Uterine diverticula complicating pregnancy rarely are reported and should be differentiated from sacculation of pregnancy and other pelvic masses.A primigravida presented for a routine second-trimester anatomical survey ultrasound examination at 22 weeks of gestation. She initially was thought to have a bicornuate, bicolic uterus with bulging membranes. However, on examination, one normal-appearing cervix was found, and no membranes were visible. Magnetic resonance imaging demonstrated a posterolateral uterine diverticulum. At 31 weeks of gestation, she had premature rupture of membranes and onset of labor. Delivery by cesarean was performed, and the presence of a uterine diverticulum was confirmed.Uterine diverticula are rare anomalies in the pregnant uterus and should be considered in the differential diagnosis of a fluid-filled pelvic mass. With close observation, successful pregnancy outcome can be achieved.