BACKGROUND Ionizing radiation poses potential health risk to fetuses, including growth retardation, organ malformations, neurological effects, fetal death, and cancer. Therefore, pregnant women rarely undergo X-ray imaging, unless absolutely necessary. However, they can be inadvertently exposed to X-rays while undergoing an examination without being aware of their pregnancy. Given that the likelihood of a fetus appearing on an abdominal radiograph is low, physicians can be unfamiliar with the appearance of a fetus on such images. If a fetus incidentally appears on an abdominal radiograph, the clinician can encounter difficulties in identifying the fetus, potentially leading to unexpected harm during subsequent imaging procedures. CASE REPORT A 41-year-old woman presented with symptoms of postprandial abdominal pain, abdominal fullness, and urinary frequency. Abdominal radiography revealed a large mass with calcifications in the pelvis and lower abdomen, raising suspicion of a large pelvic tumor, such as a teratoma. However, subsequent ultrasound unexpectedly revealed that the woman was actually 6 months pregnant. The presumed tumor was an enlarged uterus, and the presumed calcifications were the fetus's bones. Before X-ray, the woman had denied being pregnant, mistakenly attributing her condition to excess weight and irregular menstrual cycles. Fortunately, the use of ultrasound instead of computed tomography prevented radiation exposure to the fetus, and the baby was delivered in a healthy state at full term. CONCLUSIONS To avoid unintended harm to fetuses, clinicians should consider the possibility of pregnancy when evaluating a pelvic mass in women of childbearing age. To aid clinicians in accurately identifying fetuses on abdominal radiography and thereby reduce the likelihood of misdiagnosis, we propose the "circled delta sign" and the "reversed circled delta sign".