The composition of amniotic fluid changes over the course of pregnancy. Early amniotic fluid is mainly a dialysate of maternal serum that enters the amniotic cavity through the fetal membranes, and after mid-gestation, the main component of amniotic fluid is urine excreted by the fetus. In early and mid-gestation, amniotic fluid is a colorless, clear liquid. Early amniotic fluid is mainly dialysate from maternal serum entering the amniotic cavity through the fetal membranes, and partly exudes from the surface of the placenta and the surface of the umbilical cord. By mid-gestation, the major component of amniotic fluid is urine excreted by the fetus. Therefore, about 98% of amniotic fluid is water, and also contains components containing inorganic salts, proteins, glucose, enzymes, fats, and hormones. In late pregnancy, the fetal lungs are also involved in the production of amniotic fluid, and the alveoli secrete fluid into the amniotic cavity, which becomes cloudy and contains urea, uric acid, sebaceous material, hormones, and epithelial cells shed by the fetus. Therefore, amniotic fluid will contain different components at different times of gestation and can be tested to rule out whether the baby has a genetic disorder.