Polyhydramnios

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)

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