Amniotic Fluid and Its Disorders

Amniotic Fluid and Its Disorders

An overview of amniotic fluid, polyhydramnios, oligohydramnios, their causes, risks, and management.

Basic Characteristics of Amniotic Fluid

• Specific gravity: 1.008 to 1.01

• Osmolality: 260 milliosmoles per liter

• Hypotonic in comparison to maternal and fetal plasma but isotonic to fetal urine.

• pH: 7 to 7.5 (exactly 7.2)

  • Application: In premature rupture of membranes (PROM), the alkaline pH helps differentiate amniotic fluid from acidic vaginal discharge (pH around 3.5) using a litmus paper test.

• Color: Colorless; at term, it is turbid or cloudy and straw-colored.

• Odor: Odorless

  • Application: The absence of odor helps differentiate it from urine and a foul smell indicates chorioamnionitis.

  • Amniotic fluid is also called as licker amni.

Volume of Amniotic Fluid

• Volume varies with gestational age, increasing up to 34 weeks.

  • 10 weeks: 30 ml
  • 12 weeks: 50 ml
  • 16 weeks: 200 ml
  • 20 weeks: 400 ml

• Maximum volume: 1000 ml at 34 weeks (range: 32 to 34 weeks).

• At term (40 weeks): 800 ml

• Post-term (42 weeks): Drastically decreases to 200 ml.

  • Application: Decreased amniotic fluid in post-term pregnancy leads to oligohydramnios.

Source of Amniotic Fluid

• First trimester: Transudate of maternal plasma through the placenta (spiral arteries).

• 12 to 20 weeks: Fetal skin (main contributor) and fetal urine (urine production starts at 12 weeks).

• After 20 weeks: Fetal urine (main contributor due to keratinization of fetal skin).

  • Overall, fetal urine is the main contributor.

• Fetal urine does not contribute before 12 weeks.

  • Application: Oligohydramnios in the first trimester cannot be due to renal defects.

Applied Aspects Related to Amniotic Fluid Sources

• Keratinization of fetal skin occurs by 22 to 25 weeks.

• Defects in fetal skin (neural tube defects, omphalocele, gastroschisis) cause continuous transudation, leading to polyhydramnios.

• Increased fetal urine production (maternal diabetes, multi-fetal pregnancy) increases amniotic fluid.

• Decreased fetal urine production (renal agenesis, polycystic kidney disease) decreases amniotic fluid.

• Drugs affecting urine production:

  • ACE inhibitors: Decrease urine production, leading to decreased amniotic fluid.
  • Endomethacin: Decreases amniotic fluid by decreasing urine production.

Maternal Plasma Contribution

• Maternal plasma contributes via the placenta.
• A big placenta (multiple pregnancy, Rh incompatibility, diabetes) leads to increased filtration of maternal plasma and increased amniotic fluid.
• Small placenta (uteroplacental insufficiency, PIH, IUGR) leads to decreased filtration and decreased amniotic fluid.

Fetal Swallowing

• Fetus maintains amniotic fluid balance by swallowing it.
• Fetal swallowing defects lead to increased amniotic fluid.
• Rate of production and resorption helps to determine the balance of amniotic fluid.

Composition of Amniotic Fluid

• 98 to 99% water, thus having no nutritional value for the fetus.
• Functions: Distension of the uterus, space for organ development, shock absorber, temperature maintenance.
• Hormones present: Prolactin, renin, and insulin.

Abnormal Colors of Amniotic Fluid

• Green: Meconium (fetal distress, transverse lie, breach), listeria infection (due to biliverdin pigment).
• Golden: Bilirubin (fetal jaundice, Rh incompatibility).
• Tobacco juice or brown: Intrauterine demise of the fetus.
• Saffron or yellowish-green: Post-term pregnancy.
• Dark red: Concealed hemorrhage (concealed abruptio placenta).

Measurement of Amniotic Fluid

• Investigation of choice: Ultrasound.

• Amniotic Fluid Index (AFI):

  • Abdomen divided into four quadrants using linea nigra and umbilicus as landmarks.
  • Measure the largest vertical diameter of amniotic fluid pocket in each quadrant.
  • AFI is the sum of these diameters.
  • Normal range: 5 to 24 cm.
  • Oligohydramnios: AFI $<br /> less = 5$ cm.
  • Polyhydramnios: AFI $<br /> gtr = 25$ cm (some sources say $<br /> gtr = 24$ cm).

• Single Largest Vertical Pocket (SVP) or Single Deepest Pocket:

  • More sensitive method, used in twin pregnancies.
  • Measure the vertical diameter of the single largest pocket.
  • Normal range: 2 to 8 cm.
  • Oligohydramnios: Diameter $<br /> less 2$ cm.
  • Polyhydramnios: Diameter $<br /> gtr = 8$ cm.

Clinical Significance

• Moderate or severe oligohydramnios or polyhydramnios indicates a high-risk pregnancy.

• Management of high-risk pregnancies includes:

  • Fetal monitoring starting at 32 weeks.
  • Induction of labor at a specific time depending on the condition.