From the Guidelines
Normal fetal renal pelvis dilation during pregnancy is typically defined as an anterior-posterior (AP) diameter of less than 4 mm between 16 and 27 weeks of gestation and less than 7 mm between 28 weeks of gestation and delivery. According to the 2021 study published in the American Journal of Obstetrics and Gynecology 1, these parameters are based on a consensus statement that defines norms for antenatal urinary tract dilation (UTD) based on anterior-posterior renal pelvis diameter.
When evaluating UTD, it's essential to consider additional ultrasound features, including:
- The presence of calyceal dilation
- Parenchymal thickness and appearance
- Ureteral dilation
- Bladder abnormalities
- Amniotic fluid volume These features help classify UTD into A1 (low risk) or A2-3 (increased risk), guiding antenatal management and postnatal follow-up 1.
It's also important to note that UTD between 4 and 7 mm in the second trimester of pregnancy resolves in approximately 80% of cases, and consistent with the 2014 consensus statement, for fetuses with isolated UTD A1, an ultrasound examination at ≥32 weeks of gestation is recommended to determine if postnatal pediatric urology or nephrology follow-up is needed 1.
In summary, the key factors to consider when evaluating fetal renal pelvis dilation include gestational age, AP diameter, and the presence of other renal anomalies, with regular follow-up ultrasounds recommended for moderate to severe cases to monitor progression.
From the Research
Normal Fetal Renal Pelvis Dilation Parameters
The normal parameters for fetal renal pelvis dilation in pregnancy can be summarized as follows:
- Up to 1% of prenatal ultrasounds will detect fetal renal pelvic dilatation 2
- Fetuses with renal pelvic dilatation 4 mm or greater at less than 33 weeks of gestation, or 7 mm or greater at more than 33 weeks of gestation were evaluated postnatally 2
- An upper limit of 7 mm for the anteroposterior diameter in late pregnancy is considered reasonable 3, 4
- A threshold of 15 mm renal pelvic dilatation is considered significant and predictive of obstruction 2
- The third-trimester anteroposterior renal pelvis diameter of ≥7 mm is the best ultrasound criterion to predict postnatal uropathies 4
Key Findings
Some key findings from the studies include:
- The magnitude of fetal renal pelvic dilatation is predictive of obstruction 2
- The risk of postnatal renal pelvis dilatation increases with fetal renal pelvis size and earlier gestation 5
- Estimated risks of obstruction and vesicoureteral reflux (VUR) are substantially lower, particularly in the mild group 5
- Pediatric urologists had significantly higher thresholds for the detection of prenatal and neonatal renal pelvis dilatation compared to pediatric nephrologists 6
Parameters for Postnatal Evaluation
The parameters for postnatal evaluation of fetal renal pelvis dilation include:
- Renal ultrasound examinations after birth 6
- Postnatal renal pelvis dilatation was considered abnormal if the anteroposterior diameter was ≥11±1.9 mm by pediatric urologists and ≥9±2.9 mm by pediatric nephrologists 6
- Voiding cystourethrography and mercaptoacetyl-triglycine renography may be used to achieve functional evaluation during follow-up 6