Learning Objectives
Understand the production and clearance of Amniotic Fluid. Master the clinical significance of Polyhydramnios and Oligohydramnios, including their high-yield associations for the USMLE Step 1.
1. Amniotic Fluid Dynamics
Amniotic fluid is in a constant state of turnover. In the second half of pregnancy, its volume is primarily determined by a balance between fetal “output” and “input.”
| Mechanism | Process |
|---|---|
| Production (Sources) | Primarily, Fetal Urine and expelled fetal lung liquid. |
| Clearance (Removal) | Primarily, Fetal Swallowing and intramembranous absorption. |
2. Polyhydramnios (Too Much Fluid)
Occurs when there is an inability to swallow or a high-output state. It is defined as an amniotic fluid index (AFI) > 24 cm or a deep vertical pocket > 8 cm.
| Etiology | Mechanism / Examples |
|---|---|
| GI Obstruction | Esophageal or Duodenal atresia prevents the fetus from swallowing fluid. |
| Neurological | Anencephaly; loss of the swallowing reflex. |
| High Output | Maternal diabetes (osmotic diuresis), fetal anemia, or multifetal gestation. |
3. Oligohydramnios (Too Little Fluid)
Occurs when there is an inability to excrete urine or chronic fluid loss. Defined as AFI < 5 cm or a deep vertical pocket < 2 cm.
| Etiology | Mechanism / Examples |
|---|---|
| Renal Anomalies | Bilateral renal agenesis or ARPKD. |
| Obstruction | Posterior Urethral Valves (males); block urine from leaving the bladder. |
| Placental | Placental insufficiency (leads to decreased renal perfusion). |
Activity:
High-Yield Clinical Pearls:
- The Potter Link: Profound Oligohydramnios is the direct cause of Potter Sequence (pulmonary hypoplasia, facial/limb deformities).
- GI Clues: If a board question mentions the “double bubble” sign (duodenal atresia), always look for Polyhydramnios as the prenatal finding.
- Maternal Diabetes: High maternal glucose crosses the placenta, causing fetal hyperglycemia and polyuria, leading to polyhydramnios.