Polyhydramnios refers to a situation where the amniotic fluid volume is more than expected for gestational age.

It is generally defined as:

• amniotic fluid index (AFI) >25 cm, though the cutoff in some centres is being reduced to 24 cm ¹⁴

• largest fluid pocket depth (maximal vertical pocket) >8 cm ⁶

  • ^​some centres, particularly in Australia, New Zealand, and the United Kingdom, use a cut off of >10 cm ^ref

• overall amniotic fluid volume >1500-2000 mL ³

• two diameter pocket (TDP) >50 cm ²

Epidemiology

Polyhydramnios can occur in 1-1.5% of pregnancies ^ref.

Associations

• fetal macrosomia: independent of maternal diabetes ², in the idiopathic form

• mesoblastic nephroma ^ref

• Pena Shokeir syndrome ^ref

• maternal overhydration ^ref

Polyhydramnios is associated with poor outcomes if present in combination with intrauterine growth restriction (IUGR); usually seen in aneuploidies 18,13, and 21 ^ref.

Clinical presentation

The patient may clinically present as a large for dates uterus. 

Pathology

Polyhydramnios occurs as a result of either increased production or decreased removal of amniotic fluid. The aetiology of polyhydramnios can be due to a vast variety of maternal and fetal disorders. 

It is usually detected after 20 weeks (often 3^rd trimester).

Aetiology

The potential causes of polyhydramnios are variable including:

• idiopathic: 60-65%

  • ​this is a diagnosis of exclusion despite accounting for a majority of cases, also termed idiopathic polyhydramnios

• maternal: 25-30%

  • diabetes: commonly gestational diabetes

  • maternal congestive heart failure ⁸

• fetal: 10-20%

  • CNS lesions (e.g. neural tube defects): fetal CNS abnormalities tend to be the commonest out of all fetal causative associations ³

  • proximal gastrointestinal obstruction

    • gastrointestinal atresia(s) 

      • oesophageal atresia

      • duodenal atresia 

      • jejunoileal atresia

    • abdominal wall defects

      • gastroschisis

      • omphalocele

    • fetal intestinal volvulus, e.g. from an intestinal malrotation

  • fetal cervicothoracic abnormalities 

    • fetal cervical masses

      • congenital cervical teratoma/epignathus

      • large fetal goitre

    • thoracic masses

      • congenital pulmonary airways malformation (CPAM)

      • congenital high airways obstruction syndrome (CHAOS)

      • congenital diaphragmatic herniation 

  • fetal cardiovascular anomalies

    • sustained fetal tachycardia (e.g. supraventricular tachycardia (SVT), atrial flutter, ventricular tachycardia) ¹²

  • twin pregnancy-related complications

    • twin-twin transfusion: occurs in the recipient

  • hydrops fetalis: immune and non-immune

  • fetal skeletal abnormalities

    • reduced fetal movement

Classification

This classification is general consensus based on common practices at the time of writing (July 2016) but this varies according to countries and gynaecologist association guidelines.

Some classify the severity of polyhydramnios as 

• mild: a single deepest pocket at 8-11 cm or amniotic fluid index (AFI) 25-30

• moderate: a single deepest pocket at 12-15 cm or AFI 30.1-35

• severe: single deepest pocket >16 cm or AFI >35

Treatment and prognosis

The prognosis is variable dependent on associated conditions. Usually minimal or no intervention is required for idiopathic mild uncomplicated cases. Options include:

• improved maternal diabetes control

• caesarian section if there is profound macrosomia

• therapeutic amniocentesis/amnioreduction

• indometacin ^9,10

Complications

The risk of the following obstetric complications is increased when polyhydramnios is present due to over-expansion of the uterus ¹³:

• maternal dyspnoea

• premature membrane rupture

• preterm labour

• abnormal fetal presentation

• umbilical cord prolapse

• postpartum haemorrhage: due to reduced uterine myometrial tone

See also

• oligohydramnios