A rare case of heterotopic pregnancy after a single embryo transfer: A case report and literature review.

Heterotopic pregnancy (HP) is a rare but life-threatening disease. We report a rare case of HP that occurred after single embryo transfer (SET) with spontaneous natural pregnancy possibly due to sexual intercourse (SI) during assisted reproductive technology treatment and reviewed previous reports. A 39-year-old woman at 7 weeks 5 days' gestation with anti-sperm antibody who underwent a single frozen-thawed embryo transfer in her natural cycle presented with lower abdominal pain and vaginal bleeding. She had several SIs before the day of SET. A viable intrauterine fetus and an extrauterine mass at the right adnexa were detected on transvaginal ultrasonography. An emergent laparoscopic surgery showed a swollen right fallopian tube, and right salpingectomy was performed. Unfortunately, intrauterine fetal death was diagnosed at 19 weeks' gestation. In conclusion, the possibility of HP should be considered in patients with SIs around the day of SET.