Diagnostic Challenges and Uterus-Conserving Surgical Management Following Systemic Multi-dose Methotrexate for a Left Interstitial Pregnancy

Interstitial ectopic pregnancy is an uncommon form of ectopic pregnancy and can be associated with a high risk of rupture and hemorrhagic death due to its deep myometrial location.Diagnosis can be challenging because transvaginal ultrasonography (TVUS) may fail to initially identify the implantation site until late in gestation, and the presentation can closely mimic that of an early intrauterine pregnancy or spontaneous abortion.We report the case of a 27-year-old gravida 2 para 1 (G2P1) woman who presented to the emergency department (ED) with vaginal spotting and pelvic cramping at an estimated seven weeks gestational age.Serial serum beta-human chorionic gonadotropin (β-hCG) measurements demonstrated an initial suboptimal rise followed by a plateau and subsequent decline (3,954 → 6,136 → 5,863 → 5,028 mIU/mL).Repeat TVUS identified no intrauterine pregnancy and no discrete adnexal mass, constituting a pregnancy of unknown location (PUL).Following stabilization in the Rapid Decision Unit, the patient was discharged with urgent outpatient obstetrics and gynecology (OB/GYN) follow-up.Interval repeat TVUS raised concern for a left interstitial ectopic pregnancy, prompting hospital readmission for diagnostic laparoscopy, which confirmed a 3-4 cm unruptured left interstitial ectopic pregnancy.Given hemodynamic stability and the high vascularity of the interstitial mass, intraoperative conservative management was elected in lieu of immediate wedge resection.Postoperatively, she received multi-dose systemic methotrexate with leucovorin rescue under co-management with maternal-fetal medicine and was discharged in stable condition.Although β-hCG declined to undetectable levels, interval imaging identified a persistent adnexal mass and a newly developed left hydrosalpinx.Given the patient's desire for future fertility, she underwent laparoscopic left salpingectomy with wedge resection of the left uterine cornu as uterus-conserving definitive management.This case illustrates the diagnostic limitations of TVUS and the critical role of serial β-hCG kinetics in PUL, as well as the feasibility and limitations of uterus-conserving medical and surgical management for interstitial ectopic pregnancy, including the need for long-term surveillance and the role of definitive surgery when resolution after methotrexate is incomplete.