Amniotic fluid was once considered to be a stagnant pool, approximately circulating with a turnover time of one day. Adequate amniotic fluid volume is maintained by a balance of fetal fluid production (lung liquid and urine) and resorption (swallowing and intramembranous flow). Even though different hypotheses have been advanced on the mechanisms regulating this turnover, the inflow and outflow mechanism that keeps amniotic fluid volume within the normal range is not entirely clear. Regulatory mechanisms act at three levels: placental control of water and solute transfer; regulation of inflows and outflows from the fetus; and, maternal effect on fetal fluid balance. Amniotic fluid is 98-99% water. The chemical composition of its substances varies with gestational age. When fetal urine begins to enter the amniotic sac, amniotic osmolarity decreases slightly compared with fetal blood. After keratinization of the fetal skin, amniotic fluid osmolarity decreases further with advancing gestational age. The low amniotic fluid osmolarity, which is produced by the inflow of markedly hypotonic fetal urine, provides a large potential osmotic force for the outward flow of water across the intramembranous and transmembranous pathways. Within certain limits, amniotic fluid mirrors the metabolic status of the fetoplacental unit; for that reason, a study of its components and their respective variations in the different weeks of pregnancy provides useful indications, both for a correct assessment of fetal maturation and for an evaluation of kidney function parameters and placental insufficiency.