Renal pelvis dilatation

Fetal renal pelvis dilatation is identified in 1% to 5% of pregnancies and is often a transient, normal variant. The likelihood of an underlying abnormality increases with the degree of dilatation. The most common underlying abnormalities are ureteropelvic junction obstruction and vesicoureteral reflux. Mild renal pelvis dilation is also a minor marker that confers slightly increased risk for trisomy 21. The pelvis is measured anterior to posterior in the transverse plane, with the calipers placed on the inner border of the fluid collection. Images in other planes (sagittal, coronal) are obtained to exclude evidence of obstruction elsewhere along the urinary tract. If the renal pelvis measurement exceeds a threshold in the second trimester (such as 4 mm before 20 weeks), further evaluation is recommended in the third trimester, at approximately 32 weeks. If there is calyceal dilatation or cortical thinning, more frequent evaluation may be warranted. Neonatal evaluation is typically reserved for those with renal pelvis dilatation of at least 7 mm in the third trimester.