Outcomes of antenatal hydronephrosis and other renal abnormalities in a population‐based cohort

Abstract Introduction Hydronephrosis, commonly detected during second trimester ultrasound screenings, can signal underlying renal abnormalities. In 2018, Sweden introduced a national guideline recommending follow‐up for fetal renal pelves with an anteroposterior diameter (APD) ≥6 mm at the second trimester ultrasound. This study evaluates the impact of the guideline by assessing the incidence and resolution of hydronephrosis and other renal abnormalities. Secondary outcomes included postnatal renal function, frequency of febrile urinary tract infections (UTI), and the need for surgical intervention before the age of 2. Material and Methods This prospective cohort study included fetuses diagnosed with hydronephrosis or other renal abnormalities during second trimester ultrasounds between November 2019 and October 2022. Maternal and child characteristics were compared with data from the Swedish Pregnancy Register. Pre‐ and postnatal outcomes up to 2 years of age were assessed for fetuses detected antenatally. Outcomes were compared to children diagnosed postnatally within the first 2 years of life and not identified prenatally. Results Over 3 years, 31 094 second trimester ultrasounds were performed. Hydronephrosis or other renal anomalies were identified in 0.5% ( n = 155); approximately 40% resolved by week 32, and seven additional third‐trimester detections brought the total number of antenatal cases to 162. By age 2, only 42% had persistent renal abnormalities. A retrospective ICD‐10 review identified 45 additional children diagnosed postnatally following symptoms before age 2, yielding an antenatal detection rate of 78.3% (162/207). Baseline demographics were similar between antenatal and postnatal groups and matched those in the Swedish Pregnancy Register. Antenatally detected cases showed diverse anomalies—primarily multicystic dysplastic kidney, duplex kidney, pelviureteric junction obstruction, and vesicoureteral reflux (VUR)—whereas postnatally detected cases were predominantly VUR. Febrile UTI requiring admission occurred in 11.8% of antenatal versus 71.7% of postnatal children ( p < 0.0001); surgery rates were similar between groups. Conclusions The centralized structure of care enabled a comprehensive, population‐based cohort with near‐complete follow‐up. The detection rate of hydronephrosis during second trimester ultrasound is high, reaching ~80%. Given the high rate of spontaneous resolution, the national guideline threshold (APD ≥6 mm) appears to strike a reasonable balance between sensitivity and clinical practicality.