From the Guidelines
The normal renal pelvis diameter in a fetus is considered less than 4 mm between 16 and 27 weeks of gestation and less than 7 mm between 28 weeks of gestation and delivery. This is based on a consensus statement from 2014, as referenced in the study by 1. The anterior-posterior renal pelvis diameter is the key measurement used to assess urinary tract dilation (UTD) in fetuses.
Key Considerations
- UTD occurs in 1% to 2% of pregnancies and is most commonly a transient finding that is a normal variant 1.
- The presence of calyceal dilation, parenchymal thickness and appearance, ureteral dilation, bladder abnormalities, and amniotic fluid volume are additional ultrasound features that should be evaluated to fully assess and classify UTD 1.
- UTD between 4 and 7 mm in the second trimester of pregnancy resolves in approximately 80% of cases, as noted in the study 1.
Gestational Age and Renal Pelvis Diameter
- Between 16 and 27 weeks of gestation, a renal pelvis diameter of less than 4 mm is considered normal 1.
- Between 28 weeks of gestation and delivery, a renal pelvis diameter of less than 7 mm is considered normal 1. It's essential to consider these guidelines when evaluating fetal renal pelvis diameter to distinguish between normal variants and potential urinary tract abnormalities that might require monitoring or intervention after birth.
From the Research
Normal Fetal Renal Pelvis Diameter Based on Gestational Age
The normal renal pelvis diameter in a fetus varies based on gestational age. Several studies have investigated this topic to provide guidelines for prenatal and postnatal evaluation and management.
- At 20 weeks of gestation, a renal pelvis anteroposterior diameter of up to 6 mm is considered normal 2.
- At 30 weeks of gestation, a renal pelvis anteroposterior diameter of up to 10 mm is considered normal 2.
- At 33 weeks of gestation or later, a renal pelvis anteroposterior diameter of up to 7 mm is considered normal 3, 4.
- A study found that an upper limit of 7 mm for the anteroposterior diameter in late pregnancy is reasonable 3.
- Another study suggested that a fetal renal pelvic dilatation of 15 mm or greater is predictive of obstruction and may direct postnatal evaluation and management 4.
Gestational Age and Renal Pelvis Diameter
The relationship between gestational age and renal pelvis diameter is important for prenatal diagnosis and management.
- A study found that the risk of isolated renal pelvis dilation progressing to hydronephrosis is low, with a 3.3% chance of unilateral renal pelvis dilation progressing to hydronephrosis versus 26.0% in bilateral dilation 5.
- The same study found that 80% of fetuses with progression had bilateral dilation, and there was no difference in progression between right and left kidneys 5.
Prenatal and Postnatal Evaluation and Management
Prenatal and postnatal evaluation and management of fetal renal pelvis dilation are crucial for identifying potential obstructive uropathies.
- A study found that prenatal sonography proved less sensitive than postnatal sonography in revealing obstructive uropathies 6.
- Another study suggested that using thresholds of 6 mm at 20 weeks of gestation and 10 mm at 30 weeks of gestation for isolated pyelectasis, and 10 mm at 20 weeks of gestation and 12 mm at 30 weeks of gestation for hydronephrosis, could avoid a significant number of follow-up ultrasounds and invasive postnatal tests without missing cases of obstructive nephropathy requiring surgery 2.