Ruptured tubal pregnancy after low hCG prompted discontinuation of luteal support: a case report of vitrified-warmed embryo transfer in a hormone replacement cycle.

Frozen-thawed embryo transfer (FET) in hormone replacement cycles (HRC) is widely used in assisted reproductive technology (ART). However, interpreting early serum human chorionic gonadotropin (hCG) levels remains challenging, especially in the absence of endogenous luteal function. This case report describes a 30-year-old woman who underwent FET with a high-quality blastocyst under HRC. At 4+1 weeks, her serum hCG level was 6.6 mIU/mL ; hormone support was discontinued, assuming a biochemical pregnancy loss. No follow-up hCG testing or transvaginal ultrasound was done. At 8+1 weeks, she presented with acute abdominal pain and was diagnosed with a ruptured tubal ectopic pregnancy with massive hemoperitoneum. Emergency laparoscopy revealed 1,500 mL of intra-abdominal bleeding ; right salpingectomy was performed and histopathological examination confirmed a tubal pregnancy. This case highlights the diagnostic limitations of relying on a single low hCG measurement during FET cycles. Even low or borderline hCG levels are compatible with ongoing or ectopic pregnancies. Clinicians should be cautious before discontinuing luteal support and should consider continued follow-up when hCG is detectable. In ART settings where early hCG testing is routine, integrating serial hCG measurements with early imaging can avoid life-threatening delays in the diagnosis of ectopic pregnancy.