The primary function of amniotic fluid is to cushion and protect the fetus within the amniotic sac. It acts as a shock absorber that safeguards the fetus from external mechanical trauma. Additionally, it allows for fetal movements, which are crucial for musculoskeletal development. Amniotic fluid also maintains a stable temperature environment and prevents adherence of the fetus to the amniotic membrane. These protective roles are essential for normal fetal growth and development throughout gestation.

Reference: Gray's Anatomy, Standring S., 42nd Edition.