Outcome of Neonatal Hydronephrosis, a New Cut-Off to Identify Patients with Spontaneous Resolution

Background/Objectives: The anteroposterior renal pelvis diameter (APRPD) is used to assess the grade of urinary tract dilatation (UTD). There is no univocal method stratifying the risk of complications related to postnatal UTD. This study aims to identify APRPD cut-offs at birth to determine outcome stratification and second-level exams. Methods: The records of a cohort of newborns with unilateral or bilateral UTD confirmed or detected by ultrasound after birth between 2010 and 2020 were analyzed. These children underwent further examinations at 3, 6, 12, and 24 months of age. Results: We managed 500 children with postnatal UTD, with a median APRPD at 0–2 months of age of 7.7 mm [IQR 6.0–10.0]. As for UTD resolution, 279 (55.8%) patients had a complete resolution at the age of 6 months; an additional 55, for a total of 344 (68.8%), at 9–12 months; and 19, for a total of 353 (70.6%), at 24 months. An APRPD value ≤ 8.5 mm showed a sensitivity of 80.4% (95% CI [76.0–84.4]) and a specificity of 100.0% (95% CI [76.8–100.0]) in identifying candidates for spontaneous resolution within 24 months of life. An APRPD value ≤ 8.5 mm was also an independent prognostic factor of resolution at the age of 24 months (p = 0.000). Conclusions: Isolated hydronephrosis is the most frequent urinary tract abnormality detected in pregnancy. A well-structured prenatal and postnatal management plan is indeed necessary. According to our analyses, 8.5 mm can be used as a cut-off to reassure parents and clinicians of the benignity of the postnatal dilatation.