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Renal pelvic dilation refers to excessive dilation of the fetal intrarenal collecting system. Although a different terminology is used, pelviectasis or pyelectasis most often refers to mild dilation, and hydronephrosis is reserved for severe cases that are more likely to be clinically significant. Normal measurements of the anteroposterior renal pelvis diameter are <4 mm at 16 to 27 weeks of gestation and <7 mm at ≥28 weeks of gestation.
Although fetal urinary tract dilation is often mild and benign, occurring in 1% to 5% of all pregnancies, it also can be associated with genetic and structural disorders. Outcomes range from normal to significant structural anomalies requiring surgical repair.
Fetal urinary tract dilation is defined as an anteroposterior renal pelvis diameter of ≥4 mm at 16 to 27 weeks of gestation and ≥7 mm at ≥28 weeks of gestation. The severity of renal dilation can be categorized on the basis of gestational age-specific criteria. At 16 to 27 weeks of gestation, dilation is categorized as mild (4 to <7 mm), moderate (7 to ≤10 mm), and severe (>10 mm). Beyond 28 weeks of gestation, dilation is categorized as mild (7 to < 9 mm), moderate (9 to ≤15 mm), and severe (>15 mm) (Table).
The anterior-posterior (AP) renal pelvis diameter is measured in a transverse plane, with fetal spine ideally positioned at the 12-o’clock position (Figure 1). Proper caliper placement occurs at the interior margin of the renal parenchyma, measuring across the widest portion of the fluid-containing renal pelvis (Figure 2). Renal pelvic dilation can also be assessed in a coronal plane, which aids in the identification of caliectasis (Figure 3). Additional ultrasound findings that contribute to prognosis and should be reported with a finding of renal pelvic dilation include those indicative of associated congenital anomaly of the kidney and urinary tract (CAKUT): peripheral calyceal dilation, abnormal appearance of the renal parenchyma (thickness, echogenicity, and the presence of cysts), dilated ureters, abnormal bladder appearance, and oligohydramnios.
Although mild fetal pelviectasis is most often incidental and benign, it is a common and nonspecific finding that can also be associated with genetic and structural abnormalities. Isolated mild urinary tract dilation in the second trimester of pregnancy has been reported to carry a likelihood ratio of 1.6 to 2.78 for trisomy 21, although the strength of this association is debated.
Progressive, moderate, or severe urinary tract dilation is more commonly associated with vesicoureteral reflux or CAKUT, such as ureteropelvic junction (UPJ) obstruction, duplicated collecting system with obstruction, and bladder outlet obstruction (most commonly due to posterior urethral valves).
Fetuses with complete bladder outlet obstruction or significantly impaired fetal renal function will have associated oligohydramnios or anhydramnios.
The differential diagnosis of fetal pelvic dilation or hydronephrosis includes a normal variant or physiological and transient dilation as well as pathologic causes, such as vesicoureteral reflux and obstructive uropathy. Obstructive causes include those at the level of the ureter, such as UPJ obstruction, and obstruction of a prolapsed ureterocele in a duplicated collecting system, which occurs more often in female fetuses. Obstruction at the bladder outlet is most commonly caused by posterior urethral valves, which occur in males and cause a keyhole-shaped bladder. Rarer causes of fetal urinary tract dilation include urethral atresia and megacystis-microcolon-intestinal hypoperistalsis syndrome, both of which have a markedly enlarged bladder as a dominant ultrasound finding.
Fetal hydronephrosis is most commonly an isolated finding. If no further abnormalities are noted on ultrasound and the family history is unremarkable, it is reasonable to perform no genetic evaluation beyond standard aneuploidy screening. Because of the association with Down syndrome, counseling should occur in the context of an individual’s a priori risk of trisomy 21 based on maternal age, previous screening results, and additional ultrasound findings. If there are additional anomalies, consanguinity, or a family history suggestive of a specific condition, diagnostic testing with chromosomal microarray analysis (CMA) should be offered; gene panel testing or exome sequencing is sometimes useful in cases of significant hydronephrosis if CMA is normal, as CMA does not detect single-gene disorders. If exome sequencing is pursued, appropriate pretest and posttest genetic counseling by a provider experienced in the complexities of genomic sequencing are recommended.
Pregnancy and Delivery Management
Given the high likelihood of resolution when mild urinary tract dilation is noted in the second trimester of pregnancy, a single follow-up ultrasound examination in the third trimester of pregnancy at approximately 32 weeks of gestation is recommended. Normalization of the AP renal pelvic diameter suggests no further need for follow-up beyond routine obstetrical care. Fetuses with moderate or severe urinary tract dilation, and those with additional findings to suggest CAKUT, warrant closer surveillance with follow-up ultrasound examinations at 4- to 6-week intervals or sooner if clinically indicated. These patients may also benefit from prenatal consultation with a pediatric urology or nephrology specialist to formulate a plan for postnatal evaluation. Timing of delivery should not be affected by the presence of hydronephrosis, as preterm delivery has not been demonstrated to improve outcomes. The presence of hydronephrosis should not alter the mode of delivery, which should be based on the usual obstetrical or medical indications.
For fetuses with mild antenatal hydronephrosis, the risk of postnatal pathology is reported to be 11% to 15%, increasing to 27% to 45% and 53% to 88% for moderate and severe hydronephrosis, respectively.
Although the incidence of postnatal vesicoureteral reflux appears constant among fetuses with any degree of hydronephrosis, the likelihood of postnatal obstructive uropathy or abnormal renal function and the need for subsequent surgery increases with the degree of dilation and with the presence of additional findings suggestive of CAKUT.
Fetal renal pelvic dilation is a relatively common finding on prenatal ultrasound. Enlargement of the AP renal pelvis diameter beyond the gestational age-specific norms of <4 mm at 16 to 27 weeks of gestation and <7 mm at ≥28 weeks of gestation indicates a need for a thorough evaluation of the fetal genitourinary tract for evidence of CAKUT and review of screening results and genetic counseling related to an increased risk of trisomy 21. Follow-up ultrasound examination should be performed to assess for persistent or progressive renal dilation. Isolated mild pelviectasis in the second trimester of pregnancy warrants a single follow-up ultrasound examination at approximately 32 weeks of gestation. These patients can be reassured that the prognosis is favorable, with 85% to 90% having normal postnatal outcomes. In cases of moderate or severe hydronephrosis, or when there are associated findings suggestive of CAKUT, serial assessments are warranted, and pediatric urology or nephrology consultation should be offered. Obstetrical management and timing and mode of delivery should be based on standard obstetrical indications.
Mild pyelectasis: evaluating the relationship between gestational age and renal pelvic anterior-posterior diameter.