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Amniotic fluid

Role of amniotic fluid

Amniotic fluid is the fluid that surrounds the baby and it has a number of functions that are essential for healthy growth and development. The amniotic fluid:

  • provides space for the baby's growth and movement
  • plays an important part in developing the lungs so that the baby can breathe after delivery.

Source of amniotic fluid

In the first trimester of pregnancy the amniotic fluid is mainly made up of fluid supplied by the mother via the placenta.

In the second trimester the baby becomes the main producer of amniotic fluid. The two main sources of fluid are the baby's urine and fluid excreted from their lungs. The baby also removes amniotic fluid by swallowing it.

As the baby grows in size it produces more amniotic fluid and the amount increases until around 32 weeks. The fluid level then remains constant until the baby is full term (37 to 42 weeks) when the levels start to decline.

In some pregnancies there may be too much (polyhydramnios) or too little (oligohydramnios) amniotic fluid. Any changes in the amount of fluid may place the mother and baby at risk and careful assessment into the cause is required for best management of the pregnancy.

Measuring amniotic fluid

An ultrasound scan is used to measure the amount of amniotic fluid surrounding a baby. After several measurements are taken the amniotic fluid index (AFI) is calculated. In general, an AFI of five centimetres to 25 centimetres is normal. An AFI of less than five centimetres is called oligohydramnios and an AFI of 25 centimetres or more is called polyhydramnios.


Oligohydramnios means that there is too little amniotic fluid surrounding the baby.

Causes of oligohydramnios

The main causes include:

  • ruptured membranes
  • a problem with the baby's kidneys or urinary tract.
  • a problem with placental function.

For many women the cause of the oligohydramnios is often unknown.

Complications of oligohydramnios

The problems associated with oligohydramnios vary according to the cause and the stage of the pregnancy. Generally, the earlier that oligohydramnios occurs the more serious it is.

Reduced amniotic fluid can pose serious risks to the ongoing health of the developing baby including:

  • impaired lung development
  • preterm birth
  • club feet (talipes)
  • breech presentation
  • infection if the membranes are ruptured.

Treatment of oligohydramnios

There is no effective treatment for oligohydramnios. To improve ultrasound imaging, temporary replacement of the fluid via a needle (amnioinfusion) can be considered.

Ruptured membranes

Ruptured membranes are one of the main causes of oligohydramnios with an associated risk of infection and preterm birth. Management usually involves:


  • a speculum examination to confirm the diagnosis
  • an ultrasound scan
  • a period of hospital admission
  • antibiotics
  • monitoring for signs of infection such as an increased temperature, raised mother's or baby's heart rate, changes in vaginal loss, abdominal pain, flu-like symptoms or reduced fetal movements
  • consideration of delivery if infection is suspected.


Polyhydramnios means that there is too much amniotic fluid surrounding the baby.

Causes of polyhydramnios

The main causes include:

  • fetal abnormalities
  • maternal diabetes
  • placental abnormalities.

For many women the cause of the polyhydramnios is often unknown.

Complications of polyhydramnios

Depending on the severity, polyhydramnios can increase the risk of some pregnancy complications including:

  • preterm birth
  • ruptured membranes
  • cord prolapse
  • placental abruption (when the placenta separates from the uterine wall)
  • postpartum haemorrhage (severe bleeding after the birth of the baby)
  • malposition of the baby (not coming head first)
  • stillbirth.

Women with polyhydramnios may have abdominal discomfort and breathing difficulties as a result of their increased abdominal size

Treatment of polyhydramnios

The management of polyhydramnios depends upon the cause. For example, improved control of diabetes can reduce polyhydramnios.

Some cases of polyhydramnios may require removal of the excess amniotic fluid via a needle (amniodrainage) to reduce the risk of ruptured membranes and preterm birth. The procedure is performed under ultrasound guidance and may need to be repeated during the pregnancy.

Monitoring of amniotic fluid volume

If oligohydramnios or polyhydramnios is diagnosed it is likely that further ultrasound scans will be required to monitor the amniotic fluid levels as the pregnancy progresses.

Further information

This brochure will provide some information for you to consider but it is not intended to take the place of comprehensive counselling available at the Mater Centre for Maternal Fetal Medicine

For further information, we recommend that you speak to one of our staff in Maternal Fetal Medicine or alternatively contact your referring doctor.

Mater Centre for Maternal Fetal Medicine
Level 7
Mater Mothers' Hospital
Raymond Terrace
South Brisbane Qld 4101
Phone: 07 3163 1896
Fax: 07 3163 1890


Mater acknowledges consumer consultation in the development of this patient information.
Mater Doc Num: PI-CLN-430132
Last modified 09/8/2017.
Consumers were consulted in the development of this patient information.
Last consumer engagement date: 28/2/2014
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