Amniotic fluid is a biological fluid contained in the amniotic sac during gestation and in which the embryo is located. Amniotic fluid is 97% water and is constantly renewed. It also contains cells from the foetus, which will allow prenatal diagnosis through amniocentesis. Amniocentesis is the collection of amniotic fluid and the analysis of the cells collected.

The volume of the amniotic fluid is at all times a balance between the structures producing or allowing its passage into the amniotic cavity (chorion and membranes, skin, respiratory tract, and urinary tract) and those involved in reabsorption (gastrointestinal tract, respiratory tract, amnion-chorion interface on the uterine wall).

Amniotic fluid can be abnormal in quantity during pregnancy. When there is an excess of fluid, it is called hydramnios. On the other hand, when there is a decrease in the amount of fluid, it is called oligohydramnios. Finally, when there is no more amniotic fluid at all, it is called anamnios.