Amniotic Fluid and Its Disorders in Obstetrics

1. What Is Amniotic Fluid?

• Cushioning - protects the fetus from mechanical trauma
• Bacteriostatic properties - helps maintain a sterile intrauterine environment
• Space for movement - allows fetal mobility and supports normal development of lungs and limbs
• Diagnostic access - provides fetal cells and metabolic byproducts; used in amniocentesis (first performed in 1956 for sex determination)

2. Volume of Amniotic Fluid

• Volume increases at 10 mL/week early in fetal period, peaks at 50-60 mL/week at 19-25 weeks
• After 34 weeks, volume falls at 60-70 mL/week at 40 weeks
• The late-pregnancy decline is a physiologic progression, not an aberration

Figure: Amniotic fluid volumes 8-44 weeks of gestation. Dots = mean for each 2-week interval. Shaded area = 95% CI (Brace & Wolf, 1989) - Creasy & Resnik's, p. 98

3. Composition of Amniotic Fluid

• Isotonic with maternal/fetal plasma
• Minimal protein
• Very low oxygen tension
• High concentration of sugar alcohols (product of anaerobic metabolism)
• Arises as a transudate of plasma - through nonkeratinized fetal skin or across uterine decidua/placenta

• Fetal urine becomes the major component (fetus produces dilute urine)
• AF osmolality falls to ~85-90% of maternal serum osmolality (drops by 20-30 mOsm/kg with advancing gestation)
• AF urea, creatinine, and uric acid increase to 2-3x fetal plasma levels
• The entire volume of AF turns over on a daily basis - a highly dynamic system

4. Production and Resorption - Sources and Sinks

5. Ultrasound Assessment of AF Volume

• Maximum Vertical Pocket (MVP) - single deepest pocket
• Amniotic Fluid Index (AFI) - sum of MVP in four uterine quadrants

6. Disorders of Amniotic Fluid

A. Polyhydramnios (Hydramnios) - Excess AF

• Impaired fetal swallowing - neurologic abnormalities, obstructions
• GI anomalies - esophageal atresia, tracheoesophageal fistula
• Respiratory anomalies - congenital diaphragmatic hernia
• Maternal diabetes mellitus - fetal hyperglycemia leads to osmotic diuresis
• Fetal anemia - increased cardiac output and urine flow
• Twin-to-twin transfusion syndrome (TTTS) - recipient twin
• Idiopathic - no identifiable cause in many cases
• Hypoproteinemia with decreased maternal plasma oncotic pressure

• Preterm birth (PTB): 18.5% with mild, 21.8% with moderate, 14.3% with severe polyhydramnios
• Uterine overdistension triggers inflammatory cytokines (IL-1β, TNF-α, IL-6, IL-8, CCL2, PGE2, PGF2α) - causing preterm labor
• The underlying cause (congenital malformations, diabetes) determines PTB risk more than the fluid volume itself
• Maternal discomfort, dyspnea
• Malpresentation
• Cord prolapse
• Placental abruption (after rapid decompression)

B. Oligohydramnios - Reduced AF

• Renal agenesis / bilateral renal anomalies - no urine production (Potter sequence)
• Posterior urethral valves / lower urinary tract obstruction - leads to oligohydramnios, pulmonary hypoplasia, renal dysplasia, Potter facies, clubfeet
• Placental insufficiency / uteroplacental insufficiency - reduced fetal perfusion reduces urine output
• Post-term pregnancy - AF falls 33%/week after 41 weeks (physiologic)
• Rupture of membranes (PPROM/PROM) - loss of AF
• Maternal dehydration - osmotic mechanism; rehydration can reverse it

• Pulmonary hypoplasia - AF is required for normal lung development; severe oligohydramnios early in pregnancy is particularly dangerous
• Limb deformities - restricted fetal movement
• Potter sequence - renal agenesis → oligohydramnios → pulmonary hypoplasia, Potter facies (flat nose, low-set ears, epicanthal folds), limb contractures
• Fetal distress - cord compression, fetal hypoxia
• Perinatal mortality - higher with severe oligohydramnios
• Intrauterine growth restriction (IUGR) association

C. Amniotic Fluid Embolism (AFE)

1. Hemodynamic compromise (cardiovascular collapse)
2. Respiratory compromise
3. Disseminated intravascular coagulopathy (DIC)

• Sudden cardiovascular collapse and cardiac arrest
• Severe hypoxia
• DIC with hemorrhage
• Often occurs during labor, delivery, or within 30 minutes postpartum

• Early intubation + 100% O2 + PEEP
• High-quality CPR immediately for cardiac arrest
• If AFE occurs before delivery → emergent cesarean delivery to improve maternal resuscitation and fetal survival
• Large-bore IV access + arterial line
• Transesophageal echocardiography after intubation - to manage pulmonary hypertension and cardiac failure
• Pressors, inotropes, pulmonary vasodilators as needed
• ECMO - consider after prolonged arrest or severe ventricular dysfunction unresponsive to medical management
• Massive transfusion: Packed RBCs : FFP : Platelets in 1:1:1 ratio
• TEG / ROTEM point-of-care testing for coagulopathy guidance
• Note: Recombinant activated Factor VIIa is controversial - a systematic review found it may worsen outcomes in AFE

7. Summary Table