Background
- Abnormally large volume of amniotic fluid
- Amniotic fluid index >20-25
- Largest fluid pocket depth >8 or 10cm
- Overall volume >1500-2000 cm³
- 2 diameter pockets >50 cm³
- Occurs in around 1-1.5% of pregnancies
Aetiology
- Can be idiopathic (60-65%) or associated with a number of foetal or maternal abnormalities
- Foetal
- Oesophageal atresia (prevents foetal swallowing and thus excess amniotic fluid builds up)
- Can be associated with other chromosomal abnormalities e.g. Down’s syndrome
- Neural tube defects
- In multiple pregnancy- a twin-twin transfusion may be the cause
- Maternal
- Diabetes is the most common cause (20-25%) (this could be pre-existing or gestational)
- Foetal
Presentation
- Can be picked up at antenatal examination where the uterus (fundo-symphaseal height) is large for dates
- The foetal parts may be difficult to palpate
- NB Rapid presentation of polyhydramnios is twin pregnancy suggests twin-twin transfusion
Investigations
- USS for evaluation of polyhydramnios (amniotic fluid index) and foetal wellbeing
- Other tests may be required for a diagnosis
- e.g. blood glucose in diabetes
- Coomb’s test/Rhesus D if there is a suspicion of haemolytic disease/foetal hydrops
- Infection screen (cytomegalovirus; syphilis; rubella; toxoplasmosis; parvovirus B19)
- Screening for Down’s or other chromosomal abnormality (genotyping)
Management
- Manage the underlying cause where possible e.g. diabetes, infection; twin-twin transfusion (laser) etc
- Indomethacin or amniocentesis
- If foetal distress develops, delivery (induction of labour)
- There is a higher risk of caesarean section
Complications
- Cord prolapse
- Preterm labour
- The overexpanded uterus is more excitable and prone to preterm labour
- Placental abruption (particularly after rupture of the membranes where there is a rapid decompression of the uterus)
- When performing amniotomy- ensure a slow release of amniotic fluid
- Postpartum haemorrhage
- The overstretched uterus does not contract sufficiently and so bleeding can be a problem