Unilateral mild hydronephrosis with ureteral dilatation is a condition that can be detected by careful prenatal ultrasound examination. We present such a case, in a 26 year pregnant woman with hereditary antecedents (mother with Ombredan disease). The fetal ultrasound at 34 weeks indicated a left anechogenic sinuous image, identified as dilated ureter, and connected to a grade 2 hydronephrotic kidney. Normal data regarding the other kidney and the bladder made us recommend a close follow-up, considering this case as with a good prognosis. After birth, reassessment was favourable, and at 1 year follow-up the child was in good condition, and the kidney functional. We conclude that this case is a unilateral congenital megaureter, of probably obstructive origins, with a mild hydronephrosis. A good outcome was suggested by the late diagnostic, unilateral condition, and normal fetal ultrasound description regarding morphology, biometry and amniotic fluid.