Polyhydramnios is a common complication of pregnancy, where there's an excessive amount of amniotic fluid surrounding the unborn baby (foetus).
An abnormal amount of fluid (whether increased or reduced) might be suspected by your healthcare professional after 30 weeks of pregnancy, during one of your antenatal appointments, and should be monitored and investigated.
The cause of polyhydramnios is often unknown, but sometimes indicates a problem with the baby's development (see What are the causes?, below). However, most women with polyhydramnios give birth to healthy babies.
If you're pregnant and notice your belly is getting large very quickly (acute polyhydramnios), contact your doctor or midwife. It's rare for it to happen suddenly, but it can indicate an abnormality with the foetus and increases your risk of giving birth prematurely.
In most women with polyhydramnios, the excess fluid builds up slowly. After 30 weeks of pregnancy, this excess fluid should be noticeable and picked up by your GP, midwife or obstetrician during one of your antenatal appointments.
In rare cases, when polyhydramnios develops very quickly, the excess fluid may be apparent earlier in pregnancy, or might be noted at the time of the detailed scan at 18-22 weeks.
If increased amniotic fluid is suspected, you may be referred for an ultrasound scan, so that the depth of the amniotic fluid surrounding the foetus can be measured.
What are the causes?
Often, no cause can be found for polyhydramnios. However, it can be associated with any of the following:
The mother having diabetes, including gestational diabetes (pregnancy-related diabetes). Sometimes, if diabetes is the cause, the baby will be larger than expected for its stage of development.
A blockage in part of the baby's gut (a condition called gut atresia), which prevents them absorbing the usual amount of amniotic fluid. A gut atresia would often require an operation after the baby's birth (read more about gut atresia in babies).
The mother having rhesus disease, where antibodies in the mother's blood cross the placenta, sometimes causing anaemia in the foetus.
A growth of blood vessels on the placenta, called a chorioangioma.
A build-up of fluid in specific areas of the baby's body, such as the abdomen and chest cavity, called hydrops fetalis.
A genetic problem with the foetus.
What associated risks should I be aware of?
Having polyhydramnios means there's a slightly higher risk of your unborn baby having a birth defect, especially if the fluid build-up is severe. You should discuss with your healthcare professionals any other problems that the extra fluid could be linked to.
However, don't panic – the vast majority of women with polyhydramnios give birth to healthy babies.
Your labour may be affected in the following ways:
You may go into premature labour because of the additional pressure stretching the womb.
If your baby is in the wrong position, the umbilical cord may slip down into the birth canal when the membranes rupture (read more about a prolapsed umbilical cord).
You may have an increased risk of bleeding after delivery.
How is it investigated and managed?
Most women with polyhydramnios need an ultrasound investigation and a glucose tolerance test, to try to identify a cause and check for gestational diabetes.
If your condition isn't severe, this can be done in the GP's surgery and you may not need to be admitted to hospital. Read more about ultrasound scans in pregnancy.
If the ultrasound scan appears to show a problem with the foetus, doctors might discuss whether you want to have amniocentesis (where a sample of fluid is drawn out using a needle), to test for chromosomal or genetic problems in the foetus.
Your doctor may also recommend a period of bed rest to help reduce your risk of giving birth prematurely, if you're at risk.
During and after labour
Your doctors and midwives will usually recommend giving birth to the baby in hospital, where more emergency treatment options are available. They may also recommend close monitoring of the baby's heart rate in labour.
If your baby appears normal after birth, your doctor might suggest passing a tube down your baby's throat to check them for internal problems, such as a problem with the development of their gullet (oesophageal atresia).