What is the Amniotic Sac? - Definition & Function

Instructor: Adrianne Baron

Adrianne has taught high school and college biology and has a master's degree in cancer biology.

This lesson is going to discuss what the amniotic sac is and the many functions it serves inside of the amniotic egg. We will discuss how the functions are carried out in the egg.

What is an Amniotic Sac?

Pregnant women go through their day-to-day lives running errands, working, driving around and doing other activities. Even with all the actions that may include bending, stretching, suddenly stopping and bumping into things, the fetus developing inside remains safe and unbothered. Why?

The amniotic sac protects the baby while the mom runs
Picture of a pregnant runner

The reason for these actions not disturbing the fetus is the amniotic sac. This is a membrane filled with fluid that acts as a cushion to protect the developing fetus. You might hear people refer to the amniotic sac as the 'bag of waters'. This reference is pretty accurate since the fluid in the amniotic sac is 98% water and 2% cells from the baby and salts. The amniotic sac begins to develop from the point of conception and is completely formed within the first 3 weeks of pregnancy.

The amniotic fluid, or fluid within the amniotic sac, is the reason why the fetus doesn't get rammed into any other internal structures of the mother's body every time she starts and stops. This amniotic sac does more than just protect the developing fetus. There are a few other purposes that we are going to look at now.


Besides protecting the fetus from physical harm, the amniotic fluid is a great lubricant for the fetus' body parts. Think about this for a minute. An individual fertilized egg develops into a baby in just 9 months. That is very quick to develop every part of the body. As new body parts are formed, they are close together and could rub each other frequently. Amniotic fluid helps to make sure the body parts don't damage each other or grow together by allowing them to slide past each other easily.

Developing fetus inside the amniotic sac

The amniotic fluid is maintained at normal body temperature which is key to keeping the fetus warm. We all know how quickly water can go from comfortably warm to rather cold. Think about the last time you sat in a bath for an extended period of time without adding more warm water. It took a lot less time than nine months to get cold! The amniotic fluid is continuously made and supplied to the amniotic sac to keep the temperature stable at normal body temperature.

Thanks to the amniotic sac, fetuses are able to move around to work out their muscles and develop stronger bones. This can be likened to taking water aerobics. People get in the water to make it easier to move around, to add resistance and therefore increase muscle strength. That's what the little baby athlete is doing in the amniotic sac!

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