How long does fetal hydronephrosis take to resolve on its own is related to its specific condition and cannot be generalized. Fetal hydronephrosis refers to a significant separation of the fetal renal collecting system, and is a fetal abnormality that is more frequently detected by prenatal ultrasonography, usually at 18-20 weeks of gestation. Most fetal hydronephrosis is physiological, called physiologic hydronephrosis. This type of hydronephrosis often changes with fetal development, renal physiology and ureteral histology, and spontaneously resolves before the end of pregnancy or within 1 year after birth, without further intervention. If fetal hydronephrosis is caused by obstructive factors, it is pathological hydronephrosis and will not disappear on its own, which can cause serious consequences such as decreased renal function and urinary tract infection, and should be treated by active surgical intervention. In addition, regular maternity checkups during pregnancy are recommended to avoid delaying the treatment of fetal hydronephrosis.