Recallab › Question bank › Obstetrics › #50188
Oligohydramnios
At an 18-week anomaly scan the amniotic fluid is severely reduced, the fetal bladder cannot be visualised on repeated examination, and no kidneys are identified in either renal fossa. The membranes are intact and there is no history of fluid leakage. What is the most likely cause?
- A. Post-term pregnancy
- B. Twin-to-twin transfusion syndrome
- C. Preterm prelabour rupture of membranes
- correctD. Bilateral renal agenesis
Explanation
Key fact
After about 16 weeks, amniotic fluid is fetal urine. No kidneys and no bladder means no urine, and therefore no fluid.
Bilateral renal agenesis. An absent bladder on repeated scanning, with absent renal fossae and intact membranes, leaves no other explanation.
How it happens
- In the first trimester the fluid is largely a transudate across fetal skin and membranes.
- From about 16 weeks it is produced almost entirely by fetal urine.
- So severe oligohydramnios at 18 weeks points at the urinary tract.
- A bladder that never fills across repeated observations means no urine is being made.
- Absent kidneys in both renal fossae confirms it.
- Intact membranes with no leakage excludes the commonest alternative.
After sixteen weeks the liquor is the fetal urine output
Why not the others
- A. Post-term pregnancy — reduces liquor through placental insufficiency, but that is a problem of 41 weeks and beyond, not 18.
- B. Twin-to-twin transfusion syndrome — requires a monochorionic twin pregnancy - one twin with oligohydramnios and the other with polyhydramnios. This is a singleton.
- C. Preterm prelabour rupture of membranes — is the commonest cause of oligohydramnios overall, but the membranes are intact and there is no history of leakage. The bladder would also be seen to fill.
Remember
Look for the bladder. A fetal bladder that fills and empties proves the kidneys work, and takes seconds to check.
Go deeper
The consequence is Potter sequence: pulmonary hypoplasia, limb contractures and the characteristic flattened facies, all from compression and from lack of fluid for lung development. Bilateral renal agenesis is uniformly fatal.
Amniotic fluid index under 5 cm, or a deepest vertical pocket under 2 cm, defines oligohydramnios.
Other causes to consider: obstructive uropathy such as posterior urethral valves, severe growth restriction, and maternal NSAIDs or ACE inhibitors.
DC Dutta's Textbook of Obstetrics, 11th ed. (Jaypee, 2026); Williams Obstetrics, 26th ed. (McGraw Hill, 2022).