Amniotic fluid is a clear, yellow fluid which is found within the first 12 days following conception within the amniotic sac and surrounds the growing baby in the uterus.1-4
Amniotic fluid has many important functions and is vital for healthy fetal development.1,2 However, if the amount of amniotic fluid inside the uterus is too little or too great, complications can occur.
This Knowledge Center article will describe the many functions of amniotic fluid, as well as what happens when the levels of amniotic fluid are either too high or too low.
Here are some key points about amniotic fluid. More detail and supporting information is in the main article.
- To begin with, amniotic fluid consists of water from the mother's body
- Eventually, the majority of the amniotic fluid in the uterus is comprised of the baby's urine
- Amniotic fluid also contains important nutrients, hormones and antibodies
- Amniotic fluid helps protect the baby from bumps and potential injury
- As its body develops, the baby breathes and swallows the amniotic fluid
- Doctors can tell from the amount of amniotic fluid present whether or not a baby has a problem with their swallowing reflex
- Oligohydramnios refers to when amniotic fluid levels are too low
- Polyhydramnios refers to when amniotic fluid levels are too high
- Doctors can alter the levels of fluid within the uterus with a variety of procedures.
What is amniotic fluid?
While a baby is in the womb, it is situated within the amniotic sac, a bag formed of two membranes (the amnion and the chorion) in which the fetus grows and develops. Inside the sac, the baby is surrounded by amniotic fluid.5
Amniotic fluid is vital to the healthy growth and development of a fetus.
Initially, the fluid is comprised of water produced by the mother. It is soon entirely replaced by fetal urine (as it swallows and excretes the fluid), however, around 20 week's gestation.1
Amniotic fluid is not only comprised of urine, it also contains vital components such as nutrients, hormones and infection-fighting antibodies.1
When amniotic fluid is green or brown, this indicates that the baby has passed meconium (the first bowel movement) before birth. This can be problematic, causing a breathing problem called meconium aspiration syndrome that occurs when the meconium enters the lungs. In some cases, babies will require treatment after they are born.1
Amniotic fluid is responsible for:1-4
- Protecting the fetus: the fluid cushions the baby from outside pressures, acting as a shock absorber
- Temperature control: the fluid insulates the baby, keeping it warm and maintaining a regular temperature
- Infection control: the amniotic fluid contains antibodies
- Lung and digestive system development: by breathing and swallowing the amniotic fluid, the baby practices using the muscles of these systems as the grow
- Muscle and bone development: as the baby floats inside the amniotic sac, it has the freedom to move about, giving muscles and bones the opportunity to develop properly
- Lubrication amniotic fluid prevents parts of the body such as the fingers and toes from growing together; webbing can occur if amniotic fluid levels are low
- Umbilical cord support: fluid in the uterus prevents the umbilical cord from being compressed. This cord transports food and oxygen from the placenta to the growing fetus.
Under normal circumstances, the level of amniotic fluid is at its highest during weeks 34-36 of pregnancy, measuring out at an average of 800 mL. This level decreases as birth nears, averaging around 600 mL at 40 weeks of gestation.1-3
When an expectant mother's waters break, the amniotic sac tears. The amniotic fluid contained within the sac then begins to leak out via the cervix and vagina. Most women's waters break toward the end of the first stage of labor. Once the waters have broken, the maternity unit should be contacted.6
At times, there may be conditions which cause a woman to have more or less than normal amounts of amniotic fluid. Oligohydramnios is the presence of too little amniotic fluid, whereas polyhydramnios - also referred to as hydramnios or amniotic fluid disorder - is the presence of excess fluid.
What is oligohydramnios?
Low levels of amniotic fluid, referred to as oligohydramnios, is present in 4% of all pregnancies and 12% of post dates pregnancies.3
Oligohydramnios is present when the amniotic fluid index (AFI) seen on ultrasound measures less than 5 cm (a normal index is 5-25cm) and the maximum vertical pocket (MVP) is less than 2 cm.3
This may be evident in cases of leaking fluid from a tear in the amniotic membranes, measuring small for a certain stage of pregnancy or if the fetus is not moving as much as it would be expected to.
Oligohydramnios may also occur in women with a history of any of the following medical conditions:3
- Prior growth-restricted pregnancies
- Chronic high blood pressure (hypertension)
- Problems with the placenta - abruption, for example
- Multiple pregnancies - twins or triplets, for example
- Birth defects - kidney abnormalities, for example
- Pregnant past the due date.
Oligohydramnios can happen during any trimester but is a more concerning problem during the first 6 months of pregnancy. During that time, there is a higher risk of birth defects, miscarriage, premature birth or stillbirth.3
If levels of amniotic fluid are too low, the baby may need to be delivered by Cesarean section to protect it and the mother.
When oligohydramnios is present in the last trimester, the risks include slow fetal growth, labor complications (see below) and a need to deliver via Cesarean section (C-section).3
In the presence of oligohydramnios, the remainder of the pregnancy will be monitored closely to ensure the baby is developing normally. Doctors may utilize the following tests:
- Nonstress tests: a check of the baby's heartbeat when it is resting and when it is moving
- Biophysical profiling: an ultrasound scan to observe the baby's movements, muscle tone, breathing and amniotic fluid levels; this test may be followed up with a nonstress test
- Fetal kick counts: timing how long it takes for the baby to kick a certain number of times
- Doppler studies: using sound waves to check the flow of blood in the baby.
In some cases, doctors may decide that labor will need to be induced, in order to protect the mother or the child. Amnioinfusion (the infusion of saline into the uterus), increasing maternal fluids and bed rest may also be necessary.3
Risk of labor complications is common due to the risk of umbilical cord compression and may require amnioinfusion during labor; in some cases, a C-section may be warranted.3
What is polyhydramnios?
The presence of excess levels of amniotic fluid is referred to as polyhydramnios and occurs in 1% of all pregnancies. Polyhydramnios is present when the AFI is more than 24 cm and the MVP measures more than 8cm.7
Polyhydramnios can be caused by the following fetal disorders:4,7
- Gastrointestinal disorders - examples include duodenal or esophageal atresia, gastroschisis and diaphragmatic hernia
- Brain or nervous system disorders - anencephaly or myotonic dystrophy, for example
- Achondroplasia - a bone growth disorder
- Fetal heart rate problems
- Beckwith-Wiedemann syndrome - a congenital growth disorder
- Fetal lung abnormalities
- Hydrops fetalis - a condition in which an abnormal level of water builds up inside multiple body areas of a fetus
- Twin-to-twin transfusion syndrome, where one child gets more blood flow than the other
- Mismatched blood between mother and child - Rh incompatibility or Kell diseases, for example
- Poorly controlled maternal diabetes.
Too much fluid can also be produced during multiple pregnancies, when the mother is carrying more than one fetus.4
Maternal symptoms can include abdominal pain and difficulty breathing due to the enlargement of the uterus. This condition can also cause complications such as premature labor, premature rupture of membranes, placental abruption, stillbirth, postpartum hemorrhage and fetal malposition.7
Testing for maternal diabetes may be recommended and frequent ultrasounds will be obtained to monitor the levels of amniotic fluid in the uterus.7
In mild cases of polyhydramnios, the problem typically resolves without treatment. In more severe cases of polyhydramnios, fluid may need to be reduced with either amniocentesis or a medication called indomethacin; this reduces the amount of urine the baby produces.4,7
Speak with your health care provider with additional questions or to discuss your personal risks for developing abnormal levels of amniotic fluid.