What Is Amniotic Fluid?

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Amniotic fluid is a clear to slightly yellow liquid that cushions a fetus within the amniotic sac. The unborn baby floats in amniotic fluid for the duration of a pregnancy.

The amniotic fluid constantly circulates as the fetus swallows or "inhales" the amniotic fluid before releasing it by urinating.

At around 34 weeks gestation, roughly 800 milliliters (mL) of amniotic fluid surround the baby. At a full-term pregnancy at 40 weeks gestation, roughly 600 mL of amniotic fluid remain.

woman getting ultrasound to check amniotic fluid levels

Kemal Yildirim / Getty Images

Development

Amniotic fluid is present at the formation of the amniotic sac. This is a thin-walled sac that contains the fetus during pregnancy.

The development of amniotic fluid is broken into two stages:

  • Early gestation: In the period from fertilization to eight weeks, the amniotic fluid is composed mainly of water from the mother. At 10 weeks, the fetus produces urine, which enters the amniotic sac.
  • Late gestation: In the second and third trimesters, the amniotic sac expands and amniotic fluid is mainly composed of fetal urine. Alongside this, lung secretions from the fetus, as well as gastrointestinal secretion and excretions from the umbilical cord and placental surface, also contribute to the content of amniotic fluid.

Amniotic fluid is made up of 98% water and electrolytes, along with peptides, carbohydrates, and signaling molecules. The remaining 2% is made up of lipids and hormones.

Function

Amniotic fluid serves a number of purposes during pregnancy, primarily to protect the fetus from harm. The functions of the amniotic fluid include:

  • Acting as a cushion: This protects the fetus from injury should the mother's abdomen be the subject of trauma or a sudden impact.
  • Protecting the umbilical cord: Amniotic fluid flows between the umbilical cord and fetus.
  • Protecting from infection: Amniotic fluid has antibacterial properties.
  • Containing essential nutrients: These include proteins, electrolytes, immunoglobulins, and vitamins that assist in the development of the fetus.
  • Allowing for the fetus to move: Amniotic fluid also allows the developing fetus to move around in the womb, which in turn allows for proper development and growth of the musculoskeletal system, gastrointestinal system, and pulmonary system.
  • Maintaining temperature: Amniotic fluid assists in keeping a constant steady temperature around the fetus throughout pregnancy, protecting the baby from heat loss.

Complications

Complications related to amniotic fluid can occur when there is too much or too little fluid.

Oligohydramnios

Oligohydramnios occurs when there is too little amniotic fluid surrounding the fetus during pregnancy. This occurs in about 4% of pregnancies.

On average, pregnant people have roughly 0.5 quart to 1 quart (500 to 1000 mL) of amniotic fluid. Too little amniotic fluid can cause problems with the development of the fetus as well as pregnancy complications.

Too little amniotic fluid can lead to abnormal development of the baby's lungs. It may also stop the baby from growing properly. Too little amniotic fluid can put pressure on the umbilical cord, which can prevent the fetus from getting enough oxygen and nutrients.

Oligohydramnios can occur for a number of reasons. Either not enough amniotic fluid is being made, or there are issues causing the amount of amniotic fluid to decrease.

Possible causes include:

  • Poor fetal growth
  • Water breaking before going into labor
  • A pregnancy that extends past the due date
  • Identical twins who share a placenta
  • Birth defects (most commonly kidney and urinary tract issues)

Symptoms of oligohydramnios may vary between women and may also present as symptoms of other conditions. A healthcare provider will be able to provide a conclusive diagnosis.

Possible symptoms include:

  • Not enough amniotic fluid is seen during an ultrasound exam
  • Smaller uterus than expected for the stage of pregnancy
  • Leaking amniotic fluid

Oligohydramnios may be diagnosed following an ultrasound. A Doppler flow study, a special type of ultrasound, may be used to check blood flow through the fetus' kidneys and the placenta.

Treatment for oligohydramnios is focused on continuing the pregnancy for as long as is safe while keeping the mother comfortable.

This may involve:

  • Regular monitoring to see how much amniotic fluid remains
  • Amnioinfusion, where fluid is infused into the amniotic sac. This may be performed during labor if the water has broken, but not outside of labor.
  • If low amniotic fluid presents a risk for the mother and baby, early delivery may be necessary.

Polyhydramnios

Also called hydramnios, polyhydramnios occurs when there is too much amniotic fluid surrounding the fetus. When this happens, the uterus is much larger than normal.

This is a rare condition, occurring in roughly 1% of pregnancies.

Possible symptoms of polyhydramnios include:

  • Constipation
  • Indigestion
  • A feeling of tightness in the stomach
  • Enlargement of the vulva
  • Breathing issues
  • Producing less urine
  • Swollen legs, hips, thighs, ankles, and feet

For most women, the exact cause of polyhydramnios is unknown.

Moderate to severe cases may be caused by:

  • High blood glucose levels
  • Birth defects
  • Placenta issues
  • Heart rate complications in the fetus
  • Infection in the fetus
  • Identical twins with transfusion syndrome
  • Problems with the fetus' stomach

Treatment is often not necessary in mild cases. Additional appointments may be useful to monitor the growth of the uterus.

In severe cases, some of the amniotic fluid is drained from the sac using a large needle. Other options include inducing labor early.

A Word From Verywell

Amniotic fluid surrounds the fetus during pregnancy and serves an important function. It protects the fetus from harm and allows for healthy development of the baby. Complications can occur when there is too much or too little amniotic fluid. If you are concerned about your pregnancy, you should speak with your doctor.

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Article Sources
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  1. MedlinePlus. Amniotic fluid. Updated February 26, 2021.

  2. Johns Hopkins Medicine. Anatomy: fetus in utero.

  3. Mount Sinai. Amniotic fluid. Updated September 25, 2018.

  4. Johns Hopkins Medicine. Oligohydramnios. Updated March 1, 2019.

  5. Cleveland Clinic. Polyhydramnios. Updated January 21, 2019.