What Is Amniotic Fluid in Pregnancy?

Medically Reviewed on 8/5/2021
Amniotic fluid in pregnacny
Amniotic fluid is initially created from the mother’s plasma in a sac in the uterus, known as an amniotic sac, that holds the fetus. This fluid is vital for a developing baby for several different reasons. Learn those reasons here.

A pregnant woman’s uterus contains an amniotic sac, which holds amniotic fluid and the growing fetus. This fluid is clear or straw-colored and is initially created from the mother’s plasma (the pale-yellow liquid component of blood). The sac is transparent and tough.

The amniotic fluid contents are vital for a healthy, developing baby as it helps to cushion and protect the fetus. Amniotic fluid is 98 percent water and two percent salts and cells from the baby. After the fourth month, the fetus starts to make its contribution to the amniotic fluid from its urine.

  • Until the 12th week of pregnancy, amniotic fluid is mostly water with electrolytes.
  • In the second trimester, proteins, carbohydrates, lipids and urea are present, which aid in the growth of the baby.
  • From around 16 weeks of pregnancy, the baby’s kidneys begin to function, and fetal urine becomes the main source of amniotic fluid.
  • The other source of amniotic fluid is fluid excreted from the baby’s lungs.

The amniotic fluid or bag of water surrounds and protects the baby in the uterus, providing a protective barrier from the outside environment. Once the bag of water breaks, bacteria can enter the uterus through the mother’s vagina, which could cause an infection in the baby, the mother or both. Therefore, do not douche or have intercourse after the bag of water breaks.

What are the benefits of amniotic fluid?

Amniotic fluid plays a very important role in the baby’s growth and development. It also acts as a buffer to protect the fetus from external forces and injury.

The amniotic fluid is important for several reasons, including:

  • Water levels: Allows baby ample space for movement.
  • Temperature: Maintains a constant temperature for creating a healthy baby.
  • Aids healthy growth: Enables correct bone, muscle and organ development. The fetus breathes the fluid in and out, allowing it to practice breathing and aid in lung development.
  • Nutrient exchange: Enables the exchange of vital nutrients between the mother and the baby, including oxygen, via the umbilical cord. If fluid levels change, the health of the umbilical cord can be compromised.

Additionally, the amniotic fluid provides the following benefits:

  • It acts like a liquid shock absorber for the baby by distributing any force that may push on the mother’s uterus.
  • It contains some of the baby’s cells and other substances. Samples of amniotic fluid may be taken to check the health of the baby and test for genetic changes that may be a sign of a genetic disease or condition.
  • It also may be used to learn the sex of the baby (in case of emergency).

After the fourth month, the baby’s urine in the amniotic sac is the main liquid and is completely harmless. The baby swallows amniotic fluid, which then passes through their digestive system, into their kidneys and back to the amniotic sac as urine.

Doctors can tell by the amount of amniotic fluid whether the baby has difficulty with their swallowing reflex. By the time the baby is born, they consume up to 13 ounces of amniotic fluid a day.


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Does the amount of amniotic fluid in the sac matter?

During 34 weeks of pregnancy, the amniotic sac contains the most fluid, averaging 800 mL. When the baby is full-term at 40 weeks, there is approximately 600 mL of amniotic fluid. 

Conditions that affect the amount of amniotic fluid are:

  • Polyhydramnios
    • Having too much amniotic fluid in the sac is called polyhydramnios.
    • This may occur if the baby in the womb does not swallow and absorb the fluid in normal amounts.
    • The common causes of polyhydramnios include developmental problems involving the baby’s gastrointestinal or central nervous system, fetal anemia where the baby is lacking red blood cells and hydrops fetalis, which cause an abnormal amount of fluid in the sac.
    • Complications that can arise from this condition include mothers going into labor early, the placenta detaching from the uterus or wrong positioning of the baby.
    • If the condition of the baby is more severe, the doctor may treat patients with medication that controls the baby’s urine, draining amniotic fluid or inducing labor.
  • Oligohydramnios
    • A sac with low amniotic fluid is called oligohydramnios, which is associated with complications, such as maternal high blood pressure, during pregnancy or post-term pregnancy.
    • Some of the risks associated with this condition may include the underdevelopment of the baby’s muscles, limbs, lungs and digestive system, as well as preterm birth and labor complications.
    • According to the American Pregnancy Association (2019), if the mother is not full-term yet, the doctor or specialist will run tests and monitor the baby’s activity closely. If the mother is close to full-term, it is recommended that early delivery will be the safest for both the mother and the baby. Another treatment option is amniocentesis, which is an injection of fluid before the delivery to compensate for the low amniotic fluid.

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Medically Reviewed on 8/5/2021
March of Dimes. Amniotic fluid. https://www.marchofdimes.org/pregnancy/amniotic-fluid.aspx

Fitzsimmons ED, Bajaj T. Embryology, Amniotic Fluid. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2021. https://www.ncbi.nlm.nih.gov/books/NBK541089/

Griffin RM. Amniotic Fluid Volume Assessment. WebMD. https://www.webmd.com/baby/amniotic-fluid-volume-assessment