hello there 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. Mild renal pelvis dilation ie 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. Down syndrome can be a cause of renal pelvis dilatation which is confirmed by amniocentesis. As you said your amniocentesis report was normal, this can be excluded. 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.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.