False-negative qualitative human chorionic gonadotropin (hCG) test result ('hook effect') with classical ultrasound...

False-negative qualitative Human Chorionic Gonadotropin (hCG) result is a phenomenon in which large amounts of β-hCG are produced by molar pregnancy, oversaturating the test's assay system and leading to false-negative results known as the 'prozone phenomenon' or the 'hook effect'. This can lead to misdiagnosis and delay in management despite high suspicious clinical and ultrasound findings. We report a case of an 18-year-old female who presented to our health facility with amenorrhea of 16 weeks, lower abdominal pain, soft and large fundal height for gestational age, and cramping with slight per-vaginal bleeding, and a negative urinary pregnancy test (UPT). Based on clinical presentation, ultrasound findings and a positive UPT after urine dilution, molar pregnancy was diagnosed. Aspiration was performed under ultrasound guidance, and follow-up was done as per MSF guidelines. HCPs need to be familiar with some rare cases for which the possibility of finding false-negative UPT is likely.