Interictal psychosis is a psychotic symptom that is not temporally related to epileptic seizures. Pregnancy delusion is defined as a person's fixed belief that she is pregnant despite objective evidence that she is not pregnant. In this case report, pregnancy delusion was described in a patient with epilepsy. A 31-year-old woman with epilepsy was admitted to a psychiatric ward. The patient, whose pregnancy test results were never positive, believed that she was pregnant. It was learned that her identical twin had experienced reproductive-sexuality-themed psychotic symptoms 10 years ago. The patient was hospitalized for three weeks and discharged in remission with paliperidone 6 mg/day and biperiden 2 mg/day. Caution should be exercised when using antipsychotics because of their epileptogenic effects. Pregnancy delusion in epilepsy is rare. In addition to this rare condition, it is noteworthy that reproductive-sexual delusions were reported in the patient's twin brother who was diagnosed with epilepsy.