Unilateral Hydronephrosis in Third Trimester Fetal Ultrasound

Clinical Significance and Risk Stratification

The American College of Obstetricians and Gynecologists recommends that unilateral hydronephrosis detected in the third trimester is most commonly a benign, transient finding that resolves spontaneously in approximately 80% of cases, but requires postnatal follow-up ultrasound to exclude pathologic obstruction, particularly when the anteroposterior diameter (APD) exceeds 7 mm 1
An APD of less than 7 mm in the third trimester represents normal physiologic variation and requires only a single follow-up ultrasound at ≥32 weeks of gestation, according to the American College of Obstetricians and Gynecologists 1
Approximately 80% of cases with APD between 4-7 mm in the second trimester resolve completely without intervention, as stated by the American College of Obstetricians and Gynecologists 1
Isolated mild hydronephrosis carries minimal risk for aneuploidy, with a positive likelihood ratio of only 1.5 for trisomy 21, according to the American College of Obstetricians and Gynecologists 1
The presence of calyceal dilation, abnormal parenchymal thickness or appearance, ureteral dilation, bladder abnormalities, or oligohydramnios elevates the classification to UTD A2-3, indicating increased risk, as stated by the American College of Obstetricians and Gynecologists 1

For isolated UTD A1 (APD <7 mm, no other abnormalities), the American College of Obstetricians and Gynecologists recommends performing a single follow-up ultrasound at ≥32 weeks of gestation to reassess 1
No aneuploidy screening is required if prior screening was negative or low-risk, according to the American College of Obstetricians and Gynecologists 1
If no prior aneuploidy screening was performed, the American College of Obstetricians and Gynecologists recommends counseling regarding the minimal increased risk (LR 1.5) and offering noninvasive screening with cell-free DNA or quad screen 1
For UTD A2-3 or APD ≥7 mm, the American College of Obstetricians and Gynecologists recommends performing a comprehensive assessment including evaluation of calyceal dilation, parenchymal thickness, ureteral dilation, bladder morphology, and amniotic fluid volume 1
The American College of Obstetricians and Gynecologists also recommends assessing for associated structural abnormalities, particularly cardiovascular and renal anomalies, as these increase aneuploidy risk from 4% to 50% 1