Today, most of the congenital diseases (congenital anomalies) can be diagnosed during pregnancy. Diseases of the excretory system (kidneys, bladder and urinary tract) come first among anomalies that can be detected in the early period of pregnancy with ultrasound examination. Statistics indicate that enlargement of the kidneys of the baby in the womb (hydronephrosis) is detected in one out of every 100 pregnant women in the examinations made by an advanced ultrasound device and by a trained ultrasound specialist (preferably a radiologist). This issue is extremely important because in the vast majority of these mothers, there is no problem during pregnancy, the baby is born normally and looks completely healthy after delivery. Since the majority of them do not show any symptoms, they are sent home without any examination. When symptoms give rise to discomfort (such as flank pain, abdominal mass, fever, burning and stinging during urination, turbidity in the urine), they often consult a physician with serious damage to the kidneys. However, the most common cause of hydronephrosis (50%) is a narrowing of the urinary canal in front of the kidney during the embryonic period and surgical treatment is possible. Although many can go away on their own, they should be closely monitored during pregnancy and after delivery. If a deterioration in kidney function is detected, the stenosis is immediately corrected by surgery within the first month after delivery.