I. Source of amniotic fluid The membrane of the amniotic epithelium belongs to the spinous mosaic structure, which is a porous tissue layer that allows the passage of small molecules and water. According to the analysis of amniotic fluid composition in early pregnancy, it is very similar to the dialysis of maternal serum. It is likely to be a dialysate of maternal serum passing through the fetal membrane into the amniotic cavity. By late gestation the amniotic fluid composition becomes a hypotonic solution, so that the net amount of water passing in this way in late pregnancy is minimal. The umbilical cord vessels are surrounded by a large amount of hyaluronidase loose connective tissue, a structure that facilitates the absorption and conversion of water. Because of the small surface area of the umbilical cord, very little water exchange can take place. In 1970, Parmley et al. found that in early pregnancy, water can seep through the fetal skin into the amniotic cavity, and the fetal subcutaneous capillary bed is the site of water and solute exchange. In a sense, amniotic fluid is essentially the epithelium of the fetal extracellular fluid. After 24 weeks of gestation, the keratinized layer of fetal skin is formed, and water and general solutes cannot pass through. However, small molecular weight highly fat-soluble substances such as O2 and CO2 can still pass through the skin. Fetal lungs and respiration are involved in the production of amniotic fluid. In recent years, it has become clear that after 24 gestational weeks, lung type II cells can synthesize surface active substances. These substances can also be measured in amniotic fluid, thus confirming that the lungs are indeed involved in amniotic fluid production. In 1976, Pritchard et al. calculated that in late pregnancy, 600-800 ml of amniotic fluid tidal volume passes through the fetal lungs daily due to the active respiratory actions of the fetus. Thus, a large amount of hypotonic amniotic fluid enters the alveoli and passes through the capillary bed of the alveoli, allowing the recovery of a considerable amount of water per day. Many experimental and clinical data have shown that the fetus can swallow amniotic fluid. The use of the fetus to swallow amniotic fluid, which is absorbed and transported by the gastrointestinal tract, is an important way of regulating amniotic fluid. The fetal kidney is excreted at 11-14 weeks of gestation. At 14 weeks of gestation, urine is present in the fetal bladder. Fetal urine is a hypotonic solution, so in late gestation, the osmotic pressure of amniotic fluid is reduced by the addition of large amounts of hypotonic fetal urine, but uric acid, urea and creatinine do not increase accordingly. The fetal surface of the placenta is also the site of water and solute conversion between the fetus and amniotic fluid, and water, Na+, Cl-, as well as urea and creatinine pass easily through its surface. In different gestation periods, the sources of amniotic fluid are different: 1, early pregnancy: amniotic fluid is mainly the dialysate of maternal serum entering the amniotic cavity through the fetal membrane; after the formation of fetal blood circulation, water and small molecules can pass through the fetal skin that has not yet been keratinized, which is also a source of amniotic fluid. 2.After mid-term pregnancy: fetal urinary tuck excretion A amniotic cavity, so that the osmotic pressure of amniotic fluid gradually decreases, and the amount of uric acid and creatinine gradually increases; on the other hand, the fetus swallows amniotic fluid to obtain a balance of volume, and at this time the fetal skin gradually keratinizes and is no longer a source of amniotic fluid. 3, late pregnancy: the operation of amniotic fluid in addition to the excretion of fetal urine and amniotic fluid swallowing, fetal lung absorption of amniotic fluid is also a way of operation; in addition, the placenta fetal surface of the amniotic membrane is the exchange site of water and small molecule solutes, but its volume is small. The umbilical cord and amniotic surface is not an important source of amniotic fluid. The amount of amniotic fluid increases with the gestation period, at 8 weeks of gestation, there are about 5-10ml of amniotic fluid, at 11-15 weeks of gestation, the average weekly increase of 25ml. 16-28 weeks, the average weekly increase of about 50ml, to 38 weeks of gestation, the total amount of about 1000ml, and then gradually decrease. The function of amniotic fluid is to protect the fetus (1) the fetus can move freely in the amniotic fluid without being squeezed to prevent fetal malformation and fetal limb adhesion; (2) to maintain constant temperature and pressure in the amniotic cavity to reduce fetal damage caused by external forces; (3) amniotic fluid has antibacterial effects, mainly on Escherichia coli and Staphylococcus aureus; (4) the right amount of amniotic fluid to avoid direct pressure on the umbilical cord by the uterine muscle wall or the fetus (5) It is conducive to fetal fluid balance, and if the fetus has too much water in the body, it can be discharged into the amniotic fluid by way of fetal urine; (6) During contractions in labor, especially at the beginning of the first stage of labor, the amniotic fluid is directly subjected to contraction pressure, which can make the pressure evenly distributed and avoid local pressure on the fetus, and it can also keep the amniotic cavity under certain tension to support the placenta attached to the uterine wall and prevent premature abruption of the placenta. For breech position, an intact amniotic sac can prevent the occurrence of umbilical cord prolapse. After masking, the amniotic fluid can lubricate the birth canal to facilitate the delivery of the fetus. (2) After delivery, the anterior amniotic fluid sac dilates the cervical opening and vagina; (3) After rupture of membranes, the amniotic fluid rinses the vagina to reduce the chance of infection.