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Pregnancy Bliss | Reproductive Health Answers

Renal (kidney) pelvis dilatation


Renal or kidney pelvis dilatation also known as mild pyelectasis is a relatively common finding in fetal ultrasound scanning being seen in approximately 1% of all. The renal pelvis is not to be confused with the pelvis as part of the body. Renal pelvis refers to a specific part of the kidney. When the measurement of this part of the kidney in the second trimester exceeds 4 mm; it is said to be dilated. When this is found, it is usually mild. The medical term mild pyelectasis therefore refers to a dilatation between 5 and 9 mm.  When the renal pelvis measures 10-15 mm, this is described as moderate pyelectasis. Above 15 mm, that is severe.


There is an association between renal pelvis dilatation and aneuploidies (chromosomal disorders) such as Down’s syndrome. This soft marker is seen much more often in babies affected by Down’s syndrome. However, when found in isolation without other markers and without maternal risk factors (such as advanced age), mild renal pelvis dilatation is extremely unlikely to be significant. With a risk factor such as a mother over the age of 36, the risk of Down’s syndrome is just over 2% (1 in 50) compared to 0.33% (1 in 300)for a younger mother. Such a mother may want to consider having an invasive diagnostic test such as amniocentesis.



The primary concern for the prospective parents when an isolated marker of dilated renal pelvis is found is kidney function. There is, of-course, no immediate concern with the baby in the womb. However, many experts advocate a repeat scan later in the third trimester to see if the dilatation is persisting (many do not). If persistent dilatation is found, a paediatric specialist is alerted so they can take over when the baby is born.


The majority of babies found to have mild renal pelvis dilatation will need nothing more than observation and the problem resolves spontaneously. A few, usually those with moderate or severe dilatation, may need surgery for either some element of obstruction at the junction of the kidney and ureter (Pelvi-ureteric junction) or reflux of urine from the bladder up the ureter, a condition known as Vesico-ureteric reflux (VUR). These are structural anomalies which are uncommon and certainly not associated with chromosomal disorders.



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