Abstract： This article reported a prenatal diagnosis of fetal bladder prolapse caused by patent urachus and ruptured urachal cyst. On August 8, 2023 (12 weeks and 5 days of gestation), a routine prenatal ultrasound examination in Heping Hospital Affiliated to Changzhi Medical College revealed that one of the twins had an anechoic area (22.0 mm×17.0 mm) protruding into the umbilical cord at the umbilical opening, which was connected to the bladder in a bell-shaped manner through an unclosed urachus. Regular follow-up ultrasounds indicated that the cyst increased in size as the pregnancy progressed, the biggest was 38.3 mm×30.2 mm (23 +5 gestational weeks). At 33 weeks and 5 days of gestation, ultrasound showed that the cyst had disappeared, and a high-echo mass (21.3 mm×15.2 mm) was visible at the umbilical opening, while the bladder was not visible even upon repeated scans. These observations led to the diagnosis of bladder prolapse due to patent urachus and ruptured urachal cyst. Postnatally, the infant was transferred to an external hospital for surgical treatment, where the diagnosis of bladder prolapse was further confirmed and prolapse repair and umbilicoplasty were done. Postoperative pathology verified that the protruding high-echo mass was bladder tissue. Follow-up examinations at 2 months of age showed no significant complications.