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What Is Amniotic Fluid? - Levels, Function & Composition

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Kelly Robson

Kelly has taught High School Science and Applied Communications. She holds an Education Specialist Degree in Ed. Leadership.

During pregnancy, the fetus develops in a special fluid that helps to keep it protected. The amniotic fluid also aids in the development of the fetus. This lesson covers the make-up and functions of amniotic fluid.

We also recommend watching Amniotic Fluid, The Amnion, and the Yolk Sac and The Fluid Mosaic Model of the Cell Membrane

The Womb

During pregnancy, the fetal baby develops in the uterus. After about 12 days of pregnancy, the amniotic sac forms inside the uterus. The amniotic sac will hold the baby and the amniotic fluid for the remainder of the pregnancy.

Amniotic fluid fills the amniotic sac. At first, the fluid is mainly just water from the mother's body. (This may explain why women become so thirsty for the first trimester.) About 98% of the amniotic fluid is water, and the remaining 2% is salt and cells from the baby.

10-week old fetus floating in an amniotic sac filled with amniotic fluid.
Amniotic Sac and Fetus

After about 4 months or 20 weeks into the pregnancy, the fetus will develop kidneys. This will allow for the fetus to begin urinating. The fetus will be able to swallow and digest the amniotic fluid. Eventually, the amniotic fluid will become mainly urine from the fetus. Yucky, but it is perfectly okay for the baby. Besides urine, the fluid will also have some nutrients, hormones, and antibodies in it.

Protection and Development

The amniotic fluid has many different functions. Most of the functions fall under one of two categories: protection and development. As the baby floats in the fluid, the baby can move around while at the same time being protected from blows to the uterus.

Functions:

  • The fluid allows the baby to move around while it is developing. This movement allows for bone and muscle development.
  • The fetus breathes the fluid in and out, allowing it to practice breathing and to aid in lung development.
  • The fluid helps to keep the fetus nice and warm by keeping heat in. It also helps to keep the temperature consistent.
  • The fluid offers protection from any blows that may come to the uterus area. It may cushion the blow if the mother falls.
  • When the fetus swallows the amniotic fluid, it is practicing using and developing the digestive system.
  • It keeps the umbilical cord from being squeezed to hard. A big squeeze could cut off the nutritional supply from the mother to the baby.
  • The amniotic fluid also acts as a lubricant. The fetus's growing body parts are very fragile and could grow together, such as in the case of webbed fingers or toes.

Too Much, Too Little, Just Right

The amniotic fluid fills the amniotic sac throughout the entire pregnancy. At about the 34-week mark, the fluid is at its fullest - about 800mL. As the fetus grows larger in the last weeks of pregnancy, the fluid slowly begins to go down to around 600mL at the full-term mark.

Sometimes, there may not be enough fluid. This is called oligohydramnios. The low amount of fluid can be caused by pregnancies that go later, ruptured membranes, placental dysfunction, or fetal abnormalities. On the other hand, polyhydramnios is when there is too much amniotic fluid. This can be caused by multiple fetuses in the womb (twins, triplets, etc.), congenital anomalies, and gestational diabetes.

Amniotic fluid can be tested to reveal information. The fluid is removed through a process called amniocentesis. An amniocentesis can show the gender, health and development of the fetus.

Summary

The amniotic sac is inside the uterus. It holds the developing fetus and amniotic fluid. The amniotic fluid is there to protect the fetus and aid in its development. Throughout the pregnancy, the fluid contents change from water from the mother to urine from the fetus. The fluid is also used to help the fetus practice using its lungs and digestive system, and it also acts a lubricant to the fragile, developing parts. The amount of fluid is important to help make sure the fetus is healthy and protected.

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