Early US Prenatal Diagnosis and Serial Imaging Confirmation of Hydrometrocolpos with Uterine Didelphys and Cloacal Malformation: A Rare Case of Combined Müllerian Duct and Anorectal Anomaly.

Background: Hydrometrocolpos associated with uterine didelphys and cloacal malformation represents an exceptionally rare prenatal finding. These anomalies arise from complex developmental disruptions of the Müllerian ducts and anorectal structures. Early recognition through first-trimester ultrasound is critical to ensure accurate diagnosis, guide follow-up imaging, and support parental counseling and perinatal planning. Case Presentation: A female fetus was identified in the first trimester with a midline cystic pelvic mass posterior to the bladder. The finding was initially suspicious for hydrometrocolpos with an underlying anorectal malformation. At 18 weeks, ultrasound confirmed a cystic lesion with thick echogenic walls extending toward the perineum, supporting the diagnosis of cloacal malformation. By 30 weeks, advanced imaging demonstrated hydrometrocolpos with a longitudinal septum, duplicated cervix, and two uteri, consistent with uterine didelphys. Additional findings included echogenic ascites due to chemical peritonitis, secondary to urine reflux into the peritoneal cavity. These results reflected a rare and complex constellation of Müllerian and cloacal anomalies. Conclusion: This case underscores the importance of systematic serial prenatal imaging and multidisciplinary evaluation in detecting rare congenital anomalies. Early identification of key sonographic features enables optimal diagnosis, facilitates individualized perinatal care, and improves prognostic counseling for families.