Abstract Embryologically, cloacal exstrophy is thought to result from persistence and subsequent rupture of the infraumbilical cloacal membrane during the fifth embryonic week. We report a case of cloacal exstrophy in which a prenatal diagnosis was made prior to rupture of the cloacal membrane. A routine ultrasound at 17 weeks' gestation demonstrated monoamniotic twins. One twin was normal, but the other was found to have a sacral myelomeningocele, “rocker-bottom” feet, splaying of the public rami, and a large cystic mass protruding from the infraumbilical anterior abdominal wall. A repeat ultrasound was performed at 22 weeks, with the same findings. At 26 weeks, further examination showed disappearance of the abdominal cyst, a small omphalocele, no demonstrable bladder, and the suggestion of prolapsed bowel inferior to the umbilical cord insertion. After delivery at 34 weeks, the abnormal twin was found to have the typical findings of cloacal exstrophy, myelomeningocele, bilateral lower limb anomalies, and extremely foreshortened small bowel. Rupture of the presumed cloacal membrane after 22 weeks in this case is inconsistent with our current understanding of the embryology of this anomaly, and should stimulate a reexamination of the current concepts. If the characteristic features are recognized, cloacal exstrophy can be diagnosed by prenatal ultrasound, permitting prenatal counseling and appropriate perinatal management.