We present a case of a tubal ectopic pregnancy (EP) in a patient with an initially undetectable serum β-human chorionic gonadotropin (β-hCG) level. A 33-year-old woman in a same-sex relationship underwent timed donor intrauterine insemination. Her serum β-hCG level was <5 mIU/mL 14 days after the intrauterine insemination. She reported menstrual bleeding 3 days after her negative pregnancy test and returned to the office 10 days later to begin a new treatment cycle. Her serum levels of estradiol, progesterone, and β-hCG were 119 pg/mL, 6.1 ng/mL and 1157 mIU/mL, respectively. Transvaginal ultrasonography did not show an intrauterine pregnancy. Her β-hCG level increased to 1420 mIU/mL the next day. She was diagnosed with a pregnancy of unknown location and treated with methotrexate. Her β-hCG levels continued to increase despite 3 methotrexate doses, necessitating laparoscopy. The diagnostic laparoscopy demonstrated approximately 100 mL of hemoperitoneum in the posterior cul-de-sac with an intact right fallopian tube that was dilated at its distal end by the EP. A total right salpingectomy was performed. Her β-hCG level was <5 mIU/mL 3 weeks later. The current case supports that although rare, an undetectable serum β-hCG level does not completely rule out the diagnosis of an EP.
