In early gestation, amniotic fluid is maternal serum that passes through the fetal membranes into the amniotic cavity as a dialysis fluid. By mid-gestation, the main component of amniotic fluid is fetal urine. In late gestation, fetal lung fluid is also involved in the production of amniotic fluid. Every day, the fetal alveoli secrete fluid into the amniotic cavity, and the amniotic membrane, umbilical cord, Huatong glue, and fetal skin exudate, are also involved in the production of amniotic fluid. Amniotic fluid is also mixed with shed epithelial cells, fine hair, hair, a small amount of leukocytes, albumin, urate, etc. Amniotic fluid contains a large number of hormones and enzymes. Amniotic fluid is constantly exchanged in the amniotic cavity to maintain the relative balance of amniotic fluid volume.