What is the Amnion?

Mary McMahon

The amnion is a thin sac which forms around a developing fetus. Birds, mammals, and reptiles all produce amnions in their fetal development. The amnion is designed to protect the fetus as it grows, to reduce the risk of injuries which could interfere with fetal development or lead to fetal death. The size of the amnion varies during the course of development as a consequence of the fact that the amount of room needed by the fetus changes as it gets larger.

Birds produce amnions in their fetal development.
Birds produce amnions in their fetal development.

In the very early stages of development, the sac is in physical contact with the fetus. As the fetus starts to grow, the sac fills with fluid. The fluid pushes the sides of the amnion away from the fetus and keeps the fetus in a state of suspension. The sac itself is surrounded with a tough other sac known as the chorion which provides additional protection and plays a role in placental circulation in mammals.

Reptiles produce amnions in their fetal development.
Reptiles produce amnions in their fetal development.

By keeping the fetus suspended in fluid, the amnion helps to insulate the fetus from shock. Without the shock absorption provided by the fluid, the fetus could experience problems during development. This makes the sac an artificial version of the aquatic environment in which many organisms develop; since it seems likely that life originated in water, it makes sense that such environments are highly suited to fetal development.

In mammals, when it is time for the fetus to be delivered, the membranes which surround it rupture, causing a release of amniotic fluid. Once the amnion is ruptured, the clock starts ticking on delivery, and the mother can be at risk of infection. It is also possible for amniotic fluid leaks to develop during pregnancy, in which case it can indicate that the mother is experiencing a premature rupturing of membranes (PROM).

In most cases, this structure does a very good job of protecting the developing embryo from shock, infections, and toxins. However, it is possible for the fetus to be exposed to dangerous through the placenta in the case of placental mammals. This is why gestating mammals need to be careful about the environments they frequent and the foods they eat, to reduce the risk of transmitting something harmful through the placenta. In insects and birds, weakening in the walls of the egg can also contribute to problems with fetal development. Likewise, the developing fetus can absorb toxins which may have been stored along with the nutrients intended to sustain it.

The growing fetus is protected by the amnion.
The growing fetus is protected by the amnion.

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Discussion Comments


@Clairdelune - I learned this in nursing school, so let me give it a try. There are several possible reasons why amniotic fluids might become low. The baby's kidneys or the urinary tract may have problems, and there isn't enough urination.Sometimes, there is a tear in the membrane and fluid leaks out. Low fluid may happen when the pregnancy goes over time - past 42 weeks. Sometimes the level is affected by a mother who has diabetes or high blood pressure.

What can be done to treat this condition?

Sometimes fluid can be infused into the fluid sack. More fluid can be injected. Mothers can be hydrated with IV infusions.

If there is low fluid during the first half of the pregnancy, the baby may be deformed because of the pressure of a too small sack. There is a higher chance of a premature or still birth.


The amnionic fluid is incredibly important to the developing baby inside the womb. It helps to protect the baby and aids in the development of muscles,legs and arms, and the digestive system. The first of the fluid comes from the mother and after that the baby urinates to fill up the sack. Later, there is enough fluid that the baby can exercise in the fluid. He/she begins to swallow the fluid and to breathe.

Sometimes the amount of fluid is too low. If this happens in the first part of pregnancy, it can lead to serious problems. The fluid helps in the development of muscles, lungs, and the digestive system. I don't know what the cause or the treatment of this condition is. Does anyone know?

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