When the amount of amniotic fluid is less than 300 ml, it is called oligohydramnios. This is a traditional concept, but it is difficult to accurately estimate the total amount of amniotic fluid regardless of vaginal delivery or cesarean section. In recent years, due to the application of imaging, especially the application of obstetric B-ultrasound technology, the diagnosis of oligohydramnios has become more accurate, and the unified diagnostic criteria have gradually been used clinically. The standard for B-ultrasound diagnosis of oligohydramnios is amniotic fluid index (AFI) <5cm or maximum amniotic fluid pool depth <2cm. In the past, the incidence of oligohydramnios was very low, only 1:4000. In recent years, due to the improvement and popularization of prenatal monitoring of high-risk pregnancies and B-ultrasound technology, the incidence of oligohydramnios has tended to increase. The incidence of oligohydramnios is as high as 5% in some reports.