BACKGROUND: Renal fornices rupture with urinoma formation in fetuses is an unusual condition that acts as a 'pop off' mechanism to buffer renal pelvis pressure. Ureteropelvic junction obstruction (UPJO) is the most common cause of prenatal hydronephrosis, but it rarely leads to the formation of urinoma. Fetal urinoma could be indicative of poor renal function after birth. We report a rare case of UPJO-related urinoma accompanied with urinary ascites, which ruptured during vaginal delivery. CASE PRESENTATION: A 30-year-old woman was admitted to our hospital at 25 CONCLUSIONS: Prenatal identification of a large urinoma with urinary ascites requires necessary attention and appropriate management strategies. Prenatal urinoma drainage is indicated for large urinoma risking dystocia or rupture during delivery. Postnatal ascites requires active drainage to prevent creatinine and urea reabsorption and other complications. Small asymptomatic urinoma can be managed conservatively, but early surgery is recommended for large or recurrent urinoma exerting pressure on surrounding tissues.