From the Guidelines
Fetal renal dilatation is considered within the normal range if the anterior-posterior renal pelvis diameter is 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, this measurement system helps clinicians determine whether the dilation is physiologic or potentially pathologic. The study notes that urinary tract dilation (UTD) occurs in 1% to 2% of pregnancies and is most commonly a transient finding that is a normal variant.
Key points to consider when evaluating fetal renal dilatation include:
- The presence of calyceal dilation, parenchymal thickness and appearance, ureteral dilation, bladder abnormalities, and amniotic fluid volume
- Classification of UTD into A1 (low risk) vs A2-3 (increased risk), which guides antenatal management and postnatal follow-up
- UTD between 4 and 7 mm in the second trimester of pregnancy resolves in approximately 80% of cases
- 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
It is essential to note that while most cases of fetal renal dilatation are transient and resolve spontaneously, some cases may have a pathologic cause, such as vesicoureteral reflux, ureteropelvic junction obstruction, or multicystic dysplastic kidneys 1. Therefore, follow-up ultrasounds and postnatal evaluation may be necessary to monitor for progression and ensure appropriate management.
From the Research
Fetal Renal Dilatation Normal Range
- The normal range for fetal renal pelvic dilatation can vary depending on the gestational age, with different studies suggesting different thresholds 2, 3, 4, 5, 6.
- A study published in 2006 found that fetuses with renal pelvic dilatation of 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.
- Another study published in 2004 found that pediatric urologists considered postnatal renal pelvis dilatation to be abnormal if the anteroposterior diameter was ≥11 +/- 1.9 mm, while pediatric nephrologists considered it abnormal if the diameter was ≥9 +/- 2.9 mm 3.
- A study published in 2003 constructed size charts for fetal kidney measurements and defined normal limits for renal pelvic antero-posterior diameter, suggesting an upper limit of 7 mm for the AP diameter in late pregnancy 5.
- A study published in 2004 found that a renal pelvic anteroposterior diameter of less than 7.0 mm after 32 weeks was highly predictive of normal postnatal renal function in cases of mild pyelectasis 6.
Thresholds for Abnormal Fetal Renal Pelvic Dilatation
- 4 mm or greater at less than 33 weeks of gestation 2
- 7 mm or greater at more than 33 weeks of gestation 2
- ≥11 +/- 1.9 mm (pediatric urologists) 3
- ≥9 +/- 2.9 mm (pediatric nephrologists) 3
- ≥7 mm (upper limit in late pregnancy) 5
- ≥7.0 mm after 32 weeks (predictive of abnormal postnatal renal function) 6
Postnatal Evaluation and Management
- Infants with renal pelvic dilatation should be evaluated with ultrasound, voiding cystourethrograms, and renal scintigraphy 2.
- Pediatric urologists were more likely to recommend routine voiding cystourethrography and surgical therapy of dilated kidneys with low function than pediatric nephrologists 3.
- A single follow-up sonographic examination either late in pregnancy or after delivery is considered adequate for follow-up of renal pelvis dilatation detected earlier in pregnancy 4.