【Objective】 To discuss the antenatal sonographic diagnosis of fetal hydronephrosis and to evaluate the prognosis of them.Then to provide help for clinical intervention. 【Methods】 72 fetuses(91 kidneys)received B ultrasound when they were 18~40 weeks pregnant.The anteroposterior diameter and thickness of parenchyma were noted and compared.They were divided into 3 groups,mild,moderate and severe according to the anteroposterior diameter of pyelectasis, then were followed up visitation at special time. 【Results】 In group of mild hydronephrosis,there were 42 kidneys whose hydronephrosis disappeared,4 eased or had no change and 2 aggravated.However,in the group of moderate hydronephrosis,there were 15 kidneys whose hydronephrosis disappeared,2 eased or have no change and 9 aggravated.There were 8 kidneys with the parenchymal thickness less then 5 mm in this group,and 4 of them were operated.And in the group of severe hydronephrosis,2 kidneys whose hydronephrosis eased or have no change and 15 aggravated.There were 11 kidneys with the parenchymal thickness less then 5 mm in this group,and 5 of them were operated. 【Conclusion】 Most of the hydronephrosis with the anteroposterior diameter of renal pelvis less then 15 mm is a physiological change,which can disappear along with individual development.The fetal hydronephrosis which more then 15 mm and parenchymal thickness being less then 5mm shoud be obsevered periodically.If the hydronephrosis remain stabilized or become more severe,we must pay more attention or peform the operation.