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.

The Womb

During pregnancy, the fetal baby develops in the uterus. After about 12 days of pregnancy, an 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.

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 or 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.

Amniotic Fluid 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 also 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 too 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’ growing body parts are very fragile and could grow together, such as in the case of webbed fingers or toes.

Levels of Amniotic Fluid

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

Sometimes, there are different levels of amniotic fluid, which can be a problem. Oligohydramnios is a situation where there is not enough amniotic fluid. 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, such as twins or triplets, congenital anomalies, and gestational diabetes.

Amniotic fluid can be tested to reveal information. Amniocentesis is a process where amniotic fluid is removed from the amniotic sac for testing. An amniocentesis can show the gender, health, and development of the fetus.

Lesson Summary

The amniotic sac develops inside the uterus and it holds the growing fetus and the amniotic fluid. The amniotic fluid is there to protect the fetus and aid in its development, and over the course of 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.

Key Terms


Key Terms Definitions
Uterus where the fetal baby develops during pregnancy
Amniotic sac the area for a fetus to form inside the uterus
Amniotic fluid fills the amniotic sac with 98% water and 2% salt and cells
Oligohydramnios a situation where there is not enough amniotic fluid
Polyhydramnios when there is too much amniotic fluid
Amniocentesis process where amniotic fluid is removed from the amniotic sac for testing

Learning Outcomes

You should be able to accomplish the following after finishing this lesson:

  • Describe amniotic fluid
  • Identify where this fluid is located and why
  • Illustrate when serious situations arise due to too much or not enough amniotic fluid