Amniotic fluid is not all baby’s urine, and the composition of amniotic fluid varies at different times of pregnancy. More than 90% of the composition of amniotic fluid is water, with additional minerals, urea, uric acid, creatinine, fetal lipids and fetal epithelial cells. Amniotic fluid undergoes constant fluid changes in the amniotic cavity to keep the amount of amniotic fluid relatively constant. At the same time, amniotic fluid maintains a constant temperature in the uterine cavity so that the metabolic activities of the fetus can be carried out stably, and also prevents the fetus from developing adhesions and malformations. Amniotic fluid in early pregnancy mainly consists of the dialysate of maternal serum entering the amniotic cavity through the fetal membranes, which is a colorless and clarified liquid. In mid-gestation, fetal urine is an important source of amniotic fluid. By late pregnancy, fetal lung fluid is also involved in the production of amniotic fluid. Abnormal changes in amniotic fluid detected at any time of pregnancy require prompt hospitalization.