The criteria for fetal hydronephrosis are not uniform, but basically the diagnosis is greater than 10 mm. There are physiological and pathological hydronephrosis, with physiological hydronephrosis ranging from 10-15 mm. Physiological hydronephrosis is between 10-15 mm. It is caused by an increase in progesterone in the pregnant woman’s body, which affects the peristalsis of the ureter and causes hydronephrosis. It can also be caused by maternal progesterone, but once the fetus is separated from the mother, it often disappears after a few days of life when the ultrasound is performed. If it is pathological, there will be anatomical abnormalities. The general less than 1cm is not too much to worry about ~ check again after birth.