Amnionic fluid dynamics
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The document discusses amniotic fluid dynamics and regulation. Amniotic fluid provides space for fetal movement, allows fetal swallowing and breathing, and protects the fetus from trauma. Its volume increases until around 34 weeks gestation when it peaks at around 800mL, then decreases. Fluid is produced mainly through fetal urination in the second and third trimesters. Disorders of amniotic fluid volume include polyhydramnios and oligohydramnios, which are diagnosed using techniques like measuring the deepest vertical pocket or calculating the amniotic fluid index.
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Amnionic fluid dynamics
- 2. Amnionic Fluid • Creates a physical space for fetal movement • Permits fetal swallowing • Permits fetal breathing • Protects fetus from trauma • Has bacteriostatic properties Williams Obstetrics 24th edition
- 3. Volume of Amniotic Fluid • up to 8 weeks - increases at the rate of 10 ml/week • 19-25 weeks - increases at the rate of 50-60 ml/week • >25 weeks- decreasing rate • 34 weeks (GA) - peaks at about 800 mL • 40 weeks (GA) – total AF volume falls (ave: 60-70ml/week)
- 5. FIRST TRIMESTER • Isotonic with Maternal and fetal plasma • Major Source: Amniotic Membrane • Production is by Membrane Transport
- 6. SECOND TRIMESTER • Chorion Frondosum • Fetal skin • Fetal Urination – major source of AF production • Fetal Respiratory Tract Fluid • Fetal swallowing
- 7. Four Pathways in Amnionic Fluid Regulation Pathway Effect on Volume Approximate daily Volume (ml) Fetal Urination Production 1000 Fetal Swallowing Resorption 750 Fetal Lung Fluid Secretion Production 350 Intramembranous flow (across fetal vessels on placental surface) Resorption 400 Transmembranous Flow (across amnionic membrane) Resorption minimal Table 11.1 Amnionic Fluid Regulation in Late Pregnancy Williams Obstetrics 24th ed
- 8. CRITERIA FOR DISORDERS OF AMNIOTIC FLUID VOLUME
- 9. CLINICAL CRITERIA POLYHYDRAMNIOS/HYDRAMNIOS Mild - single deepest pocket 8-11cm depth - AFI 25-29.9cm Moderate - single deepest pocket 12-15cm deep - AFI 30-34.9cm Severe - single deepest pocket </= 16 cm - AFI >/= 35 cm
- 10. CLINICAL CRITERIA OLIGOHYDRAMNIOS - maximum vertical pocket </= 2cm - AFI < 5cm or less than 5th percentile - secondary to either excess loss of fluid or decresed fetal urine production
- 11. Related to the following conditions: 1. Ruptured membranes 2. Congenital anomaly (renal agenesis, cystic dysplasia, ureteral atresia) 3. Chronic conditions leading to decreased renal perfusion 4. Postterm gestation
- 12. II. SONOGRAPHIC CRITERIA A. Subjective Evaluation - convenient and simple - dependent on sonographers experience - classified as normal, low or high for gestational age
- 13. OLIGOHYDRAMNIOS CRITERIA - absence of fluid pockets throughout uterine cavity - crowding of fetal limbs - absence of pockets surrounding fetal legs - overlapping of fetal ribs (severe cases)
- 14. B. SEMIQUANTITAIVE METHODS 1. Single deepest pocket Technique - measuring deepest clear AF pocket anteroposteriorly in the uterus - Normal: >2cm but <8cm depth Oligohydrmnios <1cm depth reduced fluid volume 1-2cm depth Polyhydramnios >8cm depth - most effective in measuring AFV in multifetal gestations
- 15. 2. Amniotic Fluid Index - sum of the deepest vertical pocket in each of four quadrants - borders of the 4 quadrants: linea nigra: to divide the right from the left umbilicus: upper and lower
- 16. - Transducer parallel to the patients’ sagittal plane and perpendicular to the coronal plane - should not be angled to accommodate the uterine fundal curvature - each pocket should be clear of umbilical cord and fetal small parts
- 17. 3. Two diameter pocket technique - measures the vertical depth multiplied by the horizontal dm of the largest pocket 15.1-50 cm: Normal >50 cm: Hydramnios 0-15 cm: Oligohydramnios
- 18. OTHER EXAMINATIONS FOR AFV DETERMINATION • Dye dilution techniques • MRI • 3D Ultrasonography
- 19. CLINIC0-SONOLOGIC CORRELATIONS • HYDRAMNIOS - anatomic lesions must be sought GIT: e.g. double bubble sign CNS: anencephaly abdominal wall: omphalocoele; gastroschisis - Treatment: therapeutic amniocentesis oral Indomethacin (25 mg PO QID)
- 20. CLINIC0-SONOLOGIC CORRELATIONS • OLIGOHYDRAMNIOS - anomalies of the renal system must be sought e.g. renal agenesis; cystic dysplasia; ureteral atresia; etc - signs of pulmonary hypoplasia - IUGR - fetal distress in labor - Treatment: therapeutic amnioinfusion
- 21. COLOR FLOW DOPPLER AND AMNIOTIC FLUID EVALUATION - addition of color flow Doppler to assist in illuminating loops of Umbilical cord within amniotic fluid pockets - has been shown to lead to overdiagnosis
- 22. AMNIOTIC FLUID VOLUME IN MULTIPLE GESTATIONS - using MVP or the two diameter pocket provide best reproducibility and accuracy
- 23. • Thank You
Editor's Notes
- Physical space for fetal movement necessary for normal musculoskeletal devt. Fetal swallowing (essential for GI devt) Fetal breathing (necessary for lung devt) Guards against cord compression (protects fetus from trauma) Bacteriostatic properties maintains a sterile intrauterine environment
- Volume increases as the fetus grows and rate of change varies during the pregnancy. Amniotic fluid is often assessed for both quality and quantity. 25 weeks - amniotic volume increase starts decreasing and eventually plateaus (rate of change equals zero) 34 weeks – volume already at its max
- Membrane Transport: either by Passive transfer of water or active transport of solutes Early AF believed to arise as a transudate of plasma, through fetal non keratinized fetal skin or across uterine decidua or placental surface or both
- Chorion frondosum Is where free water exchange occurs bet fetal blood and AF Fetal skin is highly permeable before it becomes keratinized at 22-25 weeks Second half of pregnancy = fetal urine major source of AF
- Classically defined as Amniotic fluid of 2000ml or more
- Done by real time scanning of fluid throughout the uteris and observing amniotic fluid within the uterus. Subjectively, oligohydramnios crierion are: absence of fluid pockets within the cavity, crowding of fetal limbs, absence of pockets surrounding fetal legs, overlapping of fetal ribs.
- AFI currently remains the most accurate method to measure fluid volume in singleton gestations For multiple gestations, SVP provides the simplest and most reproducible results.