Low amniotic fluid is mainly associated with decreased production of amniotic fluid or increased leakage of amniotic fluid. Some of the causes of low amniotic fluid are unknown, and the common causes are: 1. Fetal malformations: Many congenital malformations, especially urinary system malformations, are associated with low amniotic fluid, such as congenital renal agenesis, renal dysplasia, polycystic kidney and urethral stenosis or atresia. The above malformations lead to reduced urine production or cannot be generated, the generated urine cannot be discharged or discharged reduced, anuria or oliguria, resulting in decreased amniotic fluid production, normal amniotic fluid absorption and finally hypohydramnios. 2, placental insufficiency: the placenta is the organ of material exchange between the fetus and the mother, and decreased placental function can lead to decreased fetal blood volume and decreased fetal kidney blood supply, finally resulting in decreased fetal urine production. 3, the role of drugs: many drugs can cause low amniotic fluid, common non-steroidal antipyretic analgesics and angiotensin-converting enzyme inhibitors two types.