Falsely Low β-hCG Due to the Hook Effect in Twin Pregnancy with Hydatidiform Mole and Co-Existing Normal Live Fetus: A Case Report.

OBJECTIVE: Coexistence of hydatidiform mole (HM) and normal pregnancy in twin pregnancies is an extremely rare phenomenon. In HM, overproduction of human chorionic gonadotropin (β-hCG) by proliferating trophoblasts often results in extremely high serum β-hCG levels, which in turn causes a hook effect during the detection of β-hCG concentrations using immunochemiluminescent methods. This case highlights the critical role of serial dilution in mitigating hook effect-induced false-negative β-hCG results.

CASE REPORT: This article reports a case of a 28-year-old woman presented to our hospital at 12 weeks' gestation with suspected HM and normal fetal coexistence. The patient became pregnant via in vitro fertilization procedure and showed a single gestational sac at 9 weeks' gestation. Unfortunately, at 13 weeks of gestation, the patient was diagnosed with twin pregnancy with HM and a co-existing normal live fetus. The pregnancy ended with a therapeutic abortion.

CONCLUSIONS: Individualized diagnosis and treatment are important when HM coexists with a normal fetus. In such cases, we strongly recommend that ancillary tests be performed in addition to traditional imaging evaluation. In particular, the laboratory should be aware of false-negative β-hCG due to the hook effect.