Prenatal ultrasound diagnosis of a cloacal malformation Cloacal malformations are a spectrum of rare congenital disorders caused by the arrest of the cloacal division during different stages in early embryogenesis. They necessitate critical intervention at birth, posing important surgical challenges. The majority unfortunately remains undetected during prenatal screening. Knowledge about sonographic red flags, however, may improve antenatal detection and therefore postnatal outcome as a result of a timely referral to multidisciplinary tertiary centers. A 29-year-old patient presents at the hospital due to decreased fetal movements in the third trimester of pregnancy. The ultrasound reveals an anechoic bilobed cystic mass with a thick midline septum within the pelvis of a female fetus. The diagnosis of a cloacal anomaly with associated duplicated vaginas and uteri is made. Severe accumulation of urine lies at the base of bilateral hydrocolpos, peritoneal ascites and a urinary tract obstruction with an adverse influence on both pulmonary and renal development. The baby is prematurely delivered by an emergency caesarean section. Her condition is poor, requiring immediate respiratory resuscitation. An explorative laparotomy on the following day confirms the diagnosis. The first steps towards anatomical correction are taken. It may be concluded that the triad of bilateral hydronephrosis, a pelvic cystic mass and an absent normal bladder within a female fetus should always raise suspicion of a cloaca.