What Is Amniotic Fluid? – Composition, Function, And Levels

What Is Amniotic Fluid

Amniotic fluid is the clear, slightly yellowish liquid that surrounds the unborn baby (fetus) during pregnancy. Usually, it is found within the first 12 days of conception.

During pregnancy, your growing baby is cushioned inside a fluid-filled bag (amniotic sac) in your uterus (womb). The wall of the amniotic sac is made up of two membranes: chorion and amnion. These membranes keep your baby safely sealed in the sac.


Composition Of Amniotic Fluid

Initially, amniotic fluid mainly comprises water with electrolytes, but around the 12-14th week of pregnancy, the carbohydrates, proteins, lipids, phospholipids, and urea aid fetal growth.1

What Happens When Your Water Breaks

The fluid serves to facilitate the exchange of nutrients, water, and biochemical products between the mother and fetus. Before or during labor, the amniotic sac breaks and the fluid drains out through the vagina. This is commonly known as your water breaking.


If your water breaks before your labor starts, you will notice either of these:

  • A slow trickle from your vagina
  • A sudden gush of water that you can’t control

Without the amniotic fluid, your baby is no longer protected and is at a risk of infection. So contact your midwife or the hospital for advice ASAP in such cases.


How Does The Amniotic Fluid Help?

The amniotic fluid is responsible for the following:

  • 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 the use of muscles of these systems.
  • 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: The 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: The fluid in the uterus prevents the umbilical cord from being compressed. This cord transports food and oxygen from the placenta to the growing fetus.

Low Amniotic Fluid Levels

As discussed earlier, the amniotic fluid is vital for healthy fetal development. However, if the amount of amniotic fluid inside the uterus is too little or too great, complications can occur.


Your baby regularly swallows amniotic fluid and passes it out as urine. So the amount of fluid in the amniotic sac normally rises and falls every day.

The amniotic fluid level increases as your pregnancy progresses:

  • You start off with just a few milliliters but will have between 800 ml (1.4 pints) and 1,000 ml (1.6 pints) of the fluid by the time you’re about 36 weeks pregnant.
  • From 38 weeks onward, the fluid gradually begins to reduce, until you’re ready to give birth.

How To Recognize Low Amniotic Fluid Levels

A very low level of amniotic fluid surrounding your baby is called oligohydramnios. Your doctor or midwife may suspect that your amniotic fluid levels are low if your tummy or baby appears smaller than expected (small for dates).

Your midwife will pick this up when she checks the size of your bump with a tape measure at one of your antenatal appointments. If your midwife has concerns, she will recommend that you have an ultrasound scan.


This is how an ultrasound works:

  1. The sonographer (the person in charge of doing the scan) will check your fluid levels by looking at the whole amniotic sac.
  2. The depth of the largest pockets of amniotic fluid in four sections of your uterus is measured here. Your score on the amniotic fluid index (AFI) is found by adding the four measurements.
  3. Another measurement may be taken by checking the deepest pocket of the amniotic fluid.
  4. Your levels are compared with the established normal levels of fluid for each week of pregnancy. In general, an AFI of 5 cm (2 in.) or less, or a deepest pocket measurement of less than 2 cm (0.8 in.), is considered to be low.

What Are The Effects Of Low Amniotic Fluid?

  • If you have low levels of amniotic fluid during your first trimester and in the early part of your second trimester, there is an increased chance of having a miscarriage or stillborn baby.
  • You might have labor complications; your baby may be in a bottom-down (breech) position and not have enough room to turn into a head-down position.
  • Your water may break early, and you may go into premature labor. Doctors will weigh the risks of this happening against the risk of infection if your baby stays in your uterus. You may be given antibiotics to prevent infections that can affect your baby. Once you’re in labor, there’s a higher chance of your baby becoming distressed.
  • The umbilical cord might get accidentally squashed by your baby while being born and you may need to give birth via C-section.

High Amniotic Fluid Levels

Too much fluid is called polyhydramnios, or hydramnios. It can be caused by the following fetal disorders:

  • Gastrointestinal disorders: For example, duodenal or esophageal atresia, gastroschisis, and diaphragmatic hernia
  • Brain or nervous system disorders: For example, anencephaly or myotonic dystrophy
  • Achondroplasia: A bone growth disorder
  • Fetal heart rate problems
  • Infection
  • Beckwith-Wiedemann syndrome: A congenital growth disorder
  • Fetal lung abnormalities
  • Hydrops fetalis: Where an abnormal level of water builds up inside multiple body areas of the fetus
  • Twin-to-twin transfusion syndrome: Where one child gets more blood flow than the other
  • Mismatched blood between mother and child: For example, Rh incompatibility or Kell diseases
  • Poorly controlled maternal diabetes
  • Multiple pregnancies: When the mother is carrying more than one fetus

How To Recognize High Amniotic Fluid Levels

Maternal symptoms can include abdominal pain and difficulty breathing due to the enlargement of the uterus. This condition can also cause other complications:

  • Premature labor
  • Premature rupture of membranes
  • Placental abruption
  • Stillbirth
  • Postpartum hemorrhage
  • Fetal malposition
  • Treatments

To monitor the levels of amniotic fluid, testing for maternal diabetes may be recommended and frequent ultrasounds obtained. In mild cases of polyhydramnios, the problem typically resolves without treatment. In more severe cases of polyhydramnios, the fluid may need to be reduced with either amniocentesis or a medication called indomethacin; this reduces the amount of urine the baby produces.

It can be worrying to be told you have low amniotic fluid. However, there’s not much you can do apart from staying well hydrated, eating healthily, and resting. Try not to let this worry stop you from sleeping. Sleep is very important for you and your baby.