

• > 25-30 AFI: Mild Polyhydramnios (Most commonly seen)
• > 30-35 AFI: Moderate Polyhydramnios
• > 35 AFI: Severe Polyhydramnios (mostly associated with anomalies)

Causes of Polyhydraamonios

• Diabetes
• Esophageal atresia
• Twin gestation
• Neural tube defects
• Cleft lip palate
• Gastroschisis
• Duodenal atresia
• Chorioangioma of Placenta

Maternal Signs & Symptoms 

• Dyspnea
• Orthopnea
• Respiratory distress
• Sudden PROM: can cause abruption: fetal demise

Management 

• Indomethacin: It is given in a dose of 25 mg TID and reduces the fetus's urinary output. However, it should not be used beyond 32 weeks as it causes premature closure of the Ductus Arteriosus.
• Therapeutic Amniocentesis: In a single setting, 1500 ml of Amniotic Fluid can be aspirated over 30 minutes. This is commonly used for polyhydramnios causing respiratory distress while fetal lung maturity is not achieved.
• Management of labour and delivery: Labour can be induced with controlled Artificial Rupture of Membranes in Labor, as the sudden release of a large quantity of amniotic fluid can cause abruption or cord prolapse. Polyhydramnios is NOT an indication for LSCS.

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