What is hydronephrosis?
Hydronephrosis is a swelling or dilation within the kidney or the ureter, which is the tube that connects the kidney to the bladder and moves the urine the kidneys produce to the bladder. Hydronephrosis generally results from a blockage at the top of the ureter near the kidney (known
as the ureteropelvic junction, or UPJ) that traps urine in the kidney, causing it to build up and stretch the kidney or ureter (see picture). Less commonly, urine backs up from the bladder and leads to hydronephrosis. Generally only one kidney is affected. The severity of the condition
depends on the extent of the blockage and the degree the kidney is stretched, and can range from mild to severe. In the most severe cases, the UPJ obstruction can lead to abnormalities in the amount of amniotic fluid, and this in turn can impair fetal lung development.
How common is hydronephrosis and what causes it?
Some studies suggest that as many as 2% of all prenatal ultrasound examinations reveal some degree of hydronephrosis, making it one of the most commonly detected abnormalities in pregnancy. It is not clear why the ureter becomes blocked during development. It is more often seen in boys than girls. Babies with severe UPJ obstructions have an overall increased chance of having a chromosomal abnormality (like Trisomy 21, or Down syndrome).
How is hydronephrosis detected during pregnancy?
Ultrasound can detect the fetal kidneys and bladder by 14 or 15 weeks gestation, though 20 weeks of pregnancy is the ideal time to detect hydronephrosis on ultrasound, since the fetus is larger and the kidneys can be visualized in detail. When a UPJ obstruction is identified, the kid-
neys are closely examined for other findings more common with UPJ, such as cysts (known as multicystic, dysplastic kidney) or an abnormal shape (commonly called a horseshoe kidney).