With the widespread development of ultrasound during pregnancy and the increased experience of sonographers, the detection rate of hydrocele in the fetal period is now about 1-5% of all pregnancies. Definition: Fetal hydronephrosis is the dilatation of the collecting system of the fetal kidney and is by far the most common fetal abnormality detected by prenatal ultrasound. It can usually be detected by ultrasound as early as 18-20 weeks of gestation. Diagnostic criteria: The width of the anterior and posterior diameters of the renal pelvis during ultrasound examination of the kidneys is used to evaluate the severity of hydrocele: a separation of the renal pelvis of less than 5 mm is considered normal; 5-10 mm is considered suspicious and requires ultrasound follow-up; greater than 10 mm is diagnosed as dilated renal pelvis. There is also another criterion: the degree of renal pelvis separation is considered together with the number of weeks of gestation: greater than 4 mm before 24 weeks of gestation or greater than 10 mm after 24 weeks of gestation, but the sensitivity and positive rate have a certain diagnostic error rate. Or the standard is greater than 4 mm before 33 weeks of gestation and greater than 7 mm after 33 weeks of gestation, but the false-positive rate is higher. Diagnostic tools: Ultrasonography has unique advantages: no radiation, dynamic observation and easy operation. Any fetal hydronephrosis found in the middle of pregnancy needs to be reviewed again in late pregnancy to confirm any progression. Among the special cases: once an enlarged fetal bladder, bilateral hydronephrosis, and low amniotic fluid are found to be highly suggestive of male posterior urethral valve signs, ultrasound needs to be repeated every four weeks, and prenatal intervention may be considered if amniotic fluid continues to decrease. It should be noted that ultrasound cannot determine renal function, and the accuracy of ultrasound results is related to the experience of the operator, sensitivity of the instrument, and the different states of the fetus. Therefore, it is recommended that if dilated renal pelvis is detected during pregnancy, follow-up review by the same physician with experience is preferable. Prognosis: According to numerous studies, most fetal hydronephrosis resolves spontaneously with fetal development; only a very small percentage progresses rapidly and requires prenatal intervention. If hydrocele is found in the fetus, it should be reviewed by ultrasound one week after birth and about 1/3 of the children may return to normal after birth.