Amniotic Fluid Volume: Oligohydramnios and Polyhydramnios

Sonographic Measurement Methods

Quantitative assessment of fluid volume is a mandatory component of every second and third-trimester obstetric ultrasound.

• Amniotic Fluid Index (AFI): The maternal abdomen is mathematically divided into four fixed quadrants centered on the umbilicus. The sonographer measures the single deepest, entirely vertical pocket of fluid in each quadrant, completely devoid of any umbilical cord or fetal parts. The sum of these four measurements is the AFI. A normal AFI strictly lies between 5 cm and 24 cm.
• Maximum/Deepest Vertical Pocket (MVP/DVP): A vastly simpler method evaluating only the single largest fluid pocket in the entire uterus. A normal DVP must absolutely lie strictly between 2 cm and 8 cm.

Oligohydramnios (Severely Reduced Fluid)

Defined precisely as an AFI less than 5 cm or a DVP strictly less than 2 cm.

• Placental Insufficiency: The most frequent cause in the third trimester. Chronic hypoxemia triggers a powerful fetal 'brain-sparing' reflex. Fetal renal blood flow is aggressively vasoconstricted, drastically reducing fetal urine output.
• Fetal Renal Anomalies: If the kidneys fail to develop entirely (Bilateral Renal Agenesis) or the urinary tract is completely mechanically blocked (Posterior Urethral Valves in male fetuses), virtually no urine enters the amniotic sac.
• Potter Sequence: Chronic severe oligohydramnios physically crushes the fetus against the uterine wall, producing highly characteristic, devastating physical deformities: severely flattened facial features (Potter facies), severe limb contractures (clubfeet), and fatal pulmonary hypoplasia because the lung physically cannot expand.

Polyhydramnios (Severely Excess Fluid)

Defined exactly as an AFI strictly greater than 24 cm or a DVP explicitly greater than 8 cm.

• Maternal Diabetes: Poorly controlled maternal hyperglycemia intensely drives extreme fetal hyperglycemia. This universally induces fetal osmotic diuresis (excessive fetal urination), representing the most common cause of polyhydramnios.
• Fetal Gastrointestinal Obstruction: High obstructions like esophageal atresia or duodenal atresia physically prevent the fetus from normally swallowing and physiologically absorbing the amniotic fluid.