The kidneys of an unborn child are used both to produce and to collect urine. The renal pelvis, situated within the kidney, collects the urine before it is passed to the bladder. It is then excreted into the amniotic fluid. Renal pelvis dilation occurs when the kidneys are retaining urine, causing the pelvis to become enlarged.
How Common Is It?
Mild renal pelvis dilation---dilation of between 5 and 9 mm---is a reasonably common finding among pregnancies of 20 weeks. Approximately one in 100 fetuses will be affected. Only 4 percent of cases of renal pelvis dilation will require further treatment after birth. If dilation is still present later in the pregnancy, your child will be offered a postnatal scan.
Why It Happens
When an unborn baby's renal pelvis becomes dilated, this indicates that the pelvis is retaining urine---either it is not leaving the kidneys or it is flushing back to the kidneys up the ureter. This could be due to some sort of blockage or compression of the ureter. It could also be the result of a slight defect in the formation of the kidneys and ureter in very early pregnancy.
Is It Serious?
A diagnosis of mild renal pelvis dilation is extremely unlikely to cause any difficulties during the remainder of the pregnancy or in delivery. In the vast majority of cases found at 20 weeks, the problem will be temporary. Very occasionally the renal pelvis can continue to dilate, but this does not necessarily indicate further problems. The condition only becomes significantly worse if the dilation is severe---greater than 15 mm---or there is also a defect detected in the kidneys.
In cases of mild renal pelvis dilation, there is a chance that, once born, your baby may be more prone to infections of the urinary tract than other children. However, these should pose no significant risk with early diagnosis and proper treatment. Some studies have linked renal pelvis dilation with cases of Down syndrome, but it is important to remember that this link is very weak. Blood screening for Down syndrome in early pregnancy is far more reliable, and an ultrasound will find 50 percent of cases. If Down syndrome is still a concern, you should consider amniocentesis.
If renal pelvis dilation is present at 20 weeks, a further scan will take place at 34 weeks to assess the situation. By this time, the pelvis may no longer be enlarged. If dilation is still present, it is likely that your child will be offered a scan two to three weeks after birth; if an infection is then diagnosed, it will be treated with antibiotics. If the dilation has become severe---greater than 15 mm---or the kidneys are affected, then the baby may need a scan and treatment immediately.