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

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