Antenatal hydronephrosis, which refers to the enlargement of the kidney due to fluid-filled sacs before birth, can be detected in a fetus through ultrasound studies as early as the first trimester of pregnancy. Although most cases do not require changes in obstetric care, surveillance and possible surgery during infancy and childhood may be necessary. There are several possible causes of antenatal hydronephrosis, including blockages in the Uretero Pelvic Junction (UPJ), the Uretero Vesical Junction (UVJ), or the urethra. Vesicoureteral reflux, which occurs when the valve between the bladder and the ureter is incompetent, is another possible cause. Other less common causes include duplications and multicystic kidneys, which are non-functional cystic kidneys. In many cases, ultrasound surveillance is all that is necessary during pregnancy. However, in rare cases where there is severe obstruction of both kidneys and insufficient amniotic fluid, drainage of the kidneys or bladder may be necessary. Postnatal ultrasound is usually performed before the child leaves the hospital, and a Voiding Cystourethrogram (VCUG) is performed to exclude vesicoureteral reflux. Children with vesicoureteral reflux are managed with antibiotics and periodic ultrasound and voiding cystogram surveillance, while those with obstruction or blockage may require surgical correction. In cases where the evidence for obstruction is marginal or the degree of blockage is mild, the tests may be repeated after a few months. If a child is diagnosed with a multicystic dysplastic kidney, which doesn’t function, the opposite kidney is usually normal. In most cases, unless the multicystic kidney is causing a problem, such as mass effect or pain, it is usually left alone and followed up with ultrasound at 6 months and a year after. However, if it is found to be enlarging during follow-up or if there is a suspicion of tumor or blockage, nephrectomy (removal of the affected kidney) may be necessary. Follow-up care for the opposite normal kidney is essential.