Polyhydramnios / oligohydramnios
Abnormal amniotic fluid volume
Overview
Too much (polyhydramnios) or too little (oligohydramnios) amniotic fluid on USS. The volume points to a cause: polyhydramnios suggests reduced fetal swallowing or excess urine; oligohydramnios suggests reduced fetal urine output or fluid loss.
Recognise
- Polyhydramnios: large-for-dates, tense uterus, difficulty feeling fetal parts, malpresentation
- Oligohydramnios: small-for-dates, easily felt fetal parts; risk of pulmonary hypoplasia/contractures if early/severe
- Quantified by amniotic fluid index (AFI) or deepest pool
Red flags
- Preterm labour (polyhydramnios); severe early oligohydramnios (pulmonary hypoplasia)
Differentials & how to tell them apart
Investigations
USS amniotic fluid index/deepest pool. Investigate the cause: polyhydramnios → maternal glucose (diabetes), fetal anomaly USS (GI atresia, neuromuscular); oligohydramnios → confirm ROM, assess fetal kidneys/growth/placenta.
Management
Identify and treat the underlying cause; surveillance
- 1Quantify fluid (AFI/deepest pool) and investigate the cause (maternal glucose, detailed fetal USS, confirm/exclude ROM).
- 2Polyhydramnios: control diabetes; amnioreduction/indometacin in selected severe cases; watch for preterm labour and cord prolapse. Oligohydramnios: manage cause; deliver if placental insufficiency/post-dates.
Key points
Use the fluid abnormality as a pointer to the cause: polyhydramnios → "fetus not swallowing or making too much urine" (GI atresia, diabetes); oligohydramnios → "fetus not making/keeping fluid" (renal agenesis, ROM, placental insufficiency).
Monitor & prognosis
Fluid trend, fetal growth/wellbeing, glucose; labour onset.
Depends entirely on the underlying cause.
Source: RCOG; NICE