With advances in ultrasound medicine, some conditions that are not easily detected early prenatally have become much simpler and easier to diagnose today. In obstetrics clinics, there are more and more cases of fetal hydronephrosis detected by ultrasound pregnancy test. So, should we terminate the pregnancy if the fetus has hydronephrosis? How to face it? Normal fetal kidneys can have mild separation of the collecting system with separation diameter up to 6mm, while hydronephrosis is defined as dilated renal pelvis ≥10mm or presence of dilated calyces after gestational age >30 weeks. There are two types of fetal hydronephrosis. One type is reproducible: hydrocele width between 10-16 mm, thicker renal parenchyma between 5-10 mm, and hydrocele disappears after the fetus is born with a change in environment. It is common in fetuses with a large urine filling in the bladder or some cause of ureteral contraction rhythm disorder. The other kind is irreducible: the width of the hydrocele is often greater than 20 mm, the renal parenchyma is 2-3 cm thick, and there may be some anatomical abnormalities in the ureter, such as congenital ureteral stenosis, and there are mostly changes in the secretory function, which should be treated promptly after birth. To sum up, if fetal hydronephrosis is found in pregnancy test, regular follow-up ultrasound is needed to observe the degree of hydronephrosis and changes in renal cortex thickness to determine the developmental changes of hydronephrosis, which is generally less than 20mm, and there is no great damage to renal function. Even if the fetal hydronephrosis is found, do not worry too much and do not need to terminate the pregnancy urgently, it should be rechecked one hour or one week after the ultrasound discovery. If the width of fetal hydronephrosis is less than 16mm or the thickness of renal parenchyma is more than 5mm, it can be regarded as normal; if the width of hydronephrosis is more than 20mm or the thickness of renal parenchyma is less than 2mm, it can be regarded as renal function damage, and the pregnancy can be terminated as appropriate. Typical case: left hydronephrosis was found at 32 weeks of gestation with 13mm separation of the renal pelvis, 14mm separation of the renal pelvis at 38 weeks, 13mm separation of the renal pelvis after birth, 8mm separation of the left renal pelvis at 3 months after birth, 6mm separation of the renal pelvis at 6 months after birth, and no obvious abnormality in both kidneys on the ultrasound after 11 months of birth.