Prenatal hydronephrosis is a condition that develops, and is usually diagnosed, before a child's birth. In this condition, the outflow of urine from the kidney is partially or completely blocked. This is usually due to a narrowing of the ureter, but can be caused by other factors as well. This condition is typically diagnosed during a prenatal ultrasound. Antenatal hydronephrosis is usually treated through careful monitoring (often the condition corrects itself), possible administration of antibiotics (to prevent infection) and sometimes surgery.