Urine production in the human kidney starts at about 10-12 weeks of gestation. The main exchange of waste products is done through the placenta while the baby (fetus) is in the uterus. If the urine flow is obstructed, high pressures on the kidney may build and damage the growing kidneys’ function. An obstruction is often associated with dilation of the urinary system above the obstruction. However, dilations will often occur on their own without definite evidence of a blockage.
Listed below are some of the common problems affecting the urinary system which may be identified with ultrasound during pregnancy.
Prenatal Hydronephrosis
This is enlargement of the collecting system of the kidneys. The collecting system is the structure that collects urine from each of the 2 kidneys, moves it in 2 hollow tubes called ureters. The ureters deposit the urine into the bladder.
A common term used to refer to hydronephrosis is “swelling of the kidney."
Keep in mind that the term hydronephrosis is not a diagnosis. It is an x-ray or ultrasound finding which means there may be an underlying blockage or other problems in the urinary system.
It is important to remember that a finding of hydronephrosis during pregnancy does not mean there is a blockage or obstruction in your baby’s urinary system. The majority of dilations found during pregnancy resolve on their own before or after birth.
After your baby is born, but not always immediately after birth, your doctor will order an ultrasound of the baby’s kidneys to determine whether there is a problem in the urinary drainage system.
Different Causes of Prenatal Hydronephrosis
The following diagnoses will not be made until after birth:
The most common cause of prenatal hydronephrosis is a narrowing or blockage at the ureteropelvic junction. It is sometimes referred to as an UPJ obstruction. This term refers to a narrowing of the ureter (the tube which moves urine from the kidney to the bladder) where the ureter is connected to the kidney. This may affect one or both kidneys and tends to improve or resolve on its own in about 75% of children.
Another common cause which occurs only in males is posterior urethral valves. This condition causes a blockage to bladder emptying that may affect both kidneys and often requires surgery soon after birth because it can affect the function of both kidneys,
Vesicoureteral reflux is a condition that can cause prenatal hydronephrosis. In this condition, the urine travels backwards from the bladder to the kidneys, which can dilate the kidneys and ureters. There is no obstruction, but a leaky valve mechanism which allows backflow. It is usually not treated with surgery immediately after birth, but observed to see if it will go away on it’s own.
Congenital megaureter is an enlarged ureter and another cause of prenatal hydronephrosis. The ureters are tubes which carry urine from the kidneys to the bladder. Megaureter leads to poor bladder emptying. After your baby is born, but not always immediately, your doctor will order an ultrasound of the baby’s kidneys to determine whether there is a problem in the urinary drainage system.
In discussing your prenatal ultrasound, your obstetrician will likely talk to you about the amount of amniotic fluid surrounding the baby. A normal volume of amniotic fluid can be a good sign of normal urinary function. When the amniotic fluid is low, this can indicate a problem with the function of the kidneys.
If a fetus does not have any amniotic fluid around it at all, there may be a serious blockage and the kidneys may not be working well. It is uncommon for this to happen. In these cases, a high risk obstetrician will evaluate whether the fetus might benefit from having a shunt (tube for drainage of urine) placed during pregnancy. This is an uncommon procedure, performed only at certain high risk centers like Ohio State University. It is not recommended for all fetuses. The fetus and mother must be carefully evaluated and tested before it is known if a shunt should be placed. Ohio State is participating in a study that may help decide which babies in the future might benefit from having a shunt placed.