Uterine synechia during pregnancy is generally diagnosed based on the presence of band-like structures in the uterine cavity. We treated a woman in the second trimester of pregnancy who had uterine synechia that was initially manifested as an intrauterine cyst just above the uterine ostium. A 26-year-old Japanese woman, gravida 1, para 0, was referred to Nagahama Red Cross Hospital because of a 68×44-mm intrauterine cyst located just above the uterine ostium found during a vaginal ultrasound examination at 26 weeks of gestation. Magnetic resonance imaging at 30 5/7 weeks of gestation revealed a transverse band-like structure attached anteriorly through the posterior at the center of the cavity, which incompletely divided the cavity into upper and lower compartments. The fetus was lying in a transverse position in the upper compartment, while the lower compartment was an 11×10×10-cm globular amniotic cavity protruding from the upper compartment. At 36 6/7 weeks of gestation, serial ultrasonography showed that the umbilical cord had penetrated into the lower compartment. Therefore, an elective cesarean delivery was conducted at 37 3/7 weeks of gestation, and a healthy female was delivered with a weight of 3,026 g and Apgar score of 9/9 (1 min/5 min). The synechia was resected during the operation. Detection of an intrauterine cyst during pregnancy may suggest the presence of uterine synechia, and accurate diagnosis can contribute to avoiding complications during pregnancy.