In the womb, the baby is surrounded by and floats in a sac filled with amniotic fluid. This fluid acts to protect the foetus from bumps and jarring, recirculates waste, and acts as a fluid for the baby to drink. If an excessive amount of fluid is present, the condition is called polyhydramnios.
Normally there is about a litre (1000 mL) of amniotic fluid at birth. A volume greater than 1500 mL is considered to be diagnostic of polyhydramnios, but it may not become apparent until 2500 mL or more is present.
Polyhydramnios occurs in about one in every 100 pregnancies, and it may be a sign that the foetus has a significant abnormality that prevents it from drinking or causes the excess production of urine. Other causes include a twin pregnancy, and diabetes or heart disease in the mother. In over half the cases no specific cause for the excess fluid can ever be found.
The condition is diagnosed by an ultrasound scan, and if proved, further investigations to determine the cause of the condition must follow. The treatment will depend upon the result of these tests, but often none is necessary.
There is an increased risk to the mother of amniotic fluid embolism, a potentially fatal complication that occurs when some of the fluid enters the mother’s blood stream, but most pregnancies proceed relatively normally, although there is an increased risk of foetal abnormality.