Polyhydramnios is the presence of excessive amniotic fluid surrounding the unborn infant.

Amniotic fluid is a clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy. It is contained in the amniotic sac.

While in the womb, the baby floats in the amniotic fluid. Amniotic fluid surrounds and cushions the infant throughout development. The amount of amniotic fluid is greatest at around 34 weeks into the pregnancy (gestation).

The amniotic fluid constantly moves (circulates) as the baby swallows and "inhales" the fluid, and then releases or "exhales" the fluid through urine.

The amniotic fluid helps:

• The developing baby move in the womb, which allows for proper bone growth
• The lungs to develop properly
• Keep a relatively constant temperature around the baby, protecting from heat loss
• Protect the baby from outside injury by cushioning sudden blows or movements

Polyhydramnios can occur if the fetus does not swallow and absorb amniotic fluid in normal amounts. This can happen due to:

• Gastrointestinal disorders, such as duodenal atresia, esophageal atresia, gastroschisis, and diaphragmatic hernia
• Brain and nervous system (neurological) problems, such as anencephaly and myotonic dystrophy
• A variety of other causes, such as poorly controlled diabetes, achondroplasia,Beckwith-Wiedemann syndrome

Polyhydramnios may also be related to increased fluid production, which occurs with:

• Certain fetal lung disorders
• Multiple gestation (for example, twins or triplets)
• Hydrops fetalis

Sometimes, no specific cause for polyhydramnios is found.