Aetiology

In most cases the excess fluid accumulates slowly resulting in chronic polyhydramnios which is usually detected after 30 weeks of gestation.

Rarely the amniotic fluid accumulates rapidly, resulting in acute polyhydramnios. There is an increased risk of premature labour before 28 weeks, and it tends to develop earlier than the chronic form, possibly by the 16th to 20th week.

There are foetal and maternal causes of polyhydramnios.

Foetal causes:

• twin pregnancy, especially uniovular twins
• anencephaly interferes with foetal swallowing
• oesophageal or duodenal atresia prevents foetal swallowing
• spina bifida
• chorioangioma of the placenta
• hydrops foetalis

Maternal causes:

• poorly controlled maternal diabetes results in foetal polyuria
• multiple pregnancy

The acute form of polyhydramnios is associated with fetal abnormality or uniovular twins.

Chronic polyhydramnios is particulary associated with gut atresia, fetal abnormality, and multiple pregnancy. Often no cause if found.